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2014 44 Ana Pérez Sanz Holoceno climate, vegetation and human aimpact in the Western Mediterranean inferred from Pyrenean lake records and climate models Departamento Director/es Ciencias de la Tierra González Sampériz, Penélope Valero Garcés, Blas Director/es Tesis Doctoral Autor Repositorio de la Universidad de Zaragoza – Zaguan http://zaguan.unizar.es UNIVERSIDAD DE ZARAGOZA Departamento Director/es Ana Pérez Sanz HOLOCENO CLIMATE, VEGETATION AND HUMAN AIMPACT IN THE WESTERN MEDITERRANEAN INFERRED FROM PYRENEAN LAKE RECORDS AND CLIMATE MODELS Director/es Ciencias de la Tierra González Sampériz, Penélope Valero Garcés, Blas Tesis Doctoral Autor 2014 Repositorio de la Universidad de Zaragoza – Zaguan http://zaguan.unizar.es UNIVERSIDAD DE ZARAGOZA Departamento Director/es Director/es Tesis Doctoral Autor Repositorio de la Universidad de Zaragoza – Zaguan http://zaguan.unizar.es UNIVERSIDAD DE ZARAGOZA Holocene climate, vegetation and human impact in the Western Mediterranean inferred from Pyrenean lake records and climate models Ana Pérez Sanz PhD Thesis Instituto Pirenaico de Ecología. Consejo Superior de Investigaciones Científicas Departamento de Ciencias de la Tierra. Facultad de Ciencias. Universidad de Zaragoza 2014 This dissertation is submitted to the Faculty of Science of the University of Zaragoza for the degree of Doctor of Philosophy. Memoria presentada para optar al grado de Doctor en Geología por la Universidad de Zaragoza. Title Holocene climate, vegetation and human impact in the Western Mediterranean inferred from Pyrenean lake records and climate models. Reconstrucción holocena de la dinámica del clima, vegetación y acción humana en el Mediterráneo Occidental a partir de registros lacustres pirenaicos y modelos climáticos. PhD Student Ana Pérez Sanz Supervisors Penélope González Sampériz Blas Lorenzo Valero Garcés Institutions Instituto Pirenaico de Ecología, IPE-CSIC Pyrenean Institute of Ecology, IPE-CSIC Departamento de Ciencias de la Tierra. Facultad de Ciencias. Universidad de Zaragoza Department of Earth Sciences. Faculty of Sciences. University of Zaragoza A mis padres    5.1.1. The onset of the Holocene: 11.7009800 cal yr BP ............................................... 181 5.1.2. The Early Holocene: 9800-8200 cal yr BP ............................................................ 182 5.1.3. The Mid-Holocene: 8200-6000 cal yr BP .............................................................. 184 5.1.4. Transitional phase: 6000-4800 cal yr BP .............................................................. 189 5.1.5. The Late Holocene: 4800-800 cal yr BP ............................................................... 190 5.2. Chronology of the Anthropoenic impact in the Southren Pyrenees ................................... 193 5.2.1. 2000-500 BC (4-2.5 cal ka BP): first intensification of human impact in lowlands ...... 193 5.2.2. 500 BC-750 AD (2.5-1.2 cal ka BP): expansion of agricultural activities in lowlands during the Iberian-Roman and Visigoth Times ................................................................. 194 5.2.3. 750-1150 AD (1.2-0.8 cal ka BP): rise of grazing practises in lowlands and increasing impact in highlands during the Muslim period .................................................................. 196 5.2.4. 1150-1650 AD (0.8-0.3 cal ka BP): threshold in land management in low and highlands during the Middle Ages ................................................................................................. 196 5.2.5. 1650-1800 AD (0.3-0.15 cal ka BP): concentration of human activities in lowlands during the second half of the Little Ice Age ..................................................................... 198 5.2.6. 1800-1950 AD (0.15-0 cal ka BP): spread of extensive agriculture practises after the Industrial Revolution .................................................................................................... 198 6.2.7. Current times ................................................................................................... 199 5.3. Timing and dynamics of vegetation belt changes in the Central Pyrenees ........................ 176 References .......................................................................................................................... 178 6. Conclusions ........................................................................................ 187 Conclusions ......................................................................................................................... 189 Climate dynamics in north-eastern Spain during the Holocene ............................................... 189 The impact of the anthropogenic activities in the Pyrenees .................................................... 190 PiControl and Mid-Holocene climate model simulations ......................................................... 191 Conclusiones ....................................................................................................................... 192 Dinámica del clima en el noreste de España durante el Holoceno ........................................... 192 El impacto de la actividad antrópogénica en los Pirineos ........................................................ 193 Simulaciones de modelos climáticos para el periodo Pre-Industrial y el Holoceno-Medio ............ 194  AGRADECIMIENTOS / ACKNOLEDGEMENTS Me siento especialmente afortunada por la etapa doctoral que me ha tocado vivir. Han sido pocas las dificultades y muchas las alegrías que he pasado durante estos años. Esto es, sin duda, debido al increíble grupo de personas que me rodean y me han rodeado a lo largo de este periodo. Con estas líneas me gustaría recordar y agradecer a todas esas personas que durante estos años han contribuido de manera profesional y personal a la elaboración de este proyecto tan importante. La presente tesis doctoral no hubiera sido posible, ni siquiera imaginable, sin la guía, el trabajo, los consejos y el apoyo de mis directores Penélope González Sampériz y Blas Valero Garcés. Lo que he aprendido a su lado está más allá de lo que podría haber imaginado. Esto es aplicable no sólo al plano científico, sino también humano. A su lado he madurado científica y personalmente. Los agradecimientos son extensibles a mis compañeros de grupo de investigación cuya calidad humana ha hecho que las pequeñas dificultades científicas que han podido aparecer a lo largo de esta tesis se diluyan y pierdan en la memoria de estos cinco años de trabajo. Me gustaría agradecer en particular a Ana Moreno y Graciela Gil que han contribuido de manera determinante al contenido científico de esta tesis, así como al resto del equipo que han participado física o moralmente a la consecución de este trabajo: Josu, Edu, Fernando, Matias, Miguel Sevilla, Miguel Bartolomé, María y Carlos. Y a otros que ya no forman parte del equipo oficial pero que han sido parte imprescindible en este proyecto: Mario, Pablo, Mayte, Carlos Sancho, Ánchel, Belén, Bea, Aida, Raquel, Pol y Laura Lasheras. Durante mi etapa investigadora en el Instituto Pirenaico de Ecología he tenido la fortuna de haber podido compartir impresiones con investigadores de otros grupos de investigación. Estoy especialmente orgullosa de haber podido ampliar mis horizontes científicos de la mano de los doctores José María García, Teodoro Lasanta, Chechu Camarero, Gabriel Montserrat y en especial Nacho López que se involucró y me ayudó en lo personal. I would like to give special thanks to Prof. Sandy Harrison for hosting me and supervising me at the Macquarie University in Sydney and introducing me in the model world. I am very grateful for the opportunity to have met her and to have worked closely with her. I will never forget the invitation to the Sydney Opera House! Si algo voy a echar de menos de mi etapa en el IPE son las conversaciones sobre lo humano y lo divino de los almuerzos junto con Pili, Hugo, María, Javi, Jesús, Nacho y Fergus como núcleo duro. No ha habido tema que se haya escapado de las garras críticas de este grupo de jóvenes investigadores: ciencia, política, religión, economía, recortes, austeridad, educación… ¡además de haber solucionado la crisis un millón de veces! Conversaciones que garantizaron una dosis bien alta de risa diaria. Por supuesto, me gustaría agradecer también al resto de compañeros del IPE con los que he compartido momentos especiales: Edu, Josu, Fabi, Enrique, Jorge, Cecilia, Paloma, Iker, Sam y Jesus V. Además, me llevo a una amiga para toda la vida: Pili. Tenerla como compañera de despacho ha sido lo mejor que me podría haber pasado. Gracias por todas las confidencias, tu buen humor y la complicidad. Y por haber hecho de esta etapa una experiencia inolvidable. Mi familia es parte fundamental de lo que soy como persona. Tengo la suerte de contar con una familia que da valor al significado de la palabra familia. Si a alguien admiro en esta vida, esos son mi madre, mi padre y mi hermano. Y sobre todo a mi yaya, Pilar Franco, que me ha enseñado que no hay nada imposible si uno se lo propone y que es el ejemplo de cómo hay que ser en la vida. A mis tíos Pili, Ibán y Susana y a mis primos Adrián y Paula, que me hacen reír tanto y siempre tienen palabras de apoyo; y a mis primitos Bruno y Claudia que alegran a cualquiera con su presencia. Me siento inmensamente orgullosa y feliz de formar parte de una familia así. Además tengo una familia no consanguínea que también es parte fundamental de lo que soy. Mis amigas de Logroño me han acompañado en todos los momentos importantes de mi vida. Es más, hacen importante cualquier momento. Miro hacia atrás y no hay ni un solo instante en el que no hayan estado presentes. No necesariamente físicamente, a veces hemos puesto distancias geográficas muy grandes de por medio, pero siempre moral y anímicamente. Siempre arrancando una sonrisa. Más bien una carcajada. Muchas gracias por todo Yvonne, Sandra, Marta, Marina, Laura V, Laura U, Bea y Ana. Me alegro muchísimo de que forméis parte de mi vida. Hace ya muchos años que salí de mi casa para llegar a la ciudad del cierzo. Aquí he encontrado otra gran familia con la que he compartido grandes experiencias. Alberto, Anika, Edu, Isabel, Juli, Marcos, Navarro, Raúl y Rubén. Por supuesto también quiero dedicar un agradecimiento especial a Moreno, Sebas, José Manuel, Hernando y demás integrantes de un grupo de cuyo nombre no quiero acordarme, que me hacen partirme de risa con sus comentarios afilados, desternillantes y mordaces. Sois la razón por la que me considero totalmente zaragozana y por la que he echado raíces aquí. Quiero agradecer a Raúl todos los buenos momentos de los últimos meses. Es increíble lo que me puedo llegar a reír a su lado. Gracias por darme unos martes de lujo. No puedo olvidarme de Alberto que ha hecho que mi vuelta a tierras mañas haya sido la mejor de las posibles y que junto con David han sido los mejores compañeros de piso del mundo haciéndome sentir como en mi propia casa. Durante seis meses he vivido en la ciudad más excitante del mundo, pero también la más lejana: Sidney. Me gustaría dedicar un agradecimiento muy especial a Álvaro y Catherine que me recibieron con las puertas de su casa abiertas y me hicieron olvidar la distancia que me separaba de mi mundo. Nunca podré olvidar lo que habéis hecho por mí. Vuestra ayuda y apoyo lo han significado todo. Muchísimas gracias por todo. It has been the most lifechanging experience. I would also like to give special thanks to Li. I am very honored that I got to meet him. I cannot forget your help and support. Quiero agradecer a Saúl todo su apoyo y ánimo constante durante gran parte de este largo camino. ¡Gracias a todos! RESUMEN La presente Tesis Doctoral aborda una detallada reconstrucción paleoclimática en el Pirineo Central durante el Holoceno a partir del estudio palinológico de dos secuencias lacustres localizadas a diferentes altitudes y que representan dos pisos de vegetación muy bien diferenciados: en primer lugar, la Basa de la Mora, localizado en el piso subalpino y, en segundo lugar, en el lago de Estaña, en piso basal del Pre-Pirineo. Además, se ha seleccionado el Holoceno Medio para estudiar la fiabilidad de los modelos climáticos a la hora de reconstruir los climas del pasado en el Mediterráneo, a partir del análisis de la expresión estacional de los climas en las simulaciones del Coupled Model Intercomparison Project (CMIP5). El estudio multiproxy (polen, sedimentología, geoquímica, quironómidos y microcarbón) de alta resolución de la secuencia de la Basa de la Mora (BSM) (42º32’ N, 0º19’ E, 1914 m s.n.m) muestra una marcada variabilidad ambiental en el Pirineo Central durante el Holoceno. El robusto modelo de edad, basado en 15 dataciones radiocarbónicas, respalda la primera reconstrucción precisa de cambios climáticos rápidos durante el Holoceno en esta área. En el Holoceno temprano se registra una cuenca altamente arbolada, con unos niveles lacustres altos y procesos intensos de run-off en la cuenca favoreció la existencia de comunidad de quironómidos dominados por taxones no lacustres (Orthocladiinae) relacionados con la entrada de arroyos fluviales. Este escenario es coherente con la alta estacionalidad en latitudes medias en el Hemisferio Norte causada por la configuración de los parámetros orbitales durante el Holoceno Temprano, que provocaría un aumento en la acumulación de nieve en las cumbres pirenaicas durante el invierno así como unas altas tasas de fusión de la nieve durante el verano. Entre 9.8 y 8.1 cal yr BP, se reconoce una gran inestabilidad climática debido al registro de profundos cambios en la cubierta vegetal y de una alta fluctuación en los procesos de erosión en la cuenca. Las variaciones entre coníferas y mesofitos has revelado la ocurrencia de al menos cuatro eventos rápidos y de corta duración registrados aproximadamente a 9.7, 9.3, 8.8 y 8.3 cal Ka BP. Entre 8.1 y 5.7, durante el Holoceno Medio, un clima más estable con abundante precipitación dio lugar a los máximos niveles lacustres, la expansión del bosque de caducifolios, la retirada de las coníferas y la intensificación de los fuegos. Hacia el 5.7 cal Ka BP un cambio climático hacia condiciones más secas contribuyó al declive regional de los arboles caducifolios, la expansión de los pinos y Juniperus y un descenso notable de los niveles del lago. A pesar de las condiciones más secas, la actividad del fuego se redujo debido a una disminución de la biomasa disponible. Dos intervalos especialmente áridos tuvieron lugar entre 2.9 y 2.4 cal Ka BP y entre 1.2 y 0.7 cal Ka BP (800-1300 AD). El segundo coincide con la Anomalía Climática Medieval y en la secuencia BSM se registra como unos de los periodos más áridos del Holoceno. La actividad antrópica es escasa e incluso nula durante la mayor parte del Holoceno, hasta al menos los últimos 700 años, cuando se reconocen los primeros signos de deforestación. La Pequeña Edad de Hielo se registra por un aumento de los niveles lacustres y por lun abandono de las actividades humanas debido a las condiciones frías en las cumbres pirenaicas. El registro palinológico del lago de Estaña (EST) (670 m s.n.m., 42°02’N, 0°32’E) proporciona la primera reconstrucción Holocena de la vegetación en piso basal de los Pirineos. La presente Tesis Doctoral presenta una comparación de la secuencia de Estaña con otras secuencias polínicas pirenaicas localizadas en pisos de vegetación más altos, permitiendo ilustrar el papel de los cambios en temperatura y precipitación que dieron lugar a un ajuste vertical de los pisos de vegetación en los Pirineos durante el Holoceno. Durante el comienzo del Holoceno, una estacionalidad alta y unas condiciones extremadme áridas dieron lugar a un paisaje estépico en Estaña, impidiendo las expansión del bosque en altitud. Entre 9.2 y 8.2 cal Ka BP, un aumento de las temperaturas de invierno junto a una mayor disponibilidad hídrica permitieron la expansión de los taxones arbóreos, principalmente Corylus, en Estaña. Este paisaje dominado por taxones mesófilos sugiere una distribución uniforme de la precipitación a lo largo del año en el piso basal de los Pirineos. Sin embargo, contrasta con un patrón de precipitación con una estación seca establecido en cotas más altas del Pirineo, indicando la existencia de un patrón hidrológico muy complejo en la región durante este periodo. Entre 8.2 y 6 cal Ka BP, la ocurrencia de inviernos cálidos y condiciones muy húmedas con una distribución de la precipitación uniforme, dio lugar al desarrollo de un bosque de tipo Mediterráneo, formado por Quercus semi-caducifolios, en Estanya y favoreció la expansión en altitud del bosque de caducifolios, el cual pudo establecerse en el piso subalpino. El periodo entre 6 y 4.8 cal Ka BP fue una fase de transición a nivel regional en el que se empezó a establecer una estacionalidad en la precipitación caracterizada por la existencia de una estación árida. Dado el carácter mediterráneo de la vegetación en Estaña, este cambio en el patrón de la vegetación sólo afecto a la vegetación mesófila del piso subalpino. El establecimiento final de unas condiciones áridas en torno al 4.8 cal Ka BP, causó la desaparición de importantes masas de árboles caducifolios en el área y favoreció la expansión de Quercus semi-caucifolio y perennifolio en Estaña y la expansión de Pinus a mayores altitudes. Los primeros signos de actividad antrópico en Estaña se registran hacia el años 3.1 cal Ka BP con la ocurrencia de la primera fase de deforestación y la aparición de polen de tipo Cerealia. El aumento del manejo del paisaje se produjo en torno al 0.8 cal ka BP debido a la expansión de las actividades agrícolas y ganaderas. Además, en la presente Tesis Doctoral también se ha analizado la expresión estacional de los climas del Mediterráneo y norte de África en las simulaciones del Coupled Model Intercomparison Project (CMIP5) para el Holoceno-Medio y el periodo Pre-Industrial. Las observaciones climáticas actuales muestran cuatro tipos distintos de regímenes de precipitación caracterizados por una distribución estacional y una cantidad total de precipitación diferente: una banda ecuatorial, caracterizada por un pico doble en la precipitación; la zona del Monzón, caracterizada por la concentración de la lluvia en verano; el desierto, caracterizado por una baja estacionalidad y cantidad total de lluvia; y la zona del Mediterráneo, caracterizado por sequía estival. En las simulaciones para el periodo PreIndustrial, la mayoría de los modelos simulan adecuadamente la posición de los climas del Mediterráneo y del ecuador pero sobrestiman la extensión de la influencia del monzón y subestiman la expansión del desierto. Sin embargo, la mayoría de los modelos fallan a la hora de reproducir la cantidad total de precipitación en cada zona. En las simulaciones para el Holoceno-Medio, los modelos simulan una reducción de la precipitación de invierno en la zona ecuatorial, y una expansión hacia el norte del monzón con un aumento significativo de la precipitación de verano y otoño. La precipitación aumenta ligeramente en el desierto, principalmente en verano y otoño, debido a una expansión hacia el norte del monzón. Por su parte los cambios en el Mediterráneo son muy pequeños, aunque hay un ligero aumento de la precipitación en primavera consistente con los datos paleoclimáticos que muestran una expansión de los arboles caducifolios y por tanto un aumento de la precipitación en la estación de crecimiento durante el Holoceno Medio. La comparación con las reconstrucciones también sugieren que la mayoría de los modelos subestiman los cambios anuales en precipitación durante el Holoceno Medio en todas las zonas salvo en la banda ecuatorial. SUMMARY The present PhD Thesis addresses a detailed paleo-climate reconstruction for the Central Pyrenees during the Holocene through the study of two lacustrine sequences placed at different altitudes, representing two marked different vegetation belts:Lake Basa de la Mora located in the subalpine belt and Lake Estaña placed in the basal belt of the Pre-Pyrenean Ranges. Additionally, this Thesis has also analyzed the simulations from the fifth phase of the Coupled Model Intercomparison Project (CMIP5) of Mediterranean climates for the midHolocene (midHolocene, 6 ka) and compare with available pollen-based climate reconstructions. High resolution multiproxy data (pollen, sedimentology, geochemistry, chironomids and charcoal) from the Basa de la Mora (BSM) lake sequence (42º32’ N, 0º19’ E, 1914 m a.s.l.) show marked climate variability in the central southern Pyrenees throughout the Holocene. A robust age model based on 15 AMS radiocarbon dates underpins the first precise reconstruction of rapid climate changes during the Holocene from this area. During the Early Holocene, increased winter snowpack and high snowmelt during summer, as a consequence of high seasonality, led to higher lake levels, a chironomid community dominated by nonlacustrine taxa (Orthocladiinae) related to higher inlet streams, and a forested landscape with intense run-off processes in the watershed. From 9.8 to 8.1 cal ka BP, climate instability is inferred from rapid and intense forest shifts and high fluctuation in surface run-off. Shifts among conifers and mesophytes reveal at least four short-lived dry events at 9.7, 9.3, 8.8 and 8.3 cal ka BP. Between 8.1 and 5.7 cal ka BP a stable climate with higher precipitation favoured the highest lake levels and a forest expansion, with spread of mesophytes, withdrawal of conifers and intensification of fires, coinciding with the Holocene Climate Optimum. At 5.7 cal ka BP a major change leading to drier conditions contributed to a regional decline in mesophytes, expansion of pines and junipers, and a significant lake level drop. Despite drier conditions, fire activity decreased as a consequence of biomass reduction. Two arid intervals occurred between 2.9 and 2.4 cal ka BP and at 1.2-0.7 cal ka BP (8001300 AD). The latter coincides with the Medieval Climate Anomaly and is one of the most arid phases of the Holocene in BSM sequence. The Holocene palynological record from Lake Estanya (EST) (670 m a.s.l., 42°02’N, 0°32’E) provides the first Holocene vegetation reconstruction from the basal belt of the southern Pyrenees. The present Thesis presents a comparison of the Estanya sequence with other Pyrenean pollen sequences placed at higher altitudes that illustrates the role of temperature and precipitation changes as main drivers of the altitudinal vegetation shifts in the southern Pyrenees during the Holocene. High continentality and dry conditions during the onset of the Holocene (11.4 and 9.8 cal ka BP) resulted in a landscape dominated by steppe vegetation in Estanya and a limited forest expansion in altitude. Between 9.2 and 8.2 cal ka BP, increase in winter temperature and in moisture conditions allowed the expansion of the arboreal taxa, mainly Corylus, in Estanya. This deciduous-dominated landscape suggests an evenlydistributed precipitation pattern in the basal level of the Pyrenees. However, it contrasts with the well-established dry season at higher altitudes and underlines a complex hydrological pattern in the region during this period. Between 8.2 and 6 cal ka BP, warm winters and more humid conditions with evenly-distributed precipitation led to the establishment of a well-developed Mediterranean forest in Estanya and favoured the upward expansion of the deciduous forest, which reached the subalpine belt. The period between 6 and 4.8 was a regional transition phase characterized by a shift in the precipitation seasonality with the establishment of a dry season. Given the Mediterranean-nature of the vegetation of Estanya, this shift affected exclusively the higher vegetation belts characterized by a larger presence of mesophytes. The final establishment of drier conditions at 4.8 cal ka BP caused the disappearance of important deciduous masses in the area and favoured the spread of semideciduous and evergreen Quercus in Estanya and Pinus at higher altitudes. The first signs of anthropogenic activity in Estanya are recorded at 3.1 cal ka BP with the occurrence of a deforestation phase and the appearance of Cerealia type. Increasing landscape management took place at 0.8 cal ka BP through the spread of grazing and farming practices. Additionally, this Thesis has also analyzed the spatial expression of seasonal climates of the Mediterranean and northern Africa in pre-Industrial (piControl) and mid-Holocene (midHolocene, 6 ka) simulations from the fifth phase of the Coupled Model Intercomparison Project (CMIP5). Modern observations show four distinct precipitation regimes characterized by differences in the seasonal distribution and total amount of precipitation: an equatorial band characterized by a double peak in rainfall, the monsoon zone characterized by summer rainfall, the desert characterized by low seasonality and total precipitation, and the Mediterranean zone characterized by summer drought. Most models correctly simulate the position of the Mediterranean and the equatorial climates in the piControl simulations, but over-estimate the extent of monsoon influence and underestimate the extent of desert. However, most models fail to reproduce the amount of precipitation in each zone. Model biases in the simulated magnitude of precipitation are unrelated to whether the models reproduce the correct spatial patterns of each regime. In the midHolocene, the models simulate a reduction in winter rainfall in the equatorial zone, and a northward expansion of the monsoon with a significant increase in summer and autumn rainfall. Precipitation is slightly increased in the desert, mainly in summer and autumn, with northward expansion of the monsoon. Changes in the Mediterranean are small, although there is an increase in spring precipitation consistent with palaeo-observations of increased growing-season rainfall. Comparison with reconstructions shows most models under-estimate the mid-Holocene changes in annual precipitation, except in the equatorial zone. Biases in the piControl have only a limited influence on midHolocene anomalies in ocean-atmosphere models; carboncycle models show no relationship between piControl bias and midHolocene anomalies. Biases in the prediction of the midHolocene monsoon expansion are unrelated to how well the models simulate changes in Mediterranean climate. 1 Introduction Outline Knowledge of Earth's past climates is essential to understand modern variability and forecast future Earth's climate. The window to past climate reconstruction is paleoenvironmental and paleoclimate analyses. The Pyrenees is a unique region to study past climate environments in north-eastern Iberia and the western Mediterranean because its geographic location and detailed, high-resolution reconstructions provide a regional framework to test climate models. 1. Introduction 8 Palaeo-environmental studies available in the southern face of the Pyrenees are summarized in figure 1.3. According to these studies the expansion of the forest in altitude was delayed at least ca. 1000 since the beginning of the Holocene due to yet severe climate conditions (Miras et al., 2007; Cunill et al., 2012), although the forest spread easily afterwards. During the Early Holocene increase in humidity was much pronounced in the Atlantic-influenced area, where it took place a large expansion of mesophytes (Montserrat-Martí, 1992; González-Sampériz et al., 2006), than in the Mediterranean-influenced area, where pine was the main forest component (Miras et al., 2007; Pérez-Obiol et al., 2012). Nevertheless, during the Mid Holocene, even the highest and most eastern sequences recorded some increase in mesophytes due to more humid environmental conditions (Miras et al., 2007; Pèlachs et al., 2007). Approximately after ca 5 cal ka BP, deciduous trees decreased probably as a result of the onset of a trend to drier conditions that, in general, has continued until the present (Miras et al., 2007). Conversely, at this time Abies expanded considerably, although whether this expansion was climate-driven or, responded to migratory paths from the glacial refugia is controversial (Montserrat-Martí, 1992; Pèlachs et al., 2009, 2011). Another subject of much debate is the spread of Fagus from ca 4 cal ka BP (MontserratMartí, 1992; Pèlachs et al., 2009). The expansion of this tree coincides with an increasing presence of anthropogenic signals, indicating that beech spread could be favored by humans (Miras et al., 2007, Pèlachs et al., 2011). In general terms, the impact of the anthropogenic activities rose during the Roman Times and gained momentum during the middle ages. Although we know the main vegetation pattern changes during the Holocene, there is still an important lack of information regarding: Shifts in vegetation belts as a result of changes in temperature. Most of the sites are located at altitudes above 1600 m. a.s.l., hindering the comparison of migration and time-evolution of the vegetation belts (fig. 1.3a). Up to now, there are only two sequences located below 1200 m. a.s.l.: Paul the Bubal, but it only has one radiocarbon date for the Holocene (Montserrat-Martí, 1992); and Montcortés, but the published pollen record only covers the last 1000 years (Rull et al., 2011). Consequently, there is a shortage of data from the low altitude vegetation belts in the Pyrenees that could complete the altitudinal transects and shed light about changes in species distributions related to temperatures shifts during the Holocene. Vegetation dynamics as a result of changes in precipitation. Most palynological studies are located in the Eastern Pyrenees while fewer sites are from the Western area (fig. 1.3a). In general, the eastern lacustrine sequences are the most recent studies, achieving better resolution and chronological control. Conversely, the scarce western-central sequences either lack a good temporal framework (MontserratMartí, 1992) or do not cover the whole Holocene (González-Sampériz et al., 2006). As a result, detailed comparisons between eastern and western areas in order to investigate variations in the limit of the Atlantic and Mediterranean forests related to changes in the precipitation regimen, are not possible. Furthermore, the eastern sequences are placed 1. Introduction 9 too far from the Atlantic influence to have recorded more oceanic conditions even during past periods of stronger westerlies. The relationship between climate and hydrological balance Palynological studies from Pyrenean sites have not been systematically integrated with sedimentological and paleolimnological studies (Pla and Catalan, 2005; Miras et al., 2007, 2010; Ejarque et al., 2010; Pérez-Obiol et al., 2012), missing an opportunity to integrate regional vegetation dynamics with local environmental conditions in order to capture common climate signals. The reconstruction of past lake level changes can characterize or constrain the features of the precipitation patterns inferred from the vegetation composition. The Quaternary Global Change research team from the Pyrenean Institute of Ecology (IPE-CSIC, http://www.ipe.csic.es/cambios-globales/) has been a pioneer team in Spain that systematically applies a multiproxy strategy to paleoclimate studies based on lake sediments. Rapid and abrupt changes during the Holocene. Because of the low sample resolution or/and because of their location in a more stable environment, available palaeo-environmental sequences from the Pyrenees do not provide evidence of the forest response to the abrupt climate changes occurred during the Holocene. Differences in patterns of human occupation between lowlands and highlands. Evidence of anthropogenic activities in the Pyrenees is ancent. Signs of human activity in the subalpine belt of central and eastern Pyrenees can be traced back to ca 8 cal ka BP (Miras et al., 2007; Ejarque et al., 2010) and can be recognised sporadically through the whole Neolithic period (Pèlachs et al., 2007; Miras et al., 2010; Cunill et al., 2012; Pérez-Obiol et al., 2012). On the other hand, in spite of the accessibility of the central Pyrenean lowlands and their more suitable climate than the highlands, there is a completely lack of information of human activities in these areas from sedimentary archives (fig. 3a). Up to now, there are only two lacustrine sequences located under 1200 m a.s.l in this region: Paul the Bubal (Montserrat-Martí, 1992) and Montcortés, (Rull et al., 2011). Some archaeological sites have provided pollen data too but they only cover short chronological periods (González-Sampériz, 2004). 1. Introduction 10 Figure 1.3. a) Schematic representation of the vegetation zones of the Southern Pyrenees including the location and altitude of all the paleoenvironmental sequences available in the area. Portalet: González-Sampériz et al., 2006; Paul de Bubal: MontserratMartí, 1992; Tramacastilla: Montserrat-Martí, 1992; BSM (Basa de la Mora): present study; Estanya: present study; Morellón, 2009; Redo: Pla and Catalan, 2005; Montcortés: Corella et al., 2010; Rull et al., 2011; Burg: Pèlachs et al., 2007; Estanilles: Pérez-Obiol et al., 2012; Pradell: Ejarque et al., 2009; BDE (Bosc dels Estanyons): Miras et al., 2007; PDP (Planells de Perafita): Miras et al., 2010; RDO (Rius del Orris): Ejarque et al., 2010. b) Vegetation description of the different vegetation belt of the Pyrenees. The definition of the vegetation belts are based on Villar, 1997, Domínguez-Llovería and Puente-Cabeza, 2003 and Ninot et al., (2007). The description includes the main trees, shrubs and herbs that for each belt, sorted out by order of abundance. Main species are highlighted in bold. 1. Introduction 11 1.1.3. Paleoclimate models for the Mediterranean region The Mid-Holocene (8.2 - 6 ka BP) provides an opportunity to examine climate-model performance in the Mediterranean region. Palaeo-environmental evidence suggests that during the Mid-Holocene the Mediterranean region was wetter than today. A more positive balance and a change in rainfall seasonality during this period is supported by the rise in lake levels (Kohfeld and Harrison, 2000; Magny et al., 2002; Roberts et al., 2008) and the expansion of deciduous trees (Prentice et al., 1996; Roberts et al., 2004; Carrión et al., 2010) recorded across the region. However, given the high complexity of interactions involved in the Mediterranean climate (Xoplaki et al., 2003; Luterbacher et al., 2006; Lionello, 2012), models have been unable to reproduce the observed MH patterns of rainfall in the Mediterranean during the Holocene. This was identified as a problem in the atmosphere-only simulations made during the first phase of the Palaeoclimate Modelling Intercomparison Project (PMIP1: see e.g. Masson et al., 1999; Guiot et al., 1999; Bonfils et al., 2004), coordinated by the Working Group on Coupled Modeling (WGCM), and also in the coupled ocean-atmosphere simulations made during PMIP2, where the spatial extent and the magnitude of the changes were not well captured (Brewer et al., 2007) A new suite of climate simulations from Coupled Model Intercomparison Project (CMIP5, Taylor et al., 2012) has been launched recently and are being analyzed as part of the third phase of the Palaeoclimate Modelling Intercomparison Project (PMIP3: Braconnot et al., 2012). The CMIP5 experiments represent a new opportunity to assess the ability of the climate models to reproduce past and present precipitation changes in the Mediterranean region in a better way. 1.2. OBJECTIVES The present PhD dissertation hast three overarching objectives. Given the high potential of the Holocene Pyrenean sequences to provide past environmental informations, this PhD dissertation aims: to investigate past precipitation and temperature changes in the Pyrenees based on vegetation dynamics and lake level variations through multi-dimensional and multi-proxy studies from sequences placed at different altitudes, in order to get a thorough understanding of the long-term climate evolution and the impact of the abrupt climate changes in the Western Mediterranean during the Holocene. Additionally, given the concerns of the precipitation over the Mediterranean region, the present thesis also aims: 1. Introduction 12 to check the ability of climate models from the CMIP5 to simulate the reconstructed more humid conditions in the Mediterranean region during the Mid-Holocene in order to measure their reliability on future climate scenarios. Finally, given the presence of human populations in the area since at least the Early Holocene, the last objective of this work is: to describe the timeline of the human activities in the Pyrenees in order to find out patterns of occupation, landscape modifications and effects of climate events on populations. 1.3. OUTLINE This PhD dissertation embraces two different research fields with specific methodologies: i) reconstruction of paleoclimates and palaeo-environments based on field and laboratory work and ii) evaluation of models analyses, based on mathematical calculations. For these reasons, methodology is described in each chapter instead of constituting an independent section. The present PhD thesis is divided in 6 chapters. The first one is the Introduction where the scientific context of the study and the main goals of the research are described Chapters 2 and 3 cover the first objective (climate reconstruction in the Pyrenees from two lake sequences). Chapter 4 includes the second objective (evaluation of Mid-Holocene model simulations). Each of them includes an introduction to the subject, the methodology, the results obtained, a general discussion of the results, and some conclusions. Chapter 5 comprises a summary and a brief discussion of all the results accomplished in previous chapters. The third objective is accomplished in this chapter. Finally, chapter 6 summarizes the main conclusions of this thesis. Next, there is an explanation of the road map followed to achieve the objectives of the present thesis. 1. First objective To study the past precipitation-temperature interactions in the Pyrenees, two lake sequences placed at two different altitudinal vegetation zones in the central part of the Pyrenees and Pre-Pyrenees were selected. The main reasons for the particular location of the sequences were: 1. Introduction 13 An altitude gradient. Different altitude means different climate conditions and, consequently, different vegetation composition. The comparison between vegetation dynamics in highlands and lowlands allows inferences on the evolution of the altitudinal belts as a response to a common change in temperature. In addition, the comparison between lake levels evolution at different altitudes provide a regional understanding of paleo-hydrological evolution. Atlantic and Mediterranean influences. The central part of the Pyrenees is close to the present limit between the Atlantic and Mediterranean climate boundary. The evolution of the Atlanticand Mediterranean-related taxa allowed to investigate past changes in the precipitation regimen related to strengthening or weakening of the Atlantic humid fronts. The selected sequences are the following: Basa de la Mora (BSM) (42º32’N, 0º19’E), at 1914 m. a.s.l. in the subalpine belt, was selected because likely would report changes in the treeline as a result of temperature variations. In addition, it is also located half-way between the Atlantic Ocean and the Mediterranean Sea, so that it is particularly sensitive to changes in precipitation regimen as a result of shifts in the strength of the Atlantic fronts. Estaña (EST) (42º02’N, 0º32’E), at 670 m. a.s.l., in the Pre-Pyrenean Range, provides the first Holocene vegetation reconstruction in the basal belt. In addition, it is located in direct contact with the semi-desert regimen of the Central Ebro Basin, so that it is rather susceptible to variability in water availability as a result of changes either in direct precipitation or in supplies from the Pyrenees. Both sequences fill a relevant palaeo-climate gap in the Pyrenees, as shown in figure 3a. Methods The study of the BSM sequence included pollen, sedimentology, geochemistry, charcoal and chironomid analyses, and it is the first multiproxy high-resolution study to provide climate reconstructions in the sub-alpine belt of the Pyrenees. Charcoal and chironomids data have been obtained in collaborations with other researchers from the Pyrenean Institute of Ecology and University of Barcelona respectively(Dr. Graciela Gil Romera, Laura Lasheras, Dr. Maria Rieradevall and Pol Tarrats). They have been used to support and put in a context the rest of analyses, but they are not a direct contribution from this thesis. The study of the EST sequence included exclusively pollen but a previous palaeo-hydrological reconstruction based on sedimentological and geochemical indicators carried out by Dr. Morellón, (2009) was used to integrate vegetation dynamics and lake evolution. 1. Introduction 14 Each sequence is described in a different chapter. In chapter 2 we focus on the sequence located in the highlands (Basa de la Mora), while in chapter 3 we discuss the sequence located in the lowlands (Estaña). These chapters are the base of the climate reconstruction achieved in this work and hence constitute the main body of the present thesis. 2. Second objective The achievement of the first objective made it clearer the different climate conditions that prevailed during the Mid-Holocene in the Central Pyrenees in terms of humidity.. This inspired the investigation on climate simulations for the Mid-Holocene. We focused on precipitation simulations because it is the most characteristic factor of Mediterranean climate, and water availability is a growing concern with large social implications. Area of interest The area analyzed to assess the reliability of the precipitation simulations over the Mediterranean during the Mid-Holocene included not only the Pyrenees, but the whole southern Europe and North Africa. The reason is that the position of the Inter Tropical Convergence Zone (ITCZ) may influence the position of the NAO centers -Azores High and Iceland Low- (Marshal et al., 2001; Souza and Cavalcanti, 2009). Given that the ITCZ is the main responsible for the monsoon precipitation over north Africa and the position of the NAO centers determines the strength of the North Atlantic westerlies -and consequently the precipitation over Europe-, the models will explore how changes in humid conditions over the Mediterranean region during the Mid-Holocene may be related to changes in the African monsoon and teleconnections with North Atlantic dynamics. Methods We analysed outputs from 12 General Circulation Models from the fifth phase of the Coupled Modelling Intercomparison Project (CMIP5) (Taylor et al., 2012). Analyses included firstly, Mid-Holocene simulations and comparison with palaeo-data recollected across the whole studied region and, secondly, simulations for present climate in order to explore whether past simulations are somewhat influenced or linked to the ability of models to simulate modern conditions. We want to know if some climate models are more reliable than others for the Mediterranean region. The second objective is accomplished in chapter 4. 1. Introduction 15 3. Third objective After a thorough reconstruction of vegetation dynamics in an altitudinal transect in the Pyrenees was completed as part of the first objective, we explored trends in land uses through the analyses of indicators related to human activities in the BSM and EST sequences. In addition, having done a detailed and multiproxy palaeo-climate reconstruction in chapters 2 and 3, we explored whether vegetation changes could have been somewhat influenced by anthropogenic pressure, or whether, conversely, climate events may have challenged or changed human activities. Approach to study human activities in the Pyrenees Agriculture, grazing and deforestation are the three main human activities able to change the landscape since long time ago. In sedimentary sequences, agricultural activities are recorded through the presence of cultivated taxa such as cereal, olive tree, vine or cannabis; grazing activities are recorded through the presence of pastoral-related taxa such as nitrophilous plants; and, finally, deforestation phases are recognized through marked drops in the arboreal component (Li et al., 2008). We examined the evolution of these three main indicators in BSM and EST sequences in order to find differences in time and use of the landscape regarding the altitude and climate conditions. The details of vegetation dynamics related to human activities are described in chapters 2 and 3 along with the general description and interpretation of pollen taxa. The main results and a general discussion about this third objective is accomplished in the Discussion part (chapter 5) but it does not constitute an independent chapter of the present thesis. 1.4. REARCH WOK AND CONTRIBUTIONS The present PhD dissertation is based on the following research work: Research papers Pérez-Sanz, A., González-Sampériz, P., Morellón, M., Valero-Garcés, B.L., Gil-Romera, G., Fontaneda-Ríos, S. Holocene altitudinal vegetation shifts at the Southern Central Pyrenees (Spain): the mid-montane site Lake Estanya. To be submitted. Pérez-Sanz, A., Li, G., González-Sampériz, P., Harrison, S.P. Evaluation of modern and MidHolocene seasonal precipitation of the Mediterranean and Northern Africa in the CMIP5 simulations. Clim. Past., 2014. Pérez-Sanz, A., González-Sampériz, P., Moreno, A., Valero-Garcés, B., Gil-Romera, G., Rieradevall, M., Tarrats, P., Lasheras-Álvarez, L., Morellón, M., Belmonte, A., Sancho, C., Sevilla-Callejo, M., Navas, A. Holocene climate variability, vegetation 1. Introduction 16 dynamics and fire regime in the central Pyrenees: the Basa de la Mora sequence (NE Spain). Quat. Sci. Rev. 73, 149–169, 2013. Pérez-Sanz, A., González-Sampériz, P., Valero-Garcés, B., Moreno, A., Morellón, M., Sancho, C., Belmonte, A., Gil-Romera, G., Sevilla, M., Navas, A. Clima y actividades humanas en la dinámica de la vegetación durante los últimos 2000 años en el Pirineo central: el registro palinológico de la Basa de la Mora (Macizo de Cotiella). Zubía 23, 17–38, 2011. Congress contributions Pérez-Sanz, A., et al., Paleoenvironmental reconstruction of Basa de la Mora glacial lake (Central Pyrenees) during the last 13 ka cal yr. BP: a high resoluton palynological study. XVIII INQUA Meeting, Bern. Quaternary International 279-280, 375, 2012 (poster contribution). Pérez-Sanz, A., et al., Reconstrucción paleoambiental de la Basa de la Mora (Pirineo Central): estudio multiproxy de alta resolución. XIII Iberian Quaternary Meeting (AEQUA), Andorra la Vella. Abstract book 91-92, 2011. (poster contribution). Pérez-Sanz, A. et al., Palaeoenvironmental reconstruction of Basa de la Mora glacial lake (Central Pyrenees) during the Holocene: preliminary results from palynological analyses. EGU General Assembly. Geophysical Research Abstracts 12, EGU20108076, 2010 (poster contribution). Pérez-Sanz, A., et al., Reconstrucción paleoambiental del Ibón de la Basa de la Mora (Pirineos centrales, NE Iberia): primeros resultados del análisis palinológico. VII Iberian Quaternary Meeting (AEQUA), Faro. Abstract book 255-258, 2009 (poster contribution). Research stays Jan. 2013 – Jul. 2013: Maquarie University, Sydney, Australia. Supervisor: Prof. Sandy Harrison. In addition, data from the present Thesis has also contributed to the next research works: Lasheras-Álvarez, L., Pérez-Sanz, A., Gil-Romera, G., González-Sampériz, P., SevillaCallejo, M., Valero-Garcés, B.L., 2013. Historia del fuego y la vegetación en una secuencia holocena del Pirineo central: la Basa de la Mora. Cuad. Investig. Geográfica 39, 77–95. Morellón, M., Pérez-Sanz, A., Corella, J.P., Büntgen, U., Catalán, J., González-Sampériz, P., González-Trueba, J.J., López-Sáez, J.A., Moreno, A., Pla-Rabes, S., Saz-Sánchez, M. á., Scussolini, P., Serrano, E., Steinhilber, F., Stefanova, V., Vegas-Vilarrúbia, T., Valero-Garcés, B., 2012. A multi-proxy perspective on millennium-long climate variability in the Southern Pyrenees. Clim. Past 8, 683–700. 1. Introduction 17 Moreno, A., Morellon, M., Martín-Puertas, C., Firgola, J., Canals, M., Cacho, I., Pérez, A., Belmonte, Á., Vegas-Vilarrúbia, T., González-Sampériz, P., Valero-Garcés, B., 2011. Was there a common hydrological pattern in the Iberian Peninsula region during the Medieval Climate Anomaly? PAGES News 19, 16–18. Moreno, A., Pérez, A., Frigola, J., Nieto-Moreno, V., Rodrigo-Gámiz, M., Martrat, B., González-Sampériz, P., Morellón, M., Martín-Puertas, C., Corella, J.P., Belmonte, Á., Sancho, C., Cacho, I., Herrera, G., Canals, M., Grimalt, J.O., Jiménez-Espejo, F., Martínez-Ruiz, F., Vegas-Vilarrúbia, T., Valero-Garcés, B.L., 2012. The Medieval Climate Anomaly in the Iberian Peninsula reconstructed from marine and lake records. Quat. Sci. Rev. 43, 16–32. Valero-Garcés, B., Morellón, M., Moreno, A., Corella, J.P., Martín-Puertas, C., Barreiro, F., Pérez, A., Giralt, S., Mata-Campo, M.P., 2014. Lacustrine carbonates of Iberian Karst Lakes: Sources, processes and depositional environments. Sediment. Geol. 299, 1–29. References Aranbarri, J., González-Sampériz, P., Valero-Garcés, B., Moreno, A., Gil-Romera, G., Sevilla-Callejo, M., García-Prieto, E., Di Rita, F., Mata, M.P., Morellón, M., Magri, D., Rodríguez-Lázaro, J., Carrión, J.S., 2014. Rapid climatic changes and resilient vegetation during the Lateglacial and Holocene in a continental region of south-western Europe. Glob. Planet. Change 114, 50–65. Aubert, S., Belet, J.-M., Bouchette, A., Otto, T., Dedoubat, J.-J., Fontugne, M., Jalut, G., 2004. Dynamique tardiglaciaire et holocène de la végétation à l’étage montagnard dans les Pyrénées centrales. C. R. Biol. 327, 381–388. Bal, M.-C., Pelachs, A., Perez-Obiol, R., Julia, R., Cunill, R., 2011. Fire history and human activities during the last 3300cal yr BP in Spain’s Central Pyrenees: The case of the Estany de Burg. Palaeogeogr. Palaeoclimatol. Palaeoecol. 300, 179–190. Beerling, D.J., Royer, D.L., 2011. Convergent Cenozoic CO2 history. Nat. Geosci. 4, 418–420. Berrocal, M., Sebastián López, M., Uriarte González, A., López-Sáez, J.A., 2012. Landscape Construction and Long-Term Economic Practices: an Example from the Spanish Mediterranean Uplands Through Rock Art Archaeology. J. Archaeol. Method Theory. Bond, G., 1997. A Pervasive Millennial-Scale Cycle in North Atlantic Holocene and Glacial Climates. Science 278, 1257–1266. Bond, G., Kromer, B., Beer, J., Muscheler, R., Evans, M.N., Showers, W., Hoffmann, S., Lotti-Bond, R., Hajdas, I., Bonani, G., 2001. Persistent Solar Influence on North Atlantic Climate During the Holocene. Science 294, 2130 –2136. Bonfils, C., de Noblet-Ducoudré, N., Guiot, J., Bartlein, P., 2004. Some mechanisms of mid-Holocene climate change in Europe, inferred from comparing PMIP models to data. Clim. Dyn. 23, 79–98. Braconnot, P., Harrison, S.P., Kageyama, M., Bartlein, P.J., Masson-Delmotte, V., Abe-Ouchi, A., OttoBliesner, B., Zhao, Y., 2012. Evaluation of climate models using palaeoclimatic data. Nat. Clim. Change 2, 417–424. Braconnot, P., Otto-Bliesner, B., Harrison, S., Joussaume, S., Peterchmitt, J.-Y., Abe-Ouchi, A., Crucifix, M., Driesschaert, E., Fichefet, T., Hewitt, C.D., Kageyama, M., Kitoh, A., Loutre, M.-F., Marti, O., Merkel, U., Ramstein, G., Valdes, P., Weber, L., Yu, Y., Zhao, Y., 2007. Results of PMIP2 coupled simulations of the Mid-Holocene and Last Glacial Maximum – Part 2: feedbacks with emphasis on the location of the ITCZ and midand high latitudes heat budget. Clim. Past 3, 279–296. 1. Introduction 24 2 La Basa de la Mora sequence. Climate at high altitudes Outline The Basa de la Mora sequence provides the most robust and complete Holocene palaeoenvironmental reconstruction carried out in the Pyrenees up to now. Placed in the sub-alpine belt of the Central Pyrenees at 1914 m a.s.l., this lake has witnessed relevant climate changes throughout the last 10,000 yr cal BP. Its sediments reveal a fascinating history of vegetation and lake-level changes that proves the high sensitivity of the Pyrenees to climate changes. 2. Basa de la Mora sequence. Climate at high altitudes 26 2. Basa de la Mora sequence. Climate at high altitudes 27 2.1. INTRODUCTION Long-term climate evolution during the Holocene has been strongly modulated by orbitallyforced insolation trends which determine heat distribution throughout the planet. In the northern Hemisphere, summer insolation sets limits on the position and strength of the Inter Tropical Convergence Zone (ITCZ), which controls the position of the north-hemisphere cell atmospheric system (Wanner and Brönnimann et al. 2012). In particular, the location of the Azores High and the Iceland Low pressure centres determines the latitudinal position and intensity of the North Atlantic Westerlies and the storm tracks, which largely govern rainfall distribution in the Western Mediterranean area (Greatbatch, 2000, Marhsall et al. 2002). During the Early Holocene, the maximum summer insolation in the Northern Hemisphere led to a rapid northward displacement in the ITCZ and its associated rain belt (Fleitmann et al., 2007). This northern position of the ITCZ was responsible for bringing moisture to the current world-largest desert in North Africa (Sahara and Sahel) (deMenocal et al., 2000). As the summer insolation decreased the ITCZ displaced southward, the monsoon system weakened and in south-western Europe the climate followed a general trend to an increasingly aridity since the Mid Holocene that led to decreased lake levels (Magny et al., 2007, 2011; Valero-Garcés and Moreno, 2011) and major shifts in the vegetation composition (Fletcher and Zielhofer, 2011; Roberts et al., 2011). However, beyond this general climate trend, many recent studies have documented the existence of rapid climate variability during the Holocene (Bond et al., 1997, 2001; Mayewski et al., 2004). Although the nature and mechanisms of these abrupt climate changes still remain unclear, weakening in the thermohaline circulation as consequence of meltwater inputs in the North Atlantic or changes in the Ocean´s dynamics has been recognised as one of the most important triggers (Renssen et al., 2007; Wanner et al., 2008). Furthermore, fluctuations in solar activity have also been responsible for climate shifts (Wanner et al., 2011). These short-living episodes of climate variability had a large impact over most of Europe, as it has been recorded in many continental palaeoclimate archives as lacustrine sediments (Magny et al., 2007), glacial deposits (Davis et al., 2009), and pollen records (Bordon et al., 2009; Magyari et al., 2012). Holocene climate reconstructions for the North Atlantic region involve mainly changes in temperature (Brooks and Birks, 2001). However, in the Mediterranean area Holocene variability is mostly related to changes in water availability as it is documented in vegetation distribution (Jalut et al., 2009; Sadori et al., 2011), lake levels (Magny et al., 2011) and stalagmite growth (Fleitmann et al., 2007; Spötl et al., 2010). The Iberian Peninsula climate integrates subtropical, Mediterranean and Atlantic influences due to its geographical location between the Mediterranean Sea and the Atlantic Ocean (Lionello et al., 2006). Moreover, the Iberian Peninsula has proven to be particularly sensitive to short-term climate shifts during the Holocene (Moreno et al., 2012a). Lakes experienced noteworthy variations in response to precipitation and evaporation shifts during the Holocene (Valero-Garcés et al., 2000; González-Sampériz et al., 2008; Martín-Puertas et 2. Basa de la Mora sequence. Climate at high altitudes 28 al., 2008; Morellón et al., 2009). Changes in sea surface temperatures (Cacho et al., 2001) and deepwater formation (Frigola et al., 2007) in the Western Mediterranean show a fast response to the North Atlantic dynamics. Other Iberian continental records highlight large Holocene variability. For example, the isotope record in the Kaite Cave stalagmite (Domínguez-Villar et al., 2008) reflects variations in the amount of precipitation related to North Atlantic dynamics and fluctuations in palaeoflood activity of Tagus River, in Central Spain have been related to changes in prevailing atmospheric circulation patterns (Benito et al., 2003). Although vegetation is a very good indicator of past climate variability, there are only a few high-resolution pollen studies from the Iberian Peninsula (e.g. Carrión et al., 2010; Fletcher et al., 2013a, Jiménez-Moreno and Anderson, 2012), documenting the fast response of vegetation to abrupt climate changes (decadalto centennial-scale) during the Holocene. A recent study has proved the high-sensitivity to current global warming of middle-latitude high mountain ranges in general, and the Pyrenees in particular, documenting an speeding up of the replacement of cold-adapted plants by thermophilic species (Gottfried et al., 2012). Past climate changes during the Holocene should have also affected the flora and landscape of the Pyrenees. Furthermore, the southern slopes of the Pyrenees are not affected by Foehn winds, and the present climate is rather complex, influenced by a progressive west-to-east decrease in precipitation, due to weakening of the Atlantic humid fronts inland. Thus, the southern Pyrenees experience both Atlantic and Mediterranean climate regimes within a relatively short distance of less than 450 Km between the Cantabrian and Mediterranean seas. The Pyrenean vegetation reflects these climate conditions, varying from humid-Atlantic forests, dominated by oak and beech, in the west, to Mediterranean forests, dominated by mainly pine and drought-resistant taxa, in the central and eastern regions. Due to these particular geographical features the Central Pyrenees play a key role in providing information about past E-W shifts of the boundary between both regimes as a result of shifts in the atmospheric components and, particularly, shifts in the Westerlies strength. In Western Europe, human disturbances in the landscape can be traced back to the Neolithic period and the climate signal is often masked by anthropogenic activities during the most recent times (Olfield, 2005; Carrión et al., 2007). Discriminating anthropogenic from natural forcings in landscape evolution has been subject of much debate during recent years (Carrión et al., 2010; Catalán et al., 2013). High-altitude sites are more useful than lowaltitude sites for detecting climate signals, since more inhospitable climate conditions limit intense human landscape intervention. 2.1.1. Objectives The main objective is to obtain a paleo-environmental reconstruction of climate, vegetation and fire dynamics from a lacustrine sequence located in the central part of the southern Pyrenees: the Basa de la Mora sequence. With this reconstruction we will tackle questions concerning: i) how the Atlantic and Mediterranean regimes have progressed along the 2. Basa de la Mora sequence. Climate at high altitudes 29 Holocene in the Pyrenees, ii) identification and timing of rapid episodes of climate change, and iii) elucidation of high mountain land-use system during last millennia. 2.2. STUDY AREA 2.2.1. Geological and geomorphological setting Lake Basa de la Mora (BSM) (42º 32’ N, 0º 19’ E, 1914 m a.s.l.) is a small, shallow glacial lake located on the north-facing slope of the Cotiella Peak (2912 m a.s.l.), the highest summit of the Cotiella Massif in the central southern Pyrenees (fig.2.1). The Cotiella Massif belongs to the homonymous nappe, located in the western part of the South Pyrenean Central Unit (Seguret, 1972). Figure 2.1. Location map of Lake Basa de la Mora in the Central Pyrenees (Spain). Map plotted by Miguel Sevilla Callejo. 2. Basa de la Mora sequence. Climate at high altitudes 30 Figure 2.2. Geomorphological map of Lake Basa de la Mora in the Central Pyrenees (Spain). From Belmonte, in prep. 2. Basa de la Mora sequence. Climate at high altitudes 31 The landscape surrounding the lake results from intense karstic and glacial activity. Lake Basa de la Mora occupies a glacial over-deepened basin enclosed by a frontal moraine (Belmonte, 2004) and surrounded by steep limestone walls (fig. 2.2). The catchment consists of Mesozoic limestones and sandy limestones affected by several thrust sheets (reverse faults). Triassic marl and evaporite formations crop out at the base of the thrust sheets, providing a hydrological seal for the lake and favouring localized surface drainage into the lake along some creeks. Triassic ophite formations in the watershed are the source of highly characteristic sediments (hematite and other Femineral with high magnetic susceptibility) within the lake deposits. The Basa de la Mora basin belongs to the watershed of the Cinca River, one of the main tributaries of the Ebro River. The lake has smooth margins, a relatively small watershed (209 ha) and a total lake surface of ca. 3 ha (fig. 2.3). It is characterized by large seasonal waterlevel fluctuations: the maximum depth varies from ca. 2.5 to 4.5 m seasonally. The lake is fed by precipitation, surface runoff, ephemeral creeks and several small springs located on the southern margin. Water losses take place through a surface outlet to the north and evaporation. The substrate, made up of non-permeable Triassic material, greatly restricts groundwater losses. Figure 2.3. Lake Basa de la Mora panoramic view. 2. Basa de la Mora sequence. Climate at high altitudes 32 2.2.2. Climate and vegetation The Pyrenees is a mountain range in south-western Europe that extends from the Atlantic Ocean in the west to the Mediterranean Sea in the east, leading to a diverse climate and plant community along a W-E transect. The precipitation in the Pyrenees results from two different mechanisms: precipitation in the east is linked to cold fronts and some summer convection storms, while precipitation in the west comes from Atlantic frontal systems (Millán et al., 2005). The Atlantic influence extends as far as the Ordesa Valley (García-Ruiz et al., 2001), ca. 150 km from the Atlantic coast and 22 km west of the BSM. Both systems are directly related to the North Atlantic Oscillation (NAO) that principally determines winter precipitation in western Europe (Trigo et al., 2002). Figure 2.4. a) Temperature map of the Pyrenees. b) Precipitation map of the Pyrenees. Source: Atlas Climático Digital de la Península Ibérica. Map drawn by Miguel Sevilla Callejo. The climate of the study area is sub-Mediterranean with continental features. Rainfall (annual average = 1360 mm) peaks during spring and autumn, following the Mediterranean pattern (García-Ruiz et al., 1985). However, summers are not as dry as is typical of the 2. Basa de la Mora sequence. Climate at high altitudes 33 Mediterranean because of frontal and convective precipitation which affects the mountainous areas in July and August. Mean air temperatures range from 0.5 to 15ºC between the coldest (January) and warmest (July) months, respectively (fig. 2. 4). The vegetation cover shows a characteristic contrast between south and north facing slopes: the southern slopes are characterised by mediterranean-type components with sclerophyllous shrubland and evergreen Quercus communities, while the northern slopes have mixed conifer/deciduous taxa forests, including Pinus sylvestris, Pinus uncinata, Abies alba, Betula alba, Corylus avellana, Fagus sylvatica, Quercus faginea and Quercus petraea (fig. 2.5). The altitudinal gradient between the valley bottoms and the Cotiella Peak, from 550 to 2900 m a.s.l., gives rise to an altitudinal distribution of vegetation, typical of mountain environments. Lowlands are occupied by crops and valley bottoms by riparian corridors (Fraxinus excelsior, Populus spp., and Salix spp.). Forests occur from the base of the foothills up to ~ 2000 m a.s.l. Below 1700 m a.s.l., the dominant species are determined by moisture availability and temperature range, mostly controlled by the slope orientation. From 1700 to 2000 m a.s.l. the forest is mainly composed of Pinus uncinata mixed with Juniperus communis shrubland and Rhododendron ferrugineum at the treeline. Above 2000 m a.s.l., steep rock formations and harsh climate prevent forest development, leading to a scrubdominated landscape formed by dwarf junipers (Juniperus communis sbsp. nana), and alpine grassland (Nardus stricta, Festuca eskiae, Caricion davallianae and Cynosurus cristatus). Lake Basa de la Mora (BSM) is located in the subalpine belt, near the treeline, so the vegetation surrounding the lake is alpine grassland, Pinus uncinata forest and Juniperus communis-Rhododendron ferugineum shrublands. Figure 2.5. 3D regional vegetation map. In order to better discern the topography, the North is plotted at the bottom of the figure. The star marks the location of the Lake Basa de la Mora. Map plotted by Miguel Sevilla Callejo. 2. Basa de la Mora sequence. Climate at high altitudes 40 (b) Lab Code Depth (cm) Sample type 14C age (yr BP) Calibrated age, 2σ (yr cal BP) Median probability (yr cal BP) Poz-29744 60 Terrestrial macrorest 385 ± 30 426-507 456 Poz-35854 172 Terrestrial macrorest 1335 ± 30 1231-1304 1276 Poz-29745 230 Terrestrial macrorest 2100 ± 30 1995-2146 2072 Poz-35853 269 Terrestrial macrorest 2615 ± 30 2718-2777 2749 Poz-35852 337 Terrestrial macrorest 3200 ± 30 3368-3469 3419 Poz-35804 422 Terrestrial macrorest 3815 ±35 4089-4299 4206 Poz-29743 502 Terrestrial macrorest 5185 ± 35 5893-6002 5942 Poz-35803 562 Terrestrial macrorest 5840 ± 40 6533-6745 6657 Poz-35802 677 Terrestrial macrorest 6450 ± 40 7288-7430 7367 Poz-29746 795 Charcoal 7330 ± 50 8014-8214 8125 Poz-35801 943 Terrestrial macrorest 7930 ± 50 8628-8983 8778 Poz-29747 1011 Charcoal 7950 ± 50 8640-8990 8817 Poz-29779 1167 Terrestrial macrorest 8780 ± 50 9581-9941 9798 Poz-35856 1198 Bulk sediment 10710 ± 60 12547-12743 12627 152235 1206 Pollen concentrates 13080 ± 100 15181-16476 15828 Table 2.1. AMS radiocarbon dates from core BSM08-1A-1U. Rejected dates are shown in brown and italics. Figure 2.13. SEM image from Facies 5 of the BSM sediment core. A) authigenic crystals of carbonate partially disolved. B) authigenic crystals of gypsum and a diatom . C) detrital carbonate grains and diatom remains. D) detrital carbonate grains and authigenic grains of carbonate and gypsum. 2. Basa de la Mora sequence. Climate at high altitudes 41 The first group of sediments (Facies 1, 2 and 3) are banded to laminated silicate and carbonate fine silts dominated by clay minerals (20-30 %) and quartz (5-15 %) with minor amounts of calcite (< 25 %) and with presence of hematite, pyrite and clinochlorite. Facies 3 has the highest MS, and relatively high carbonate content. Facies 1 and 2 are more silicaterich, but Facies 2 is finer, with lower MS, better-defined lamination and higher TOC content than Facies 1. The second group (Facies 4, 5 and 6) is dominated by massive carbonates (ca. 6% TIC; 60-80% calcite). Facies 5 and 6 have mottled textures and abundant gastropods, indicating littoral deposition. These facies dominate the littoral core (BSM-2A) almost entirely. Facies 4 has a higher TOC content (up to 3%) dominated by macrophyte and terrestrial remains. Facies 5 contains authigenic crystals of carbonate and gypsum, partially dissolved, pointing to deposition in ephemeral lake conditions with rapid fluctuations of lake level and salinity (fig. 2.13). Diatoms (pennate, benthic) only occur in the carbonate-rich Facies 5. Facies 6 has a slightly banded texture and lower TOC content than the other carbonate facies. The BSM sequence has been divided into three main sedimentary units according to sedimentary facies, MS, TIC and TOC percentages and the mineralogical and geochemical composition (XRF) (fig. 2.14). i. Unit 3 (1168-491 cm depth; 9800-5700 cal yr BP) corresponds to the lowermost part of the sequence and it is characterized by banded carbonate – poor sediments with high values of MS and relatively low TOC percentages (Facies 1, 2 and 3). TIC percentages and Ca, Sr and S values are low throughout Unit 3 while Si, K, Ti values (and particularly Fe and Mn) are high. The lowermost Sub-unit 3b (1168-690 cm depth, 9800-7450 cal yr BP) is composed of laminated Facies 1 and a thin interval of Facies 3. Magnetic Susceptibility (MS) reach the highest values of the sequence and are positively correlation with Mn (table 2.3). The high MS values are related to the presence of paramagnetic minerals eroded from ophite outcrops. Values of Ca and TIC are relatively low, but also display a strong positive correlation with MS. TOC percentages are the lowest in the sequence while TOC/N ratios are the highest. Subunit 3a (690-491 cm depth, 7450-5700 cal yr BP) is composed of Facies 2 and has finer lamination, lower MS and higher TIC and TOC values. Sub-unit 3a MS values are still high but decrease progressively. MS is significantly positively correlated with Mn and Fe (table 2.3). Ca values are very low and not significantly correlated with MS. TOC percentages increase, showing a significant negative correlation with MS, while TOC/N ratios decreases. 2. Basa de la Mora sequence. Climate at high altitudes 42 ii. Unit 2 (491-93 cm depth; 5700-680 cal yr BP) is made up of carbonate-rich Facies 5 and 6 with intercalations of organic-rich Facies 4. Thus, Unit 2, although highly variable, is characterized by the lowest values of MS and the highest content in TIC of the whole sequence. The high values of TIC in Unit 2 (up to 8%) are related to precipitation of authigenic carbonates. Sr and S elements increase considerably in this unit. TOC percentages also vary greatly during this period but, in general, they are relatively high and increase upwards. Relatively low TOC/TN values (< 12) indicate the dominance of lacustrine organic matter (Meyers, 2003). Si, Ti, Fe and Mn show parallel trends to MS (table 2.3). Unit 2 can be subdivided into three subunits, following the facies association. Thus, BSM 2c (491-350 cm depth; 5700-3540 Facies Facies description Clastic, laminated facies 1 Gray banded to laminated quartz and carbonate silts. Mostly composed by clay minerals (45 %), calcite (17 %) and, quartz (7 %) and low organic matter (<1%). High MS (100 SI). Laminated intervals are composed of up to 1 cm thick couplets of (1) black, carbonate siltysands with high quartz content, abundant hematites, chlorite and maphic minerals and occasional terrestrial and macrophyte remains and (2) gray carbonate silts with lower silicate minerals content and rare organic matter. 2 Dark gray laminated carbonate silts. Mineralogical composition similar to Facies 1, but better laminated higher organic content (1-2 %) and lower MS (average 40 SI). Couplets composed of mmthick laminae of (1) black, carbonate silty-sands with abundant terrestrial and macrophyte remains and (2) brown carbonate silts with less siliciclastic minerals and lower organic matter. 3 Light gray banded carbonate silts. Dominant carbonate content (TIC, X %; calcite, 40 %); quartz (6 %) and significant amounts of hematites, pyrite, clinochlorite, other maphic. Low organic matter (1%). Very high MS (>150 SI). Interpretation Clastic dominated deposition in distal, deeper setting. Laminated facies reflect flooding episodes reaching the centre of the lake. More abundant carbonate (Facies 3) or organic matter (Facies 2) reflects changes in watershed and littoral environments. Carbonate and organic-rich facies 4 Black, massive, carbonate silts. Composition is dominated by calcite (45 %), quartz (10 %), clay minerals (10 %) and organic matter (>2%) of terrestrial and macrophyte origin. Abundant pyrite and rare hematites. Low MS (25 SI). Occasional presence of pennate diatoms. 5 Light gray, massive, carbonate silts. Composition is dominated by calcite (70 %), with relatively low quartz and clay minerals (7 %) and organic matter (<2%); occasional pyrite and rare hematites. Low MS (25 SI). Organic matter is terrestrial, macrophyte and lacustrine origin. Mottling is common. Abundant gastropods and presence of pennate diatoms. 6 Light brown, banded, carbonate silts. Composition is dominated by calcite (30 %), clay minerals (15 %) and relatively low quartz (9 %) and organic matter (<2%) mostly terrestrial and macrophyte remains. Interpretation Carbonate dominated deposition in littoral environments with higher carbonate and organic productivity (Facies 5) deeper, with more frequent anoxic conditions (Facies 4) and transitional (Facies 6). Table 2.2. Facies description and interpreted depositional environment of BSM sequence. 2. Basa de la Mora sequence. Climate at high altitudes 43 cal yr BP) is constituted by the alternation of cm-thick intervals of Facies 4 and 6 and displays an upward TIC increase (up to 8%). TOC percentages are highly variable but generally low (1-2 %). BSM 2b (350-240 cm depth; 3540-2200 cal yr BP) represents a 1 m-thick interval of Facies 5 with the highest TIC, Ca and calcite values and the lowest TOC and MS of the sequence (fig. 2.14). Higher Sr values occur as a result of more abundant biogenic aragonite. Finally, BSM 2a (240-93 cm depth; 2200-700 cal yr BP) comprises rhythmic sequences of about 20 cm-thick composed of thin layers of Facies 1->, Facies 4 -> Facies 5 (detritalorganic-carbonate). iii. Unit 1 (93-0 cm depth; 698 cal yr BP-2007 AD) comprises carbonate – poor Facies 1 and organic-rich Facies 4. As a consequence, all geochemical indicators show high variability. Facies 1 lamination is less well defined than in Unit 3. MS values increase again and show strong positive correlation with Si, Ti, Mn and Fe, while the correlation with Ca and TIC and TOC is strongly negative (table 2.3). TOC/N ratios increase at the base of the unit and decrease towards the top: TOC percentages show the opposite pattern. Unit 1 (0-93 cm) Unit 2 (93-491 cm) Sub-unit 3a (491-690 cm) Sub-unit 3b (6901168) MS MS MS MS r p r p r p r p Si 0.726 < 0.001 0.546 < 0.001 0.146 0.148 -0.346 < 0.001 Ti 0.699 < 0.001 0.688 < 0.001 0.280 0.005 -0.388 < 0.001 Mn 0.688 < 0.001 0.545 < 0.001 0.543 <0.001 0.451 < 0.001 Fe 0.806 < 0.001 0.582 < 0.001 0.643 < 0.001 0.162 0.013 Ca -0.660 < 0.001 -0.564 < 0.001 0.179 0.075 0.671 < 0.001 TIC -0.700 < 0.001 -0.591 < 0.000 0.404 <0.001 0.689 < 0.001 TOC -0.746 < 0.001 -0.409 < 0.001 -0.609 < 0.001 -0.494 < 0.001 Table 2.3. Correlation values between Magnetic Susceptibility and other geochemical parameters in the different sedimentary units. 2. Basa de la Mora sequence. Climate at high altitudes 44 Figure 2.14. Main sedimentological features, geochemical and physical properties of the BSM sequence plotted in depth, indicating the location and results of radiocarbon dates. 2. Basa de la Mora sequence. Climate at high altitudes 45 2.4.3. Modern pollen rain The modern pollen rain results agree largely with the regional vegetation around the Lake BSM. Despite the different distance to the lake (from 0 to 500 m), 7 out of 8 samples present stunning similar percentages of pollen highlighting the homogeneity of the pollen rain composition and its dispersal. Furthermore, the mean values of the modern pollen-rain samples compares fairly well with pollen percentages from the core-topmost sample (BSM08-1B-1G-1,12-13) (fig. 2.15). The AP values represent around 70% of the total, with Pinus accounting from approximately 60% in agreement with the dominance of this tree in the area and its high pollen productivity. Despite the distance of current formations to the lake (more than 5 km) (fig. 2.5 and Appendix I, fig A2.1), all types of Quercus are present in the pollen rain with values up to 5%. The mesophytes (Betula, Corylus and other Mesophytes grouo) show very low values or are even absent in some samples in agreement with their position in the bottom of the valleys. Regarding the main anthropogenic species it is remarkable the presence of Olea, with values ranging from 1 to 5%, and the occasional appearance of Vitis, in spite of the distance of the olive and wine fields to the lake, indicating the long-distance transport of these two species, particularly Olea (Cañellas-Boltá et al., 2009). Conversely, there is a large underestimation of all herbaceous species. The spare sample (T15) shows an overrepresentation of Pinus, reaching almost 85%, and an underrepresentation of the rest of trees. Nevertheless, this sample is located the farthest from the lake (500 m) and in any case, those values disguise the real vegetation Figure 2.15. Modern pollen rain from 8 mosses samples collected across a distant transect from the lake shore to up 500 m. Sample highlighted in orange represents the core-topmost sample. Other Mesophytes group includes: Alnus, Carpinus, Salix, Ulmus, Populus, Acer, Fraxinus and Juglans. Deciduous forest group includes: Betula, Corylus, deciduous Quercus Alnus, Carpinus, Salix, Ulmus, Populus, Acer, Fraxinus, Fagus, Tilia and Juglans. Nitrophilous group includes: Plantago and Rumex. 2. Basa de la Mora sequence. Climate at high altitudes 46 composition. Despite the current grazing activity recognized in the area, only Urticaceae, Rumex and Plantago proportions (nitrophilous taxa) appear significant, while Artemisia values are remarkably low or even absent. This taxon is usually considered as anthropogenic indicator also in Late Holocene records, but in the case of the BSM sequence it seem not be related to the occurrence ofrecent human pressure given that, depite the current moderate grazing activities in the area, it is absent in current vegetation formations. Additionally, coprophilous fungi, other usual indicator of human pressure in pollen diagrams (LópezMerino et al., 2011; Morales-Molino, 2013) are not a relevant component of the modern pollen rain. In general, these results suggest that in the BSM sequence, tree values around 5% seem represent middle distance presence while tree values under 5% represent long-distance presence. They also suggest that the grazing activity cannot be directly related to the presence of Artemisia nor coprophilous fungi as often used in palynological interpretations, but to the presence of nitrophilous (like Rumex or Plantago) plants instead. The matching values between the modern pollen rain and the vegetation present in the area validate the fossil pollen rain as representative of local, nearby and relatively regional past vegetation landscapes. 2.4.4. Pollen and charcoal data The pollen record can be divided into six zones (BSM-0 to BSM-V: fig. 2.16). In BSM-I to BSM-V (9.8 ka cal BP-present), the 5 cm-resolution pollen analyses provide a temporal resolution of 22 to 150 years per sample. Statistical results for pairwise comparison between vegetation and geochemical parameters are shown in table 2.4. The maximum number of charcoal particles counted was 3098, with a mean of 307 and a SD of 453. The patterns of charcoal influx are consistent with the pollen zones. Appendix I (figs A.2.2, A2.3 and A2.4), located at the end of the thesis, shows all taxa found for the BSM sequence. BSM-0 (1209-1167.5 cm depth; before 9800 cal yr BP) This zone is characterised by scarce representation of the herbaceous component (NAP) and particularly the steppe taxa group (Artemisia, Chenopodiaceae, Helianthemum, Plantago, Rumex, which rarely exceed 5-10 %), and abundant representation of arboreal pollen (AP), dominated by conifers (mainly Pinus) and deciduous forest taxa (Betula, Corylus, Alnus, Salix, Ulmus, Populus, Acer, Fraxinus, Fagus, Tilia and deciduous Quercus), with values around 25-30 % (fig. 2.16). Representation of Poaceae and aquatics (Cyperaceae, Ranunculus, Myriophyllum and Potamogeton) in this zone is not significantly different to the rest of the sequence. The pollen spectra of this zone are not consistent with a pre-Holocene deposit as would be inferred from the two dates (15.8 cal ka BP and 12.6 cal ka BP) from this interval. Pollen content, together with the lack of sedimentological evidences for a hiatus, indicates that these dates are too old, due to a possible reservoir effect. Both 2. Basa de la Mora sequence. Climate at high altitudes 47 palynological and sedimentological data suggest these are Holocene sediments, but given the absence of chronological control the record from this zone is not further considered in this study. BSM-I (1167.5-815 cm depth; 9800-8200 cal yr BP) Arboreal pollen varies between 60 and 80% of the total pollen abundance, and in some cases it exceeds 85%. Pinus is the main arboreal taxon, but deciduous taxa are well represented by Betula, Corylus and deciduous Quercus, with some significant fluctuations in Betula. Juniperus is also present with percentages above 6%. Evergreen Quercus and Mediterranean shrubs (Pistacia, Rhamnus, Phillyrea, Buxus, Sambucus, Ephedra fragilis and E. distachya) are present in relatively low but continuous percentages. The first Tilia appearance is recorded at 870 cm depth (8500 cal yr BP); this timing is consistent with other records from the region (Montserrat-Martí 1992; González-Sampériz et al., 2006; Miras et al., 2007; Pèlachs et al., 2007). Poaceae dominates the herbaceous stratum, while the abundance of Helianthemum significantly declines and Artemisia decreases in importance. Myriophyllum is the dominant aquatic. A significant change is found towards at the end of the zone (860 - 815 cm depth; 8400-8200 cal yr BP) characterized by a sharp decline in Betula, Corylus and deciduous Quercus, the virtual disappearance of Other Mesophytes (fig. 2.16) and the total absence of Tilia. Pinus increases to its maximum in the whole sequence, reaching 75%, and Helianthemum reappears at this time. This is a phase of high variability in fire activity, although charcoal counts are very low. Pinus and Juniperus show a positive correlation with MS within this zone, while Betula, Corylus, Quercus faginea, evergreen Quercus and Myriophyllum are negatively correlated with MS (table 2.4). Thus, MS is correlated negatively with moisture-adapted and temperate taxa, but positively with more drought-resistant taxa such as Pinus and Juniperus. BSM-II (815-491 cm depth; 8200-5700 cal yr BP) After the short, abrupt vegetation change previously described, forest contracts slightly but there is considerable compositional variability. Pinus decreases to 35% and Juniperus is also highly reduced in abundance. Deciduous taxa, mainly Betula, Corylus and deciduous Quercus, show large and more continuous expansion reaching their maximum values in the sequence (fig. 2.16). Tilia reappears and is constantly present at moderate levels throughout the zone. Evergreen Quercus declines to its minimum values, while Mediterranean Shrubs fluctuate in abundance. The first isolated appearance of Abies occurs at 646 cm (7200 cal yr BP). The NAP is mainly composed by Poaceae, Artemisia and Lamiaceae, as in the rest of the record. Aquatic plants are well represented by Cyperaceae, Pedicularis, Ranunculus and Potamogeton, although Myriophyllum is dominant and reaches its highest values in the sequence. Deciduous Quercus and Tilia abundances show a strong negative correlation with MS (table 2.4). There is an increasing trend of fire activity, although the variability is high (Lasheras et al., 2013). 2. Basa de la Mora sequence. Climate at high altitudes 48 BSM-III (491-389 cm depth; 5700-3900 cal yr BP) The beginning of this zone is characterized by a steep decline in deciduous forest taxa, mainly Betula (abruptly reduced by nearly 60%) and deciduous Quercus. In contrast, Pinus expands rapidly and Juniperus and evergreen Quercus increase slightly (fig. 2.16). Fagus appears for the first time, chronologically fitting the regional expansion (Montserrat-Martí, 1992; Pla and Catalán, 2005). The base of the zone is characterised by the permanent presence of Abies in the area, after its initial appearance shortly before. Poaceae, Artemisia, Lamiaceae and Chenopodiaceae are still the main NAP taxa and Rumex rises. No significant changes are recorded on the aquatic component except a decrease in Myriophyllum and a short-term disappearance of Potamogeton at the base of the zone. The conifer/mesophyte ratio is inverted at the top of the zone, just before the transition from Sub-unit 2c into Sub-unit 2b. Fire activity reaches a maximum towards the end of this zone. BSM-IV (389-93 cm depth; 3900-700 ca yr BP) The beginning of this zone is characterized by a change in forest composition. Pinus recovers and becomes the dominant arboreal taxon, Abies reaches its maximum abundance and Betula exhibits its minimum values (fig. 2.16). Juniperus and evergreen Quercus increase, but Corylus and Other Mesophytes only experience a slight increase. Tilia decreases progressively and disappears at top of the zone. In contrast, Fagus reaches its highest levels, at a time consistent with other records from the region (Pla and Catalán, 2005; Pérez-Obiol et al., 2012). A sudden and abrupt rise of Artemisia and further decrease in mesophyte taxa accompany the Pinus-dominant landscape. The NAP, of which Poaceae and Artemisia constitute the main elements, accounts for 40% of the pollen sum. There are two peaks of Artemisia in this zone, the most modern (ca. 1000-1300 A.D) of which (when Artemisia reaches its maximum value in the whole sequence) coincides with the disappearance of Abies and Tilia. The aquatic component is markedly reduced in BSM III & BSM IV (93-491 cm) BSM II(491-815 cm) BSM I (815-1168 cm) MS MS MS r p r p r p Pinus 0.453 0.003 0.444 0.006 0.464 0.001 Juniperus 0.351 0.023 ― ― 0.339 0.021 Betula -0.573 < 0.001 ― ― -0.517 < 0.001 Corylus ― ― ― ― -0.292 0.049 Tilia ― ― -0.537 0.002 ― ― Dec. Quercus ― ― -0.528 0.001 ― ― Quercus fag. -0.401 0.009 ― ― -0.373 0.018 Ever. Quercus -0.378 0.014 ― ― -0.505 < 0.001 Artemisia ― ― -0.433 0.007 ― ― Cyperaceae ― ― 0.414 0.017 ― ― Myriophyllum ― ― ― ― -0.592 < 0.001 Table 2.4. Correlation between MS and pollen taxa in the different pollen zones. 2. Basa de la Mora sequence. Climate at high altitudes 49 abundance, with low values of Myriophyllum and the absence of Potamogeton during the most of the zone contrasting with an increase in Cyperaceae. Cultivated taxa like Olea, Vitis, Castanea and Cerealia type appear more continuously. Although there are some marked peaks of Pinus in this zone, the general trend is for relatively stable pine forest during the last phase of sedimentary Sub-units 2b and 2a. An abrupt decrease in charcoal concentration lasting several centuries was followed by a new abrupt increase in fire activity at the end of the zone. BSM-V (93-0 cm depth; 700 cal yr BP-present, 1250-2008 cal AD) This zone is characterized by important changes in both pollen and sedimentological records (Unit 1). The most relevant feature is the increase in Olea and Fraxinus. Pinus increases up to the 70%, but with very short episodes of were abundance is much lower (40%). The expansion of pine is coincident with the decline of Abies, Betula, Corylus and Other Mesophytes (fig. 2.16). Deciduous Quercus and, especially evergreen Quercus increase in abundance in the topmost part of the sequence. The NAP is still dominated by Poaceae, but Artemisia drops dramatically while Asteraceae and Chenopodiaceae reach their maximum values. Myriophyllum becomes less important and Cyperaceae dominates the aquatic assemblage. Variations in MS at this time are not correlated with vegetation composition changes. Fire activity is very high during most of the zone, but ceases in the top part of the record. 2.4.5. Chironomids A total of 6422 chironomid head capsules were picked up, individually mounted and identified from 71 samples of the core BSM08-1A by Pol Tarrats and Maria Rieradeval (Universidad de Barcelona) (Tarrats, 2011). Total chironomid biodiversity was represented by 18 taxa (up to 9 taxa per sample), belonging to three chironomid subfamilies: Tanypodinae, Orthocladiinae and Chironominae. Tanytarsus gr. lugens was the most abundant all through the core, followed by Procladius, Chironomus and Paratanytarsus. Chironomus or Paratanytarsus are not shown in the diagram (fig. 2.17) because they are present through the entire sequence and show no clear pattern of changes through the Holocene. The chironomid assemblage indicates that the lake has been always relatively shallow and oligotrophic, although relatively rich in organic matter. Quantitative analysis of the Chironomidae allows the sequence to be divided into 4 zones: CHZ-1: Chironomid Zone 1 (1168.5-491 cm depth; 9895 - 5700 cal yr BP) Low values characterize this zone. Tanytarsus gr. lugens abundance is relatively low although with some fluctuations. Procladius reaches its maximum relative abundance within the core (30-60%), whereas Pentaneurini tribe appears through the entire zone although with a highly fluctuating distribution. The Orthocladiinae tribe is quite diverse, with an early representation of Psectrocladius gr. limbatellus and Corynoneura and a moderate representation of Orthocladiinae indet. (5-7%), which include several taxa related to water runoff and seepages (e.g. Smittia). 2. Basa de la Mora sequence. Climate at high altitudes 56 Figure 2.17. Diagram plotted in age, including selected pollen taxa, geochemical parameters, chironomid taxa and microcharcoal influx curves of Basa de la Mora sequence compared to NAO summer insolation curve for latitude 24ºN, regional phases of deforestation (Fletcher et al., 2013b) and phases of increased storm activity (Sabatier et al., 2012) in the Western Mediterranean. Note: Orthocladiinae (s.t.) means sum of rheophilous (see page taxa). Blue horizontal bars represent humid phases whereas yellow and orange bands represent arid phases and further arid events respectively. 2. Basa de la Mora sequence. Climate at high altitudes 57 2.5.3. The end of the Middle Holocene (5700-3900 cal yr BP): transition phase The evolution of the landscape in southern Europe from 6 ka (or even earlier) onwards has been widely assumed to be influenced by both climate and human forcings (Oldfield and Dearing, 2003; Vannière et al., 2008; Roberts et al., 2011, Sadori et al., 2011). Many palynological studies show a clear increase of anthropogenic indicators from the Middle Holocene, pointing to an intensification of human activities and a subsequent change in the vegetation composition related to forest clearance for pastures and agriculture fields (Jalut et al., 2009). However, some of these taxa are naturally found in xeric Mediterranean ecosystems (De Beaulieu et al., 2005) and this makes it difficult to discriminate between climate and anthropogenic forcings. The spread of xeric vegetation across the Mediterranean region during Middle-Holocene does not necessarily imply anthropogenic degradation of the landscape (Collins et al., 2012). In addition, fire activity in Mediterranean areas increased significantly at this time and its impact on vegetation composition has to be taken into consideration (Colombaroli et al., 2007, 2008, 2009; Vannière et al., 2008, 2011). Increased fire activity can result from anthropogenic activities but also reflects the climatic shift towards arid conditions (Carrión et al., 2001a, 2010; Fernández et al., 2007; Fletcher and Sánchez-Goñi 2007; González-Sampériz et al., 2008; Morellón et al., 2008; Jalut et al., 2009; Corella et al., 2010; Anderson et al., 2011). The expansion of heliophytes (like Artemisia, Chenopodiaceae, Asteraceae, Rumex, Plantago, and Poaceae Mediterranean species similar to Cerealia type) observed during this period is favoured by increased fire, increased aridity, and anthropogenic activity. Overall, the complex changes found in Mediterranean areas at the end of the Mid-Holocene are not necessarily related to intense human pressure, but could equally well be explained by the trend towards drier conditions. There is a sharp change in the vegetation cover and sedimentological features in the BSM sequences at 5.7 cal yr BP. The pollen record in BSM-IV (fig. 2.17) is characterized by a pronounced increase in pine and decrease in mesophytes, mainly Betula, in combination with a rise in Juniperus, deciduous and evergreen Quercus and heliophytes (Artemisia and Chenopodiaceae). The replacement of mesophytes by conifers suggests a change from humid to drier conditions or, at least, a significant shift in the seasonal distribution of the precipitation since reduced summer rainfall is unfavourable to the broad-leaf taxa. The sedimentary shift is defined by an increase in carbonates, indicating lower lake levels (Subunit 2c). Lower values of MS suggested reduced sediment transport as consequence of lower run-off and inflow streams, which in turn indicates reduced precipitation or meltwater inputs. The decrease in allochthonous sediments is reflected in lowered sedimentation rates and deposition of carbonate Facies 6, which reflects high carbonate productivity in a littoral setting with low and fluctuating water level. The decline in Myriophyllum is consistent with a reduction in water level (figs. 2.16 and 2.17). Moreover, the sharp decrease in Procladius and the near disappearance of non-lacustrine Orthocladiinae taxa also indicates reduced runoff and stream inflow during this period. The increase in chironomid abundances, mainly Tanytarsus, could indicate increased decomposition rates in the sediments. 2. Basa de la Mora sequence. Climate at high altitudes 58 Both biological and sedimentological indicators are consistent with a trend to increased aridity and a persistent arid phase between 5.6 to 4.6 cal ka PB (fig. 2.15). Similar vegetation changes have been recognised in other Pyrenean sequences (Pelachs et al., 2007), in southern Spain (Jiménez-Moreno and Anderson, 2012) and in Mediterranean records (Carrión et al., 2010). Fletcher et al., (2013b) have identified a major phase of deforestation in the Western Mediterranean during this period. The coincidence between lowered lake levels and forest decline supports the idea of climate as the main forcing. A major climate shift has been recognised in many other regions at this time, including the end of wet conditions in the Sahara between 6 and 5.5 cal ka BP (deMenocal et al., 2000; Kröpelin et al., 2008), and lake-level and vegetation changes indicating drier conditions in eastern North America (Shuman et al., 2001; Zhao et al., 2012; Menking et al., 2012). The similarities in climate changes between such different geographic areas during the MidHolocene suggest broad-scale changes in the coupled ocean-atmosphere circulation. This large-scale and synchronous climate shift may be related to changes in global atmospheric circulation. The weakened summer insolation in North Hemisphere led to a southward shift in the Inter Tropical Convergence Zone (ITCZ) and thus, the summer Asian monsoon also weakened considerably (Wanner and Brönnimann, 2012). Readjustment of these two main climatic system drivers led to the establishment of similar conditions to present atmospheric tele-connections (ENSO) since ca 5.5 ka (Wanner et al., 2008; Carré et al., 2012; Fletcher and Moreno 2012). Southward movement of the ITCZ favoured southward shift of the subtropical North Atlantic high pressure and led to increased summer aridity in the Iberian Peninsula (González-Sampériz et al., 2008; Morellón et al., 2009; Corella et al., 2010; Valero-Garcés et al., 2011; Carrión et al., 2010; Valero-Garcés and Moreno, 2011). As the North Atlantic high-low pressure system moved away, westerlies became weaker and lost their capacity to penetrate inland. A change towards wetter conditions is observed in the BSM sequence between 4.5 and 3.9 cal ka BP, marked by increased abundance of mesophytes, and the recovery of Betula and deciduous Quercus values (fig. 2.17). This humid period corresponds well with a phase of increased storm activity recorded in the Gulf of Lion (Sabatier et al., 2012), suggesting stronger and southward migration of the westerlies. However, the total AP decreases during this phase. This reduction of the arboreal pollen in the BSM sequence occurs at the same time as the first deforestation phase recognised in the Pyrenean sequence of Tramacastilla at ca. 4000 BP (Montserrat-Martí, 1992). However, no other indicator of anthropogenic pressure was found during this period in the BSM sequence suggesting that the vegetation shift was mainly climate driven. The high regional fire activity detected during this period is the culmination of a previous trend. Although there was an initial dry phase when fire occurrence was linked to the presence of pine forest, higher charcoal influx values during this subsequent humid phase are linked with the spread of mesophyte forest. The fact that fire is high during both humid and arid spells, reflects on the one hand more permanent drying conditions than any time before in the Holocene leading to frequent fire-conducive conditions coupled with relatively high fuel availability from mesophyte vegetation, and on the other 2. Basa de la Mora sequence. Climate at high altitudes 59 hand, the strengthening of fire activity during any interval of mesophyte forest expansion when fire-conducive conditions occur (Lasheras et al., 2013). 2.5.4. The Late Holocene (3700-700 cal yr BP): aridity crises Complex societies developed across the Mediterranean during the Late Holocene and human pressure on the landscape intensified and expanded (Carrión et al., 2007, Bal et al., 2011; Finné et al., 2011; Magyari et al., 2012). High altitude palaeoenvironmental records, where anthropogenic activities would have been limited due to both severe weather and difficult access, provide an opportunity to isolate the climate signal influencing vegetation evolution in recent times (Pérez-Sanz et al., 2011). The BSM sequence reveals a well forested landscape during most of the late Holocene (AP abundance around 70%, BSM-IV), indicating negligible anthropogenic pressure until ca 1150 cal yr BP, when the first evidence of forest management is found. The trend towards increased aridity that started during the Mid-Holocene transition intensified considerably at 3700 cal yr BP. The pollen record (BSM-V) is characterized by a sharp fall of Betula and the disappearance of birch from this area (fig. 2.15). The expansion of conifers (Pinus and Juniperus, which reaches its maximum proportions of the whole record), indicates either reduction in annual mean precipitation or a significant change in the seasonal distribution of precipitation (Franco-Mugica et al., 2000). The Pinus expansion in BSM is coeval with an expansion in other high altitude Pyrenean sites (Pèlachs et al., 2011), which suggests it is more likely to be controlled by changed climate than by human action. At ca. 2900 cal yr BP, Artemisia starts to spread rapidly and Myriophyllum decreases strongly (BSM-V). Traditionally, the Artemisia expansion has been explained by an increase in pastoral activity during the Late Holocene. However, as we have indicated before, in reference to current pollen rain data in BSM area, modern values of Artemisia rarely reach 2% even though there is moderate pastoral activity in the BSM area (figs. 2.15). Given that there is no evidence for major deforestation at the time of the Artemisia expansion, it seems unlikely that this represents an interval of more intense anthropogenic activity than today. This suggests that the Artemisia expansion at the Basa de la Mora site indicates a climatically-induced expansion of dry steppe. There is evidences for a period of intensified aridity across the Mediterranean at around 2900-2400 cal yr BP (Jalut et al., 2000). The deposition of carbonate-rich massive Facies 5, characterized by the presence of authigenic calcite crystals, gastropods, pennate diatoms (fig. 2.13) and mottling textures, indicative of bioturbation, provides evidence for lowered lake levels and the development of a larger palustrine area at the time of the expansion of dry steppe. Facies 5 characterises most of littoral core BSM-2A-1U, supporting our interpretation of the depositional environment. The presence of partially dissolved authigenic crystals of calcite and gypsum in Facies 5 suggests the lake was ephemeral and may have desiccated at times. The strong negative correlation between MS and TIC (table 2.3) indicates that decreased runoff, and thus reduced external water supply into the lake, led to increased concentration of the lake water 2. Basa de la Mora sequence. Climate at high altitudes 60 and authigenic carbonate precipitation. Furthermore, the negative correlation between MS and drought-resistant taxa such us Pinus and evergreen Quercus and the positive correlation between MS and Betula strengthen the link between lack of run-off and precipiation deficit (table 2.4). Intercalation of organic Facies 4 supports the development of a palustrine area with high accumulation of organic matter. In addition, the high percentages of TOC and low TOC/N ratio indicate increased lacustrine productivity, consistent with shallower conditions. This expansion of littoral areas is consistent with the very high abundance of Cyperaceae while Myriophyllum values remain relatively unchanged (Fig. XX). The higher percentages (up to 20%) of Psectrocladius gr. Limbatellus (Fig. XX) than in previous zones also indicates an increase lacustrine productivity, as this genus is associated with productive environments and/or littoral areas with abundance of biofilm primary production on stones or macrophytes (Rieradevall et al., 1999; Brodersen et al., 2001). There is no charcoal in the BSM between 3.2-1.5 cal ka BP (Lasheras et al., 2013). An interval of two millennia without fire is highly unusual as fire activity is registered in most southern European sequences during this time (Colombaroli et al., 2010; Tinner et al., 2005: Vescovi et al., 2007; Vannière et al., 2008). Arid pulses could prevent forest development at high altitudes and, therefore, limiting charcoal production through fires but, considering the absence of any other clear biotic or abiotic indicators, it seems more likely that the lack of microcharcoal is linked to taphonomical issues affecting charcoal preservation during oxic periods and/or short sub-aerial exposure events (Facies 5). Failure in laboratory procedures linked to the use of Thoulet solution differential flotation processes may have played a role in the absence of microcharcoal in these samples . Currently, new analyses are in progress in order to establish the presence of charcoal in this interval. There is a common pattern to the evolution of vegetation across the Western Mediterranean (including southern Iberia, northern Africa and Italy) during this interval. A general phase of forest decline has been recorded in marine record MD95-2043 from the Alborán Sea between 3.7 and 2.9 cal ka BP (Fletcher et al., 2013b). In Zoñar sequence, low values of AP (< 10%) and an expansion of steppe taxa occurred between 4 – 2.9 cal ka BP (Martín-Puertas et al., 2008). At Sierra de Gádor (Carrión et al., 2003), Pinus and evergreen oak expand at the expense of deciduous Quercus after 3940 cal yr BP. In Sierra de Baza, there was a replacement of mesophytic by more xeric taxa around 3800 cal yr BP (Carrión et al., 2007), while in El Cañizar de Villarquemado, mesophytes and deciduous Quercus decreased and steppe herbs increased between 4000-3800 cal yr BP (Aranbarri et al., 2014). A similar pattern has been recorded in Italian sequences, with an expansion of sclerophyllous taxa between 3.9-3.4 ka (Sadori et al., 2010). These changes can all be attributed to both drier climate conditions and human activities, especially considering that several civilizations collapsed at ca. 4000 cal yr BP (i.e., Akkadians: Cullen et al., 2000). 2. Basa de la Mora sequence. Climate at high altitudes 61 Figure 2.18. Selected records from Central and Mediterranean Iberia covering last 2000 years that indicate variations in aridity. From top to bottom: A) Zr/Al ratio from AlgerianeBalearic basin core; B) Rb/Al ratio from Zoñar Lake; C) Sr (cps) from Arreo Lake; D) the aridity reconstruction of Estanya Lake (axis 2 from the Principal Component Analyses applied to the XRF dataset in two cores); E) the number of paleoflood events in Taravilla Lake; F) Si (cps) from Basa de la Mora Lake; G) the number of detrital layers per year from Montcortès Lake and H) Fe (cps) from the Tagus prodelta. Note that all records are plotted to indicate arid conditions towards the bottom. RP: Roman Period; DA: Dark Ages; MCA: Medieval Climate Anomaly; LIA: Little Ice Age. From Moreno et al., 2012. 2. Basa de la Mora sequence. Climate at high altitudes 62 Peaks in Artemisia and high TIC percentages in BSM record (fig. 2.17) mark two periods of increased aridity at 2.9-2.4, and at 1.2-0.7 cal ka BP (800-1300 AD). Both episodes are characterized by high TIC and TOC percentages and low TOC/TN ratios suggesting high precipitation of carbonates and high bioproductivity and content of autochthonous organic matter. These episodes are separated by a relative humid period between 2.1 and 1.5 cal ka BP (fig. 2.17). The arid phase between 2.9-2.4 ka cal BP is synchronous with a dry episode recorded in both western (Ferrio et al., 2006; Aguilera et al., 2012) and eastern Iberia, that led to a prominent decline in deciduous Quercus pollen in the Amposta sequence (PérezObiol et al., 2011). Increased water level can be inferred from the significant reduction of TIC percentages between 2.1 and 1.5 cal ka BP. An episode of more humid conditions has been recognized in Iberia (Corella et al., 2010; Martín-Puertas et al., 2008, 2009; Currás et al., 2012), coinciding with the Iberian civilization and the Roman occupation and thus is called the Iberian-Roman Humid Period (IRHP). The NW Mediterranean region also registers an intensification of rainfall reflected by higher storm activity in the Gulf of Lion (Sabatier et al., 2012). However Fletcher et al., (2013b) report another phase of forest decline in Western Mediterranean at this time (fig. 2.17). Since wetter conditions should have positively affected forest development in the Mediterranean, where water is the greatest limiting factor, it is possible that depletion in tree mass could be related in some areas of Iberia to higher land use by the Romans (García-Bellido, 1985). However, we do not observe great exploitation of the subalpine belt at BSM suggesting that the vegetation composition, which runs in parallel with sedimentological features, is still primarily controlled by climate in this area. The second arid period recorded in BSM sequence matches the well-known Medieval Climate Anomaly (MCA: 900-1300 AD), a period of aridity recognized in most of south-western Europe (Seager et al., 2007; Mann et al., 2009) which led to notable agro-economic crisis in medieval societies. In Spain, it resulted in a major water deficit leading to lower lake levels and expansion of thermophytes and steppe taxa (Moreno et al., 2012b) (fig. 2.18). In the BSM sequence, this phase coincides with the first signal of deforestation, indicated by abrupt decreases in pine percentages (fig. 2.17). Charcoal influx increased ca. 1700 cal BP, most likely because of either warmer conditions or strengthened regional fire activity in the lowlands (Lasheras et al., 2013). Both episodes of depleted water availability correspond with maxima in reconstructed North Atlantic Oscillation (NAO) indexes (fig. 2.17). This indicates that there is a fast response of palaeoenvironmental changes in the BSM record to changes in the North Atlantic. The persistence of a positive NAO index during 2.9-2.4, and at 1.2-0.7 cal ka BP, led to maximum winter precipitation in Scandinavia and to minimum winter precipitation in the Iberian Peninsula (Trouet et al., 2009). 2. Basa de la Mora sequence. Climate at high altitudes 63 2.5.5. The last centuries (700 cal yr BP-present): anthropogenic impact In contrast to most Pyrenean studies that indicate intensified human disturbance during at least the last two millennia (Riera et al., 2004; Pèlach et al., 2011; Guiter et al., 2005), the effects of anthropogenic pressure are only detected in the BSM sequence during the last 700 cal yr BP (Pérez-Sanz et al., 2011) (fig, 2.19, BSM-V-A). As seen in figures 2.16, 2.17 and 2.19, the increase in Olea marks an expansion of agricultural practises in the lowlands (Cañellas-Boltà et al., 2009) whereas large, short-term reductions in Pine indicate phases of deforestation and expansion of grazing lands at higher altitudes (fig. 2.19, BSM-V-B). Parallel to Olea, Fraxinus also spreads. Fraxinus has traditionally been used in the region for hedgerows (Gómez and Fillat, 1981). Its parallel expansion to Olea marks the regional establishment of modern and intense agro-pastoral activities. The drop in Artemisia synchronous with clear evidence of increasing anthropogenic pressure in the highlands supports the idea that Artemisia is not an indicator of human activities in the BSM sequence. The expansion of Olea and Fraxinus ceased, and deforestation temporarily stopped, between 1600 and 1850 AD (fig. 2.19; BSM-V-C) coinciding with the second half of the Little Ice Age. This interval is characterized by the coldest conditions in the southern Pyrenees (GonzálezTrueba et al., 2008; Morellón et al., 2012) (fig. 2.20). In BSM sequence a sharp decrease in evergreen Quercus coincides with these colder conditions. The rapid recovery of pine after intervals of deforestation emphasizes the fact that human disturbance at high altitudes was not strong and climatic conditions were the main determinant of vegetation changes. High values of MS and strong negative correlation with TIC during this period (fig. 2.19; BSM-V-C) indicate increased sediment delivery to the lake and decreased carbonate productivity, both indicative of higher lake levels and increased runoff. The abundance of allocthonous organic matter, shown by low TOC and high TOC/N ratios, also supports the inference of high sediment delivery from the catchment. Fire activity was high for most of this period, confirming the occurrence of either regional fires linked to husbandry or local fires correlated with the occasional pine deforestation (Lasheras-Álvarez et al., 2013). Although it is difficult to distinguish between human and climate-induced fires in this period, all other records indicate an intensification of anthropogenic activities after 700 yrs BP. A general decrease in temperature coinciding with the Little Ice Age (LIA i.e. 1300-1850 AD) has been recorded throughout Europe. Higher storm activity occurred in the NW Mediterranean, (Sabatier el al., 2012) (fig. 2.17) while stronger climatic variability has been recognised in Iberia, although generally cold and humid conditions dominated (Benito et al., 2003; Valero-Garcés et al., 2008; Morellón et al., 2012; Moreno et al, 2008, 2012b) (fig. 2.20). 2. Basa de la Mora sequence. Climate at high altitudes 64 Figure 2.19. Comparison of selected curves (pollen –Pinus, Evergreen Quercus, Fraxinus, Olea, Artemisia, Potamogeton-, geochemical proxies-MS, TIC, TOC, TOC/Nand chironomids –Psectrocladius gr. limbatellus-) from Basa de la Mora sequence with global and regional records (Estanya salinity (Morellón et al., 2011); NH temperature reconstruction (Mann et al., 2003) and Solar Irradiance (Sthinhiber et al., 2009) for the last 750 years, indicating the main climate and historical periods and the interpretation of local land use. Bands in rose mark the intense periods of anthropogenic activities. 2. Basa de la Mora sequence. Climate at high altitudes 65 Figure 2.20. Selected records from the Southern Pyrenees reviewed in this paper, from top to bottom: Capdella tree-ring based mean annual rainfall (30× moving average) (Saz Sánchez, 2003), GER-SOB summer temperature tree-ring based reconstruction (original data and 20× moving average) (Büntgen et al., 2008), diatom alkalinity-based summer–autumn temperature and chrysophyte-based winter– spring temperatures reconstruction in Lake Redon (Pla and Catalán, 2005; Catalan et al., 2009), phases of advance and retreat of the Pyrenean glaciers (González Trueba et al., 2008), calcite sublayering (Fine–Coarse (F–C) or Coarse–Fine (C–F)) (Corella, 2011) and diatom C:P ratio (Scussolini et al., 2011) in Lake Montcortés, Si (cps) content in Lake Basa de la Mora (original data and 30× moving average) (Moreno et al., 2012), Sr (cps) content in Lake Arreo sequence (original data and 30× moving average) (Corella, 2011) and XRF-based salinity reconstruction and humid/arid phases in Lake Estanya (Morellónn et al., 2011). (B) Supplementary regional and global records, from top to bottom: NH temperature reconstruction (Mann and Jones, 2003), solar irradiance (Steinhilber et al., 2009), and NAOms reconstruction (Trouet et al., 2009). Vertical yellow bars represent the chronology of the grand sunspot minima and temporal divisions Medieval Climate Anomaly (MCA), Little Ice Age (LIA) and Industrial Era (IND. ERA) are also indicated at the uppermost part of the figure. From Morellón et al. 2012. 2. Basa de la Mora sequence. Climate at high altitudes 72 González-Sampériz, P., Utrilla, P., Mazo, C., Valero-Garcés, B., Sopena, M., Morellón, M., Sebastián, M., Moreno, A., Martínez-Bea, M., 2009. Patterns of human occupation during the early Holocene in the Central Ebro Basin (NE Spain) in response to the 8.2 ka climatic event. Quaternary Research 71, 121–132. González-Sampériz, P., Valero-Garcés, B.L., Moreno, A., Jalut, G., García-Ruiz, J.M., Martí-Bono, C., Delgado-Huertas, A., Navas, A., Otto, T., Dedoubat, J.J., 2006. Climate variability in the Spanish Pyrenees during the last 30,000 yr revealed by the El Portalet sequence. Quaternary Research 66, 38–52. González-Sampériz, P., Valero-Garcés, B.L., Moreno, A., Morellón, M., Navas, A., Machín, J., DelgadoHuertas, A., 2008. Vegetation changes and hydrological fluctuations in the Central Ebro Basin (NE Spain) since the Late Glacial period: Saline lake records. Palaeogeography, Palaeoclimatology, Palaeoecology 259, 157–181. González-Trueba, J.J., Moreno, R.M., Martinez de Pison, E., Serrano, E., 2008. `Little Ice Age’ glaciation and current glaciers in the Iberian Peninsula. The Holocene 18, 551–568. Gottfried, M., Pauli, H., Futschik, A., Akhalkatsi, M., Barančok, P., Benito Alonso, J.L., Coldea, G., Dick, J., Erschbamer, B., Fernández Calzado, M.R., Kazakis, G., Krajči, J., Larsson, P., Mallaun, M., Michelsen, O., Moiseev, D., Moiseev, P., Molau, U., Merzouki, A., Nagy, L., Nakhutsrishvili, G., Pedersen, B., Pelino, G., Puscas, M., Rossi, G., Stanisci, A., Theurillat, J.-P., Tomaselli, M., Villar, L., Vittoz, P., Vogiatzakis, I., Grabherr, G., 2012. Continent-wide response of mountain vegetation to climate change. Nature Climate Change 2, 111–115. Greatbatch, R. J., 2000. The North Atlantic Oscillation. Stochastic Environmental Research and Risk Assessment 14, 213-242. Guiter, F., Andrieu-Ponel, V., Digerfeldt, G., Reille, M., Beaulieu, J.-L., Ponel, P., 2005. Vegetation history and lake-level changes from the Younger Dryas to the present in Eastern Pyrenees (France): pollen, plant macrofossils and lithostratigraphy from Lake Racou (2000 m a.s.l.). Vegetation History and Archaeobotany 14, 99–118. Haas, J.N., Richoz, I., Tinner, W., Wick, L., 1998. Synchronous Holocene climatic oscillations recorded on the Swiss Plateau and at timberline in the Alps. The Holocene 8, 301–309. Heiri, O., Wick, L., Van Leeuwen, J.F.., Van der Knaap, W.O., Lotter, A.F., 2003. Holocene tree immigration and the chironomid fauna of a small Swiss subalpine lake (Hinterburgsee, 1515 m asl). Palaeogeography, Palaeoclimatology, Palaeoecology 189, 35–53. Hély, C., Braconnot, P., Watrin, J., Zheng, W., 2009. Climate and vegetation: Simulating the African humid period. Comptes Rendus Geoscience 341, 671–688. Hofmann, W., 1986. Chironomid analysis, in: Handbook of Holocene Paleoecology and Paleohydrology. Wiley and Sons, Chichester, pp. 715–727. Hoffman, J.S, Carlson, E., Winsor, K., Klinkhammer, G.P., LeGrande, A.N., Andrews, J.T., Strasser, J.C., 2012. Linking the 8.2 ka event and its freshwater forcing in the Labrador Sea. Geophysical Research Letters, 39, L18703, doi:10.1029/2012GL053047. IPCC, 2007. Climate Change 2007: impacts, adaptation and vulnerability., Contribution of working group II to the fourth assessment report of the intergovernmental panel on cliamte change. ed. Cambridge University Press, Reino Unido. Jalut, G., Dedoubat, J.J., Fontugne, M., Otto, T., 2009. Holocene circum-Mediterranean vegetation changes: Climate forcing and human impact. Quaternary International 200, 4–18. Jalut, G., Esteban Amat, A., Bonnet, L., Gauquelin, T., Fontugne, M., 2000. Holocene climatic changes in the Western Mediterranean, from south-east France to south-east Spain. Palaeogeography, Palaeoclimatology, Palaeoecology 160, 255–290. Jiménez-Moreno, G., Anderson, R. S., 2012. Holocene vegetation and climate change recorded in alpine bog sediments from the Borreguilles de la Virgen, Sierra Nevada, southern Spain. Quaternary Research 77, 44–53. Kaplan, M.R., Wolfe, A.P., 2006. Spatial and temporal variability of Holocene temperature in the North Atlantic region. Quaternary Research 65, 223–231. 2. Basa de la Mora sequence. Climate at high altitudes 73 Kobashi, T., Severinghaus, J.P., Brook, E.J., Barnola, J.M., Grachev, A.M., 2007. Precise timing and characterization of abrupt climate change 8200 years ago from air trapped in polar ice. Quaternary Science Reviews 26, 1212–1222. Kröpelin, S., Verschuren, D., Lézine, A.-M., Eggermont, H., Cocquyt, C., Francus, P., Cazet, J.-P., Fagot, M., Rumes, B., Russell, J.M., Darius, F., Conley, D.J., Schuster, M., Von Suchodoletz, H., Engstrom, D.R., 2008. Climate-Driven Ecosystem Succession in the Sahara: The Past 6000 Years. Science 320, 765 –768. Lasanta-Martínez, T., Vicente-Serrano, S.M., Cuadrat-Prats, J.M., 2005. Mountain Mediterranean landscape evolution caused by the abandonment of traditional primary activities: a study of the Spanish Central Pyrenees. Applied Geography 25, 47–65. Lasheras-Álvarez, L., Pérez-Sanz, A., Gil-Romera, G., González-Sampériz, P., Sevilla-Callejo, M., ValeroGarcés, B.L., in press. Historia del fuego y la vegetación en una secuenica holocena del Pirineo Central: La Basa de la Mora. Cuad. Investig. Geográfica 39, 77–95. Leira, M., Santos, L., 2002. An early Holocene short climatic event in the northwest Iberian Peninsula inferred from pollen and diatoms. Quaternary International 93-94, 3–12. Lionello, P., Malanotte-Rizzoli, P., Boscolo, R., Alpert, P., Artale, V., Li, L., Luterbacher, J., May, W., Trigo, R., Tsimplis, M., Ulbrich, U., Xoplaki, E., 2006. The Mediterranean climate: An overview of the main characteristics and issues, in: Developments in Earth and Environmental Sciences. Elsevier, pp. 1–26. López-Merino, L., Cortizas, A.M., López-Sáez, J.A., 2010. Early agriculture and palaeoenvironmental history in the North of the Iberian Peninsula: a multi-proxy analysis of the Monte Areo mire (Asturias, Spain). Journal of Archaeological Science 37, 1978–1988. López-Merino, L., Moreno, A., Leira, M., Sigró, J., González-Sampériz, P., Valero-Garcés, B.L., LópezSáez, J.A., Brunet, M., Aguilar, E., 2011. Two hundred years of environmental change in Picos de Europa National Park inferred from sediments of Lago Enol, northern Iberia. J. Paleolimnol. 46, 453–467. López-Moreno, J.I., 2005. Recent variations of snowpack depth in the Central Spanish Pyrenees. Artic, Antartic, and Alpine Research 37, 253–260. López-Moreno, J.I., Stähli, M., 2008. Statistical analysis of the snow cover variability in a subalpine watershed: Assessing the role of topography and forest interactions. Journal of Hydrology 348, 379–394. Lucio, J.V., 1982. Estudio del Medio Físico del Sobrarbe. Aprovechamiento de los pastos estivales en el Valle de Gistain. Explotación actual y capacidad potencial. ICONA, Huesca. Magny, M., De Beaulieu, J.-L., Drescher-Schneider, R., Vannière, B., Walter-Simonnet, A.-V., Miras, Y., Millet, L., Bossuet, G., Peyron, O., Brugiapaglia, E., Leroux, A., 2007. Holocene climate changes in the central Mediterranean as recorded by lake-level fluctuations at Lake Accesa (Tuscany, Italy). Quaternary Science Reviews 26, 1736–1758. Magny, M., Vannière, B., Calo, C., Millet, L., Leroux, A., Peyron, O., Zanchetta, G., La Mantia, T., Tinner, W., 2011. Holocene hydrological changes in south-western Mediterranean as recorded by lakelevel fluctuations at Lago Preola, a coastal lake in southern Sicily, Italy. Quaternary Science Reviews 30, 2459–2475. Magyari, E.K., Chapman, J., Fairbairn, A.S., Francis, M., Guzman, M., 2012. Neolithic human impact on the landscapes of North-East Hungary inferred from pollen and settlement records. Vegetation History and Archaeobotany 21, 279–302. Mann, M.E., Zhang, Z., Rutherford, S., Bradley, R.S., Hughes, M.K., Shindell, D., Ammann, C., Faluvegi, G., Ni, F., 2009. Global Signatures and Dynamical Origins of the Little Ice Age and Medieval Climate Anomaly. Science 326, 1256–1260. Marshall, J., Kushnir, Y., Battisti, D., Chang, P., Czaja, A., Dickons, R., Hurrell, J., McCartney, M., Saravanan, R., Visbeck, M., 2002. North Atlantic climate variability: phenomena, impacts and mechanisms. International Journal of Climatology 21, 1863–1898. Martín-Puertas, C., Valero-Garcés, B.L., Brauer, A., Mata, M.P., Delgado-Huertas, A., Dulski, P., 2009. The Iberian–Roman Humid Period (2600–1600 cal yr BP) in the Zoñar Lake varve record (Andalucía, southern Spain). Quaternary Research 71, 108–120. 2. Basa de la Mora sequence. Climate at high altitudes 74 Martín-Puertas, C., Valero-Garces, B.L., Pilar Mata, M., Gonzalez-Samperiz, P., Bao, R., Moreno, A., Stefanova, V., 2008. Arid and humid phases in southern Spain during the last 4000 years: the Zonar Lake record, Cordoba. The Holocene 18, 907–921. Mayewski, P.A., 2004. Holocene climate variability. Quaternary Research 62, 243–255. Menking, K.M., Peteet, D.M., Anderson, R.Y., 2012. Late-glacial and Holocene vegetation and climate variability, including major droughts, in the Sky Lakes region of southeastern New York State. Palaeogeography, Palaeoclimatology, Palaeoecology 353-355, 45–59. Meyers, P.A., 2003. Applications of organic geochemistry to paleolimnological reconstructions: a summary of examples from the Laurentian Great Lakes. Organic Geochemistry 34, 261–289. Millán, M.M., Estrela, M.J., Miró, J., 2005. Rainfall Components: Variability and Spatial Distribution in a Mediterranean Area (Valencia Region). Journal of Climate 18, 2682–2705. Miras, Y., Ejarque, A., Riera, S., Palet, J.M., Orengo, H., Eubab, I., 2007. Dynamique holocène de la végétation et occupation des Pyrénées andorranes depuis le Néolithique ancien, d’après l’analyse pollinique de la tourbière de Bosc dels Estanyons (2180 m, Vall del Madriu, Andorre). Comptes Rendus Palevol 6, 291–300. Montserrat-Martí, J., 1992. Evolución glaciar y postglaciar del clima y la vegetación en la vertiente sur del Pirineo: estudio palinológico., Monografías del Instituto Pirenaico de Ecología-CSIC. Zaragoza. Moore, P.D., Webb, J.A., 1978. An illustrated guide to pollen analysis. Hodder and Stoughton, London. Moore, P.D., Webb, J.A., Collinson, M.E., 1991. Pollen Analysis, Second. ed. Blackwell Scientifc Publications. Oxford. Morales-Molino, C., 2013. Dinámica vegetal desde el Tadiglaciar en la cuenca del Duero: interacciones con el clima, fuego e impacto humano. Universidad Politécnica de Madrid, Madrid. Morales-Molino, C., Postigo-Mijarra, J.M., Morla, C., Garcia-Antón, M., 2012. Long-term persistence of Mediterranean pine forests in the Duero Basin (central Spain) during the Holocene: The case of Pinus pinaster Aiton. The Holocene 22, 561–570. Morellón, M., Pérez-Sanz, A., Corella, J.P., Büntgen, U., Catalán, J., González-Sampériz, P., GonzálezTrueba, J.J., López-Sáez, J.A., Moreno, A., Pla-Rabes, S., Saz-Sánchez, M. á., Scussolini, P., Serrano, E., Steinhilber, F., Stefanova, V., Vegas-Vilarrúbia, T., Valero-Garcés, B., 2012. A multiproxy perspective on millennium-long climate variability in the Southern Pyrenees. Climate of the Past 8, 683–700. Morellón, M., Valero-Garcés, B., Moreno, A., González-Sampériz, P., Mata, P., Romero, O., Melchor Maestro, Navas, A., 2008. Holocene palaeohydrology and climate variability in northeastern Spain: The sedimentary record of Lake Estanya (Pre-Pyrenean range). Quaternary International 181, 15–31. Morellón, M., Valero-Garcés, B., Vegas-Vilarrúbia, T., González-Sampériz, P., Romero, Ó., DelgadoHuertas, A., Mata, P., Moreno, A., Rico, M., Corella, J.P., 2009. Lateglacial and Holocene palaeohydrology in the western Mediterranean region: The Lake Estanya record (NE Spain). Quaternary Science Reviews 28, 2582–2599. Moreno, A., López-Merino, L., Leira, M., Marco-Barba, J., González-Sampériz, P., Valero-Garcés, B.L., López-Sáez, J.A., Santos, L., Mata, P., Ito, E., 2011. Revealing the last 13,500 years of environmental history from the multiproxy record of a mountain lake (Lago Enol, northern Iberian Peninsula). Journal of Paleolimnology 46, 327–349. Moreno, A., González-Sampériz, P., Morellón, M., Valero-Garcés, B.L., Fletcher, W.J., 2012. Northern Iberian abrupt climate change dynamics during the last glacial cycle: A view from lacustrine sediments. Quaternary Science Reviews 36, 139–153. Moreno, A., Pérez, A., Frigola, J., Nieto-Moreno, V., Rodrigo-Gámiz, M., Martrat, B., González-Sampériz, P., Morellón, M., Martín-Puertas, C., Corella, J.P., Belmonte, Á., Sancho, C., Cacho, I., Herrera, G., Canals, M., Grimalt, J.O., Jiménez-Espejo, F., Martínez-Ruiz, F., Vegas-Vilarrúbia, T., ValeroGarcés, B.L., 2012b. The Medieval Climate Anomaly in the Iberian Peninsula reconstructed from marine and lake records. Quaternary Science Reviews 43, 16–32. Moreno, A., Valero-Garcés, B.L., González-Sampériz, P., Rico, M., 2008. Flood response to rainfall variability during the last 2000 years inferred from the Taravilla Lake record (Central Iberian Range, Spain). Journal of Paleolimnology 40, 943–961. 2. Basa de la Mora sequence. Climate at high altitudes 75 Muñoz Sobrino, C., Ramil-Rego, P., Gómez-Orellana, L., 2007. Late Würm and early Holocene in the mountains of northwest Iberia: biostratigraphy, chronology and tree colonization. Vegetation History and Archaeobotany 16, 223–240. Muñoz Sobrino, C., Ramil‐rego, P., Gómez-Orellana, L., Varela, R.A.D., 2005. Palynological data on major Holocene climatic events in NW Iberia. Boreas 34, 381–400. Nesje, A., Bjune, A.E., Bakke, J., Dahl, S.O., Lie, Ø., Birks, H.J.B., 2006. Holocene palaeoclimate reconstructions at Vanndalsvatnet, western Norway, with particular reference to the 8200 cal. yr BP event. The Holocene 16, 717–729. O’Brien, S.R., Mayewski, P.A., Meeker, L.D., Meese, D.A., Twickler, M.S., Whitlow, S.I., 1995. Complexity of Holocene Climate as Reconstructed from a Greenland Ice Core. Science 270, 1962– 1964. Oldfield, F., Dearing, J.A., 2003. The role of human activities in past environmental change, in: Paleoclimate, Global Change and the Future, IGBP. Berlin, pp. 143–162. Ortu, E., Peyron, O., Bordon, A., De Beaulieu, J.L., Siniscalco, C., Caramiello, R., 2008. Lateglacial and Holocene climate oscillations in the South-western Alps: An attempt at quantitative reconstruction. Quaternary International 190, 71–88. Pantaleón-Cano, J., Yll, E.-I., Pérez-Obiol, R., Roure, J.M., 2003. Palynological evidence for vegetational history in semi-arid areas of the western Mediterranean (Almería, Spain). The Holocene 13, 109– 119. Pausas, J.G., Paula, S., 2012. Fuel shapes the fire-climate relationship: evidence from Mediterranean ecosystems. Global Ecology and Biogeography 21, 1074–1082. Pèlachs, A., Julià, R., Pérez-Obiol, R., Soriano, J.M., Bal, M.-C., Cunill, R., Catalan, J., 2011. Potential influence of Bond events on mid-Holocene climate and vegetation in southern Pyrenees as assessed from Burg lake LOI and pollen records. The Holocene 21, 95 –104. Pèlachs, A., Soriano, J.M., Nadal, J., Esteban, A., 2007. Holocene environmental history and human impact in the Pyrenees. Contributions to Science 3, 421–429. Pérez-Obiol, R., Bal, M.-C., Pèlachs, A., Cunill, R., Soriano, J.M., 2012. Vegetation dynamics and anthropogenically forced changes in the Estanilles peat bog (southern Pyrenees) during the last seven millennia. Vegetation History and Archaeobotany 21, 385–396. Pérez-Obiol, R., Jalut, G., Julia, R., Pelachs, A., Iriarte, M.J., Otto, T., Hernandez-Beloqui, B., 2011. MidHolocene vegetation and climatic history of the Iberian Peninsula. The Holocene 21, 75–93. Pérez-Sanz, A., González-Sampériz, P., Valero-Garcés, B., Moreno, A., Morellón, M., Sancho, C., Belmonte, A., Gil-Romera, G., Sevilla, M., Navas, A., 2011. Clima y actividades humanas en la dinámica de la vegetación durante los últimos 2000 años en el Pirineo central: el registro palinológico de la Basa de la Mora (Macizo de Cotiella). Zubía 23, 17–38. Pla, S., Catalán, J., 2005. Chrysophyte cysts from lake sediments reveal the submillennial winter/spring climate variability in the northwestern Mediterranean region throughout the Holocene. Climate dynamics 24, 263–278. Prat, N., Real, M., Rieradevall, M., 1992. Benthos of Spanish lakes and reservoirs. Limnetica 8, 221–229. Rasmussen, S.O., Vinther, B.M., Clausen, H.B., Andersen, K.K., 2007. Early Holocene climate oscillations recorded in three Greenland ice cores. Quaternary Science Reviews 26, 1907–1914. Reille, M., Lowe, J.L., 1995. Atlas. Pollen et spores d’Europe et d’Afrique du nord. Éditions du Laboratorie de botanique historique et palynologie, Marselle. 530p. Reimer, P.J., Baillie, M.G.L., Bard, E., Bayliss, A., Beck, J.W., Blackwell, P.G., Bronk Ramsey, C., Buck, C.E., Burr, G.S., Edwards, R.L., Friedrich, M., Grootes, P.M., Guilderson, T.P., Hajdas, I., Heaton, T.J., Hogg, A.G., Hughen, K.A., Kaiser, K.F., Kromer, B., McCormac, F.G., Manning, S.V., Reimer, R.W., Richards, D.A., Southon, J.R., Talamo, S., Turney, C.S.M., Van der Plicht, J., Weyhenmeyer, C.E., 2009. INTCAL09 and MARINE09 radiocarbon age calibration curves, 050,000 years cal. BP. Radiocarbon 51, 1111–1150. Renssen, H., Goosse, H., Fichefet, T., 2007. Simulation of Holocene cooling events in a coupled climate model. Quaternary Science Reviews 26, 2019–2029. 2. Basa de la Mora sequence. Climate at high altitudes 76 Renssen, H., Seppä, H., Crosta, X., Goosse, H., Roche, D.M., 2012. Global characterization of the Holocene Thermal Maximum. Quaternary Science Reviews 48, 7–19. Renssen, H., Seppä, H., Heiri, O., Roche, D.M., Goosse, H., Fichefet, T., 2009. The spatial and temporal complexity of the Holocene thermal maximum. Nature Geoscience 2, 411–414. Riera, S., Wansard, G., Julià, R., 2004. 2000-year environmental history of a karstic lake in the Mediterranean Pre-Pyrenees: the Estanya lakes (Spain). Catena 55, 293–324. Rieradevall, M., Bonada, N., Prat, N., 1999. Community structure and water quality in the Mediterranean streams of a natural park (St. Llorenc del Munt, NE Spain). Limnetica 17, 45–56. Rieradevall, M., Brooks, S.J., 2001. An identification guide to subfossil Tanypodinae larvae (Insecta: Diptera: Chironomidae) based on cephalic setation. Journal of Paleolimnology 25, 81–99. Roberts, N., Eastwood, W.J., Kuzucuoglu, C., Fiorentino, G., Caracuta, V., 2011. Climatic, vegetation and cultural change in the eastern Mediterranean during the mid-Holocene environmental transition. The Holocene 21, 147–162. Rubiales, J.M., García-Amorena, I., Hernández, L., Génova, M., Martínez, F., Manzaneque, F.G., Morla, C., 2010. Late Quaternary dynamics of pinewoods in the Iberian Mountains. Review of Palaeobotany and Palynology 162, 476–491. Sabatier, P., Dezileau, L., Colin, C., Briqueu, L., Bouchette, F., Martinez, P., Siani, G., Raynal, O., Von Grafenstein, U., 2012. 7000years of paleostorm activity in the NW Mediterranean Sea in response to Holocene climate events. Quaternary Research 77, 1–11. Sadori, L., Jahns, S., Peyron, O., 2011. Mid-Holocene vegetation history of the central Mediterranean. The Holocene 21, 117–129. Sadori, L., Mercuri, A.M., Mariotti Lippi, M., 2010. Reconstructing past cultural landscape and human impact using pollen and plant macroremains. Plant Biosystems - An International Journal Dealing with all Aspects of Plant Biology 144, 940–951. Saether, O.A., 1979. Chironomid communities as water quality indicators. Holartic Ecology 2, 65–74. Sancho, C., Peña, J.L., Muñoz, A., Benito, G., McDonald, E., Rhodes, E.J., Longares, L.A., 2008. Holocene alluvial morphopedosedimentary record and environmental changes in the Bardenas Reales Natural Park (NE Spain). Catena 73, 225–238. Santos, L., Romani, J.R.V., Jalut, G., 2000. History of vegetation during the Holocene in the Courel and Queixa Sierras, Galicia, northwest Iberian Peninsula. J. Quat. Sci. 15, 621–632. Schnurrenberger, D., Russell, J., Kelts, K., 2003. Classification of lacustrine sediments based on sedimentary components. Journal of Paleolimnology 29, 141–154. Scussolini, P., Vegas-Vilarrúbia, T., Rull, V., Corella, J.P., Valero-Garcés, B., Gomà, J., 2011. Middle and late Holocene climate change and human impact inferred from diatoms, algae and aquatic macrophyte pollen in sediments from Lake Montcortès (NE Iberian Peninsula). J. Paleolimnol. 46, 369–385. Seager, R., Graham, N., Herweijer, C., Gordon, A.L., Kushnir, Y., Cook, E., 2007. Blueprints for Medieval hydroclimate. Quaternary Science Reviews 26, 2322–2336. Seguret, M., 1972. Etude tectonique des nappes et seris décollées de la partie centrale du versant sud des Pyrénees. Caractère synsédimentaire, rôle de la compression et de lagravité, Publications USTELA. Série Géologie Structurale nº 2, Montpellier, 155 p. Shuman, B., Bravo, J., Kaye, J., Lynch, J.A., Newby, P., Webb, T., 2001. Late Quaternary Water-Level Variations and Vegetation History at Crooked Pond, Southeastern Massachusetts. Quaternary Research 56, 401–410. Spötl, C., Nicolussi, K., Patzelt, G., Boch, R., 2010. Humid climate during deposition of sapropel 1 in the Mediterranean Sea: Assessing the influence on the Alps. Global and Planetary Change 71, 242– 248. Steinhilber, F., Beer, J., Fröhlich, C., 2009. Total solar irradiance during the Holocene. Geophys. Res. Lett. 36. 2. Basa de la Mora sequence. Climate at high altitudes 77 Stockmarr, J., 1971. Tablets with spores used in absolute pollen analysis. Pollen Spores 13, 614–621. Stoll, H.M., Moreno, A., Mendez-Vicente, A., Gonzalez-Lemos, S., Jimenez-Sanchez, M., DominguezCuesta, M.J., Edwards, R.L., Cheng, H., Wang, X., submitted. Growth rates of speleothems in NW Iberian Peninsula over the last two glacial cycles and relationship with climate. Quaternary Research Talon, B., 2010. Reconstruction of Holocene high-altitude vegetation cover in the French southern Alps: evidence from soil charcoal. The Holocene 20, 35–44. Tarrats, P., 2011. Paleoenvironmental reconstruction of Basa de la Mora Lake (Central Pyrenees) during the Holocene using chironomid record (insecta: diptera). Universidad de Barcelona, Barcelona. Tarrats, P., Rieradevall, M., González-Sampériz, P., Pérez-Sanz, A., Valero-Garcés, B., Moreno, A., 2014. Relating actual with subfossil chironomid assemblages. Holocene habitat changes and paleoenvironmental reconstruction of Basa de la Mora Lake (Central Pyrenees). Presented at the Erupean Geoscience Union, Vienna. Tinner, W., Conedera, M., Ammann, B., Lotter, A.F., 2005. Fire ecology north and south of the Alps since the last ice age. The Holocene 15, 1214–1226. Tinner, W., Hu, F.S., 2003. Size parameters, size-class distribution and area-number relationship of microscopic charcoal: relevance for fire reconstruction. The Holocene 13, 499–505. Tinner, W., Hubschmid, P., Wehrli, M., Ammann, B., Conedera, M., 1999. Long-term forest fire ecology and dynamics in southern Switzerland. Journal of Ecology 87, 273–289. Tinner, W., Lotter, A., 2001. Central European vegetation response to abrupt climate change at 8.2 ka. Geology 29, 551–554. Trigo, R., Osborn, T., Corte-Real, J., 2002. The North Atlantic Oscillation influence on Europe: climate impacts and associated physical mechanisms. Climate Research 20, 9–17. Trouet, V., Esper, J., Graham, N.E., Baker, A., Scourse, J.D., Frank, D.C., 2009. Persistent Positive North Atlantic Oscillation Mode Dominated the Medieval Climate Anomaly. Science 324, 78–80. Valero-Garcés, B.L., González-Sampériz, P., Delgado-Huertas, A., Navas, A., Machín, J., Kelts, K., 2000. Lateglacial and Late Holocene environmental and vegetational change in Salada Mediana, central Ebro Basin, Spain. Quaternary International 73/74, 29–46. Valero-Garcés, B.L., Moreno, A., 2011. Iberian lacustrine sediment records: responses to past and recent global changes in the Mediterranean region. Journal of Paleolimnology 46, 319–325. Valero-Garcés, B.L., Moreno, A., Navas, A., Mata, P., Machín, J., Delgado Huertas, A., González Sampériz, P., Schwalb, A., Morellón, M., Cheng, H., Edwards, R.L., 2008. The Taravilla lake and tufa deposits (Central Iberian Range, Spain) as palaeohydrological and palaeoclimatic indicators. Palaeogeography, Palaeoclimatology, Palaeoecology 259, 136–156. Vannière, B., Colombaroli, D., Chapron, E., Leroux, A., Tinner, W., Magny, M., 2008. Climate versus human-driven fire regimes in Mediterranean landscapes: the Holocene record of Lago dell’Accesa (Tuscany, Italy). Quaternary Science Reviews 27, 1181–1196. Vargas-Yáñez, M., Jesús García, M., Salat, J., García-Martínez, M.C., Pascual, J., Moya, F., 2008. Warming trends and decadal variability in the Western Mediterranean shelf. Global and Planetary Change 63, 177–184. Venables, W.N., Smith, D.M., R Development Core Team, 2008. An introduction to R notes on R, a programming environment for data analysis and graphics. Dept. of Statistics and Mathematics, Wirtschaftsuniversität Wien, Wien, Austria. Vescovi, E., Ravazzi, C., Arpenti, E., Finsinger, W., Pini, R., Valsecchi, V., Wick, L., Ammann, B., Tinner, W., 2007. Interactions between climate and vegetation during the Lateglacial period as recorded by lake and mire sediment archives in Northern Italy and Southern Switzerland. Quaternary Science Reviews 26, 1650–1669. Wanner, H., Beer, J., Bütikofer, J., Crowley, T.J., Cubasch, U., Flückiger, J., Goosse, H., Grosjean, M., Joos, F., Kaplan, J.O., Küttel, M., Müller, S.A., Prentice, I.C., Solomina, O., Stocker, T.F., Tarasov, P., Wagner, M., Widmann, M., 2008. Midto Late Holocene climate change: an overview. Quaternary Science Reviews 27, 1791–1828. Wanner, H., Brönnimann, S., 2012. Is there a global Holocene climate mode? PAGES news 20, 44-45. 2. Basa de la Mora sequence. Climate at high altitudes 78 Wanner, H., Solomina, O., Grosjean, M., Ritz, S.P., Jetel, M., 2011. Structure and origin of Holocene cold events. Quaternary Science Reviews 30, 3109–3123. Wiederholm, T., 1983. Chironomidae of the Holarctic region. Keys and diagnoses. Part I. Larvae, Wiederholm. ed. Museum of Zoology and Entomology, Lund University. Zhao, C., Yu, Z., Ito, E., Zhao, Y., 2010. Holocene climate trend, variability, and shift documented by lacustrine stable-isotope record in the northeastern United States. Quaternary Science Reviews 29, 1831–1843. 2. Basa de la Mora sequence. Climate at high altitudes 79 APPENDIX Figure A2.1. Vegetation map of hte Pyrenees. Map plotted by Miguel Sevilla Callejo. 2. Basa de la Mora sequence. Climate at high altitudes 80 Figure A2.2. Arboreal pollen from Basa de la Mora sequence. 2. Basa de la Mora sequence. Climate at high altitudes 81 Figure A2.3. Non-Arboreal pollen from Basa de la Mora sequence. 3. Estanya sequence. Climate at low altitudes 88 1.1.1. Objectives This chapter aims to investigate the altitudinal vegetation shifts in the southern Central Pyrenees in order to evaluate the role of temperature and humidity as main drivers of the vegetation dynamics during the Holocene. In order to achieve this goal, we present the first Holocene pollen sequence from the lowermost vegetation belt of the southern Pyrenees: the Lake Estanya sequence (670 m a.s.l.) (fig 3.1). The Estanya results are compared with the available regional palaeoenvironmental data from high mountain areas in the region (Montserrat-Martí, 1992; Pla and Catalan, 2005; González-Sampériz et al., 2006; Miras et al., 2007; Pèlachs et al., 2007; Ejarque et al., 2009; Rull et al., 2011; Pérez-Obiol et al., 2012; Pérez-Sanz et al., 2013) (fig 3.1) in order to reveal the altitudinal vegetation shifts in the central southern Pyrenees during the Holocene and the possible mechanisms beyond those changes. Figure 3.1. Geographical position of Lake Estanya and the rest of sequences from northern Spain named in the text. The red square marks the area represented in figure 2. Sequences from west to east are: 1: Las Pardillas (Sánchez-Goñi and Hannon, 1999); 2: Villarquemado (Aranbarri et al., 2014); 3: Portalet (GonzálezSampériz et al., 2006); 4: Tramacastilla (Montserrat-Martí, 1992); 5 : Paul de Bubal (Montserrat-Martí, 1992); 6: Chiprana (Valero-Garcés et al., 2000); 7: Basa de la Mora-BSM (Pérez-Sanz et al., 2013); 8: Redo (Pla and Catalan, 2005); 9: Montcortés (Rull et al., 2011); 10: Burg (Pèlachs et al., 2007); 11: Estanilles (Pérez-Obiol et al., 2012); 12: Pradell (Ejarque et al., 2009); 13: Planels de Perafita-PDP(Miras et al., 2010) 14: Bosc dels Estanyons-BDE (Miras et al., 2007); 15: Riu dels Orris (Ejarque et al., 2010). Map plotted by Saul Fontaneda. 3. Estanya sequence. Climate at low altitudes 89 1.2. SETTINGS Lake Estanya is a low altitude karstic lake (fig. 3.2) placed in the Southern Pre-Pyrenees (42°02’N, 0°32’E, 670 m a.s.l.), in the Mediterranean bioclimatic regime (fig. 3.1). Mean temperature variation ranges from 4°C in the coldest month (January) to 24° C in the warmest (July). The scarce mean annual precipitation (470 mm/year) is seasonally distributed with a long dry season during the summer. Additionally, Lake Estanya is located in a transition zone between the nearby semi-desert area of the Ebro River Basin, (350 m a.s.l.) and the high peaks of the Pyrenees (up to 3400 m a.s.l.). As a result of this sharp altitudinal gradient both temperature and precipitation varies greatly in a relative small area, from 14°C and less than 350mm/yr at the Ebro Basin, to around 5°C and more than 2000mm/yr at 2000 m asl. The Lake Estanya surrounding vegetation is formed by Mediterranean communities, represented by Quercus rotundifolia, Buxus sempervirens and Juniperus oxycedrus, and by submediterranean associations, dominated by Quercus faginea and Quercus cerrioides. These communities are mixed with patches of cereal crops. The lake is bordered by a hygrophyte band formed by Phragmites australis, Typha angustifolia, Juncus spp., and Scirpus spp. Figure 3.2. Lake Estanya panoramic view. 3. Estanya sequence. Climate at low altitudes 90 Given its position in the lowermost altitudinal zone of the Pyrenees, Estanya sequence provides vegetation information from both the basal (up to 700 m a.s.l.) and part of the submontane belt (700-1100 m a.s.l.) (fig.3.3 a, b). Figure 3.3. a) Schematic representation of the vegetation zones of the Southern Pyrenees including the location and altitude of all the paleoenvironmental sequences available in the area.From west to east: Portalet: González-Sampériz et al., 2006; Paul de Bubal: Montserrat-Martí, 1992; Tramacastilla: MontserratMartí, 1992; BSM (Basa de la Mora): Pérez-Sanz et al., 2013; Estanya: present study; Morellón et al., 2009a,b; Redo: Pla and Catalan, 2005; Montcortés: Corella et al., 2010; Rull et al., 2011; Burg: Pèlachs et al., 2007; Estanilles: PérezObiol et al., 2012; Pradell: Ejarque et al., 2009; BDE (Bosc dels Estanyons): Miras et al., 2007; PDP (Planells de Perafita): Miras et al., 2010; RDO (Rius del Orris): Ejarque et al., 2010. b) Vegetation description of the different vegetation belt of the Pyrenees. The definition of the vegetation belts are based on Villar (1997), Domínguez-Llovería and Puente-Cabeza (2003) and Ninot et al., (2007). The description includes the main trees, shrubs and herbs that for each belt, sorted out by order of abundance. Main species are highlighted in bold. 3. Estanya sequence. Climate at low altitudes 91 1.3. MATERIAL AND METHODS The fieldwork (coring and sampling), sedimentological and geochemical analyses and agedepth model for Lake Estanya sequence have been previously published in Morellón et al., 2009a. In the present Thesis radiocarbon dates for the Holocene have been calibrated with the INTCAL09 curve (Reimer et al., 2009) (table 3.1). Sedimentological and paleohydrological evolution of Lake Estanya for the last 22 cal ka BP has been published elsewhere (Morellón et al., 2009b) (fig. 3.4). The pollen sequence of the last 800 years was included in Morellón et al., 2011. The Lateglacial diatom-vegetation relationships have been considered in a recent work (Vegas-Vilarrúbia et al., 2013) (fig. 3.5). Here we present the palynological data concerning the whole Holocene period from the same set of cores used by Morellón et al (2009, 20119. Previous sedimentological, geochemical, biological and pollen studies in short cores from Estanya cover the last 2000 years (Riera et al., 2004) The palynological analyses of the whole sequence were carried out every 10 cm, and laboratory procedures followed the classic chemical method (Moore et al., 1991), modified according Dupré (1992), including use of HCl, KOH, HF digestion and gravitational separation with Thoulet solution (2.0 gr/cm3 density). Lycopodium clavatum tablets were added to calculate pollen concentrations (Stockmarr, 1971). Pollen sum was always higher than 300 terrestrial grains and taxa number not less than 20. Aquatic plants, ferns and algal remains were excluded of percentages calculation. Pollen diagrams included in this paper only concern the Holocene sequence and have been drawn using GRAPHER 4® and graphic design software ADOBE ILLUTRATOR CS4®. Lab Code Depth (cm) Sample type 14C age (yr BP) Calibrated age, 2σ (yr cal BP) 137Cs 14 -13 Poz-24749 28.5 Phragmites stem 155 ±30 198 ± 32 Poz-12245 54.5 Terrestrial macrorrest 405 ±30 472 ± 43 Poz-12246 170 Terrestrial macrorrest 895 ±35 823 ± 88 Poz-15972 189.5 Bulk organic matter 2120 ± 30 1175 ± 242 Poz-12247 233 Salix leave 3315 ±3 3572 ± 324 Poz-12248 330 Gramineae seed 5310 ± 60 6097 ±116 Poz-15973 349 Bulk organic matter 6230 ± 40 6195 ± 289 Poz-15974 393 Bulk organic matter 8550 ± 50 8647 ± 345 Poz-9891 432 Wood fragment 8510 ± 50 9497 ± 49 Poz-17190 493 Plant macroremain 8830 ± 50 9834 ± 136 Poz-17191 564 Bulk organic matter 10 680 ± 60 11443 ± 384 Table 3.1. AMS radiocarbon dates from Lake Estanya core. Dates have been updated from Morellón et al., 2009b with the INTCAL09 curve. 3. Estanya sequence. Climate at low altitudes 92 1.4. RESULTS The Holocene Estanya pollen record shows great variety of arboreal, herbaceous and aquatic taxa. Pinus, semi-deciduous Quercus, Corylus, evergreen Quercus, and Juniperus are the main components of the arboreal pollen (AP) (fig. 3.6). The herbaceous taxa (NAP) are Figure 3.5. Diatoms-vegetation relationship fin Lake Estanya during the Late Glacial. From Vegas-Vilarúbia et al. 2013. Figure 3.4. Sedimentological, compositional and mineralogical profiles; depositional environments and lake level fluctuation of the Lake Estanya sequence for the last 22 cal ka BP. From Morellón et al. 2009b. 3. Estanya sequence. Climate at low altitudes 93 widely represented by Poaceae, Artemisia, Chenopodiaceae and increasing Ruderals in recent times (see fig. 3.6 for taxa included in the vegetation groups). The aquatic component, including hydrophytes and hygrophytes, keeps relatively low values; Potamogeton is rather remarkable at some levels, and Cyperaceae and Ranunculus rise at the top of the sequence reaching up to 20%. Based on major changes on tree taxa, six main pollen zones have been described (fig. 3.6). Correlation with the sedimentological units defined by Morellón et al., (2009a) is also shown in figure 3.6. Appendix II (figs A.3.1, A3.2 and A3.3), located at the end of the thesis, shows all taxa found for the BSM sequence EST-I (570–490 cm; 11.700-9800 cal yr BP cal BP) This zone is dominated by Juniperus (up to 55%). Other significant taxa within the AP are Pinus, semi-deciduous and evergreen Quercus and Betula. The most noticeable taxa on the herbaceous component are Poaceae, Artemisia and Chenopodiaceae (fig. 3.6). Both the hygrophytes and the hydrophytes display the minimum values of the Holocene. EST-II (490–385 cm; 9800-8200 cal yr BP ) The onset of this zone is characterized by a sharp decline of Juniperus while Corylus and Pinus spread. Semi-deciduous Quercus, evergreen Quercus, Other Mesophytes and Mediterranean Shrubs also increase at this moment while Tilia and Fagus appear for the first time. In the NAP, Poaceae expands and reaches its maximum along the sequence (up to 20%) while Artemisia compared to the previous zone. The aquatic components increase their presence, particularly Cyperaceae and Potamogeton (fig. 3.6). EST-III (385–320 cm; 8200-6000 cal yr BP) The decline of Corylus and increase of semi-deciduous Quercus, evergreen Quercus and Pinus define this period. The herbaceous component is reduced and experiences some significant changes (fig. 3.6). Poaceae decreases to less than 10% and Artemisia and Chenopodiaceae contents strongly vary, reaching both their maxima and minima within this zone. The aquatic component reflects the drop of Potamogeton, which even disappears in some intervals. EST-IV (320-260 cm; 6000-4800 cal yr BP) This zone is defined on the basis of the expansion of semi-deciduous and evergreen Quercus, and Abies, while a decrease in Pinus and a further reduction in Corylus and Other Mesophytes are also recorded. In the herbaceous component Poaceae and Artemisia are reduced while in the aquatic component the hydro and hygrophytes curves barely change (fig. 3.6). 3. Estanya sequence. Climate at low altitudes 94 Figure 3.6. Selected curves from Estanya pollen diagram represented in depth. Other Mesophytes include: Alnus, Carpinus, Salix, Ulmus, Populus, Acer Fraxinus and Juglans. Mediterranean Shrubs include: Pistacia, Rhamnus, Phyllyrea, Buxus and Sambucus. Ruderals include: Cichorioideae, Carduae, Asteroideae, Centaurea, Plantago, Rumex, Brassicaceae, Urticaceae, Geraniaceae, Malvaceae. Hygrophytes include: Epilobium, Aristolochia, Ranunculus, Thalictrum, Lythrum, Juncus, Utricularia, Cyperaceae, Typha Pedicularis and Sparganium. Hydrophytes include: Lemna, Nymphaea, Myriophyllum, Potamogeton and Ruppia. 3. Estanya sequence. Climate at low altitudes 95 EST-V (260-170 cm; 4800-800 cal yr BP) Thermophilous taxa such as semi-deciduous Quercus and evergreen Quercus reach the maximum expansion of the Holocene in this zone. Conversely, mesophytes such as Betula and Corylus keep decreasing and Tilia disappears (fig. 3.6). Nevertheless Fagus peaks at this time. The herbaceous component is characterized by the sequence’s minima of Poaceae and Artemisia, and, more interesting, the first appearance of Juglans, Cerealia and Vitis and the continuous presence of Rumex. The Hydrophytes keep low values and the Hygrophytes decrease towards the middle of the zone and increase upwards. EST-VI (170-0 cm; 800-0 cal yr BP, 1150-1950 AD) This zone is characterized by a decrease in the AP values and the large expansion of the anthropic component such as Olea, Vitis, Juglans, Cerealia and Cannabis related to agricultural practises and Rumex, Urticaceaea and Plantago related to pastoral practises. All of them reach the maximum values in the 100-top cm. On the aquatic component is remarkable the expansion of Ranunculus, Cyperaceae and Potamogeton. A charcoal–rich layer found at 110 cm depth -and previously identified by Morellón et al., (2009a) - has resulted sterile - white band in figures 6 and 7likely due to differential pollen conservation (Morellón et al., 2011) (fig. 3.7). 1.5. DISCUSSION The comparison between the vegetation changes in the Lake Estanya sequence and the vegetation changes recorded in other Pyrenean sequences placed at higher altitudes illustrates the vertical vegetation shifts in the southern Pyrenees and allow us to infer the regional climate evolution during the Holocene. Figure 3.7. Selected pollen taxa from Lake Estanya sequence of the last 800 years. From Morellón et al. 2011. 3. Estanya sequence. Climate at low altitudes 96 1.5.1. The onset of the Holocene (11.700-9800 cal yr BP): high continentality Extremely high values of Juniperus and Artemisia and scarcity of temperate trees such as semi-deciduous and evergreen Quercus characterize the beginning of the Holocene in Lake Estanya (fig. 3.8). The absence or low presence of temperate trees is also observed both in inner continental Iberian sequences like Villarquemado (Aranbarri et al., 2014), Ayoó de Vidriales (Morales-Molino and García-Antón, 2013), Fuentillejo, (Vegas et al., 2010), or Espinosa del Cerrato (Franco-Múgica et al., 2001) among others, and higher altitude sequences located in the eastern part of the Pyrenees like Estanilles (Pérez-Obiol et al., 2012) or La Pouretère (Aubert et al., 2004), which are characterized by the dominance of conifers (mainly Pinus but also Juniperus). The large proportion of junipers in Estanya and the limited expansion of the temperate forest at higher altitudes, in contrast to more Atlantic-influenced sequences from northern Spain like El Portalet (González-Sampériz et al., 2006), Enol lake (Moreno et al., 2011) or Monte Areo (López-Merino et al., 2010) among others, point to severe climate conditions in the region at the beginning of the Holocene. Current Juniperus thurifera communities in Iberia, including those from the nearby area of the central Ebro Basin, are found under harsh environmental conditions characterized by extreme temperatures, low precipitation and poor soil development (Blanco-Castro, 2005). The environmental conditions recorded at the onset of the Holocene in Estanya and other inner continental sequences (Carrión et al., 2010 and references therein) show some similarities to the current juniper community niche. Firstly, the onset of the Holocene was characterized by a maximum in summer and a minimum in winter insolation in the Northern Hemisphere (Kutzbach and Webb, 1993), resulting in extremely high continentality with maximum contrast between summer and winter temperatures. Secondly, linked to the high summer temperature, high evaporation-rates were likely to occur resulting in strong summer water deficits. Thirdly, the long-lasting glacial period produced gelifraction processes causing a lack of well-developed soil horizons at the Holocene onset. Analogue vegetation composition with dominance of Juniperus and Artemisia has been also recognised in other sequences from the semi-arid Central Ebro Basin (González-Sampériz et al., 2008; Davis and Stevenson, 2007) highlighting the widespread severe climate conditions in extensive areas of North-Eastern Iberia. Particularly cold winter temperatures affecting the Pyrenees at this time are supported firstly, by the absence of temperate trees both in Estanya and at higher altitude sequences (i.e. La Paul de Bubal by Montserrat, 1992: fig. 3.3 (find references therein)), and secondly by the downwards shift of the treeline as observed in the Estanilles sequence (2250 m a.s.l.) (Pérez-Obiol et al., 2012) (fig. 3.3), where pine proportions reached the lowest values at the beginning of the Holocene. Further evidence of limited forest expansion in altitude in the Pyrenees is also provided by the absence of pine stomes in the Bosc del Estanyons sequence (BDE hereafter) (2200 m a.s.l.) (Miras et al., 2007) (fig. 3.3) and by the absence of wood charcoals until ca 10.5 cal ka BP in Plaus de Boldís-Montarenyo area (2000-2200 m a.s.l.) (Cunill et al., 2012). These cold conditions still recorded in the Pyrenees are in agreement 3. Estanya sequence. Climate at low altitudes 97 with the Alborán SST reconstruction curve from Cacho et al., 2001 (fig. 3.8c), supporting the role of low temperatures in both ocean and atmosphere to avoid forest development across inner Iberia during the first millennia of the Holocene. In addition, the dense juniper landscape at lowlands indicates a year-round water shortage accompanying the high seasonal contrast. Nevertheless, rainfall reconstructions during the first stages of the Holocene show a large variability: while Lake Estanya displays the lowest lake levels of the Holocene (Morellón et al., 2009b), the river discharge into the western Mediterranean seems to have reached very high values at this time, according to the highest values of the K/Al recorded in the marine sequence MD99-2343 (fig. 3.8b) by Frigola et al., 2007. Particularly dry conditions have been also recognised across the northern Mediterranean coast during the first millennium of the Holocene (Magny et al., 2013). This water shortage was responsible for a long delay on the establishment of well-developed forests across the region. The increased runoff into the Mediterranean (Frigola et al., 2007) may be a result of the scarce vegetation in the watersheds as well as of melting processes in the Late Glacial-inherited Pyrenean glaciers, rather than a direct increase in the precipitation. Hence, the absence of a forested landscape in Estanya may be a result of a combination of factors such as high continentality, low effective humidity and absence of well-developed soil, as well as in many areas of Mediterranean Iberia (Carrión et al., 2010). 1.5.2. The Early Holocene (9800-8200 cal yr BP): increasing humidity The expansion of the forest in Estanya took place at ca. 9.8 cal ka BP and was characterized by a marked increase in broad-leaf taxa (mainly Corylus) and a slight increase in Quercus species and Mediterranean shrubs (fig. 3.6). The marked shift from a continental steppe landscape, dominated by Juniperus and Artemisia, toward a wooded landscape, dominated by Corylus, suggests both more humid conditions and an increase in winter temperatures. The presence of deciduous taxa in higher altitude sequences in the Pyrenees like i.e., Paul de Bubal, El Portalet, Lake Burg, BSM, Lake Racou, BDE or Estanilles (figs. 3.1 and 3.3: references therein) is also relevant during this period, indicating the upwards treeline shift due to the occurrence of milder temperatures. In addition, both the chrysophyte cysts-based temperature anomaly curve from the Pyrenean Lake Redo (figs. 3.1, 3.3 and 3.8d) and the SST reconstruction from the Alboran Sea (fig. 3.8c), reflect the fast increase in mean annual temperatures at the western Mediterranean area, likely as a result of meaningful warmer winters. The large spread of deciduous taxa in Estanya sequence and the palaeohydrological reconstruction carried out by Morellón et al., (2009a) showing a water level rise at this moment (fig. 3.8h), indicate that the precipitation would have increased significantly. In agreement with increased water availability, in altitude, the palaeohydrological reconstruction from Lake Basa de la Mora (BSM) (fig. 3.2) highlights the occurrence of high lake levels at this moment too (Pérez-Sanz et al., 2013). Accordingly, more humid conditions 3. Estanya sequence. Climate at low altitudes 104 and higher water salinity were recorded in Estanya at this time (Morellón et al., 2011), as well as in other regional sequences (Morellón et al., 2012). Figure 3.11. Reconstructed dry conditions for the MCA from Iberian records. A) Zr/Al ratio from Algerian–Balearic basin core; B) Rb/Al ratio from Zoñar Lake; C) Sr (cps) from Arreo Lake; D) the aridity reconstruction of Estanya Lake (axis 2 from the Principal Component Analyses applied to the XRF dataset in two cores); E) the number of paleoflood events in Taravilla Lake; F) Si (cps) from Basa de la Mora Lake; G) the number of detrital layers per year from Montcortès Lake and H) Fe (cps) from the Tagus prodelta. From Moreno et al. 2012. 3. Estanya sequence. Climate at low altitudes 105 Nevertheless, apart from the last stage of this period when the landscape underwent some human-related changes, in general, the vegetation changes recorded in Estanya sequence between 4800 and 800 cal yr BP are in agreement, firstly, with the palaeohydrological reconstruction carried out in the same lake by Morellón et al., (2009b), and secondly, with changes observed in other environmental reconstructions from higher altitudes in the Pyrenees (Pérez-Sanz et al., 2011), indicating that, despite some human influence, vegetation shifts were primarily climate-driven during this period. 1.5.7. The last centuries (800-0 cal yr BP / 1150-1950 AD): crossing a threshold in landscape management Increasing landscape management in Estanya took place at 0.8 cal ka BP through the spread of grazing and farming practices (Riera et al., 2004; Morellón et al., 2011). The sharp increase in Olea and in ruderal and nitrophilous plants such as Rumex, along with the appearance of Cannabis and a sharp decline in the AP values (fig. 3.10, EST-VI), marks a threshold in the agro-pastoral activities at this moment, coinciding with the origin of the Crown of Aragon and the beginning of the Middle Ages (Carreras-Ares, 1996). This marked human-induced vegetation change has been documented in other Pyrenean sequences (Pèlachs et al., 2007; Miras et al., 2010; Ejarque et al., 2010), indicating a massive use of the forest at all levels in order to open the landscape for extensive agro-pastoral systems. However, despite the mentioned increase of human activities, some vegetation sequences also show imprints of the colder and more humid conditions that characterized the latter half of the Little Ice Age (LIA: 1500-1850 AD) in the Pyrenees (Morellón et al., 2012) (fig. 3.10). Firstly, in the lowlands, though the Estanya sequence records the highest expansion of Vitis and Cerealia type and the intensification of Cannabis and Olea cultivation at this point, it also shows an increase in mesophytes and in the aquatic component, in contrast to the drop in these taxa and the increase in Juniperus recorded during the MCA (fig. 3.6 and 3.10). This increase in water-demanded taxa coincides with a more positive water balance in the lake (Morellón et al., 2011; Morellón et al., 2012) in agreement with more humid conditions. On the other hand, pollen sequences placed at higher altitudes show a partial recovery of the forest and a decrease in cultivated taxa (Pérez-Obiol et al., 2012; Pérez-Sanz et al., 2011, 2013) indicating a decline in human pressure on the highlands. During the LIA, the Pyrenees recorded a remarkable development of glacial systems pointing out a drop in temperatures and an increase in moisture (González-Trueba et al., 2008; Chueca-Cía et al., 2005). These cold conditions could be responsible for a temporal abandonment of human activities in the highlands (Pérez-Sanz et al., 2011), which indeed intensified in the lowlands, likely further favoured by an increase in water availability. These facts prove that both humans and climate forcings have been always implied in the vegetation shifts occurred in the Pyrenees, even during the last centuries. Since the onset of the 20th century and especially during the second half of that century, it took place an important change in the social organization in Spain that led to a major 3. Estanya sequence. Climate at low altitudes 106 migration from the villages into the cities (Lasanta-Martínez et al., 2005). The Estanya pollen sequence only records de first half of the 20th century but it shows evidence of this major socio-economical shift. Agricultural-related taxa such as Olea, Cerealia type and Vitis decreased considerably since their maximum values recorded between the 17th and 19th centuries (fig. 3.10). Additionally, the AP values increased slightly in agreement with less agricultural pressure in the region. However, both nitrophilous plants and Cannabis increase (fig. 3.10) pointing out yet some important anthropogenic activity in the area. At a more regional scale, higher altitude sequences of the southern Pyrenees show a progressive increase in forest recovery (Ejarque et al., 2009; Cunill et al., 2012; Pérez-Obiol et al., 2012; Pérez-Sanz et al., 2013) indicating the spread of pine formations in the subalpine belt in agreement with reduced anthropogenic pressure also in the highlands. Nevertheless, recent works have demonstrated that the upwards migration of the treeline and the increase in tree density, that the subalpine forest of the Pyrenees have been experiencing lately, is also a result of the current Global Change and the recent rise in temperatures (Camarero and Gutiérrez, 2004; Batllori and Gutiérrez, 2008). This fact proves the fast response of the Pyrenean forests to climate changes, particularly under low-degree anthropogenic pressure, supporting the climate-driven vegetation shifts reconstructed in this chapter for the regional Holocene evolution. 1.6. CONCLUSIONS i. The synchrony and consistency of the altitudinal vegetation shifts indicates that the vegetation dynamic in the Pyrenees has been climate-driven during most part of the Holocene until approximately 0.8 cal Ka BP (1150 AD), when the anthropogenic activities caused high-degree degradation on the landscape. ii. Severe climate conditions characterized by extremely seasonal contrast and water deficit determined the presence of steppe communities in the lowlands and limited the forest development at high altitudes from the onset of the Holocene until 9.8 cal ka BP. iii. After 9.8 cal ka BP, the mesophyte communities spread in the lowlands. This expansion during a period of wet and cold winters but relative dry summers could have been favoured by high water inputs as snowmelt from the Pyrenees iv. Between 8200 and 6000 cal yr BP, an increase in winter temperatures along with a change in the regimen precipitation with a more evenly distribution of the rainfall favoured the vegetation belts to rise in altitude, leading to the establishment of a welldeveloped Mediterranean forest in the lowlands and a deciduous forest in the subalpine belt. v. A change in the precipitation pattern with a development of a dry season was responsible for the substitution of the deciduous forest of the subalpine belt by pine-dominant formations at ca. 6 cal Ka BP. The Mediterranean forest located in the lowlands was 3. Estanya sequence. Climate at low altitudes 107 hardly affected until 4.8 cal ka BP, when a trend toward drier conditions along with a moderate decrease in winter temperatures resulted in semi-deciduous Quercus in Estanya and the opening of the pine forest at higher altitudes. vi. The first signs of anthropogenic activities are recognised at ca. 3.1 cal ka BP with the appearance of Cerealia type and a likely deforestation phase. The large spread of agropastoral activities are recorded at ca 0.8 cal ka BP, when the expansion of grazing and farming-related taxa and a sharp decrease in the forest cover indicate a threshold in sustainable land management. vii. Climate has secondarily controlled altitudinal vegetation shifts during the periods of most intense anthropogenic pressure, as it has also determine the location and intensity of human activities in the lowlands and highlands. References Aguilera, M., Ferrio, J.P., Pérez, G., Araus, J.L., Voltas, J., 2012. Holocene changes in precipitation seasonality in the western Mediterranean Basin: a multi-species approach using δ13C of archaeobotanical remains. J. Quat. Sci. 27, 192–202. Anderson, R.S., Jiménez-Moreno, G., Carrión, J.S., Pérez-Martínez, C., 2011. Postglacial history of alpine vegetation, fire, and climate from Laguna de Río Seco, Sierra Nevada, southern Spain. Quat. Sci. Rev. 30, 1615–1629. Aranbarri, J., González-Sampériz, P., Valero-Garcés, B., Moreno, A., Gil-Romera, G., Sevilla-Callejo, M., García-Prieto, E., Di Rita, F., Mata, M.P., Morellón, M., Magri, D., Rodríguez-Lázaro, J., Carrión, J.S., 2014. Rapid climatic changes and resilient vegetation during the Lateglacial and Holocene in a continental region of south-western Europe. Glob. Planet. Change 114, 50–65. Aubert, S., Belet, J.-M., Bouchette, A., Otto, T., Dedoubat, J.-J., Fontugne, M., Jalut, G., 2004. Dynamique tardiglaciaire et holocène de la végétation à l’étage montagnard dans les Pyrénées centrales. C. R. Biol. 327, 381–388. Batllori, E., Gutiérrez, E., 2008. Regional tree line dynamics in response to global change in the Pyrenees. J. Ecol. 96, 1275–1288. Benito, G., Sopeña, A., Sánchez-Moya, Y., Machado, M.J., Pérez-González, A., 2003. Palaeoflood record of the Tagus River (Central Spain) during the Late Pleistocene and Holocene. Quat. Sci. Rev. 22, 1737–1756. Blanco Castro, E., 2005. Los Bosques ib Cacho, I., Grimalt, J.O., Canals, M., Sbaffi, L., Shackleton, N.J., Schönfeld, J., Zahn, R., 2001. Variability of the western Mediterranean Sea surface temperature during the last 25,000 years and its connection with the Northern Hemisphere climatic changes. Paleoceanography 16, 40–52. Camarero, J.J., Gutiérrez, E., 2004. Pace and Pattern of Recent Treeline Dynamics: Response of Ecotones to Climatic Variability in the Spanish Pyrenees. Clim. Change 63, 181–200. Carreras-Ares, J.J., 1996. Historia de Aragón II.: economía y sociedad: resumen de las lecciones impartidas en los cursos 1987-88 y 1988-89. Institución Fernando el Católico, Zaragoza. Carrión, J.S., 2002. Patterns and processes of Late Quaternary environmental change in a montane region of southwestern Europe. Quat. Sci. Rev. 21, 2047–2066. Carrión, J.S., Fernández, S., González-Sampériz, P., Gil-Romera, G., Badal, E., Carrión-Marco, Y., LópezMerino, L., López-Sáez, J.A., Fierro, E., Burjachs, F., 2010. Expected trends and surprises in the Lateglacial and Holocene vegetation history of the Iberian Peninsula and Balearic Islands. Rev. Palaeobot. Palynol. 162, 458–475. 3. Estanya sequence. Climate at low altitudes 108 Chueca-Cía, J., Julián-Andrés, A., Saz-Sánchez, M.A., Creus-Novau, J., López-Moreno, J.I., 2005. Responses to climatic changes since the Little Ice Age on Maladeta Glacier (Central Pyrenees). Geomorphology 68, 167–182. Corella, J.P., Moreno, A., Morellón, M., Rull, V., Giralt, S., Rico, M.T., Pérez-Sanz, A., Valero-Garcés, B.L., 2010. Climate and human impact on a meromictic lake during the last 6,000 years (Montcortès Lake, Central Pyrenees, Spain). J. Paleolimnol. 46, 351–367. Cunill, R., Soriano, J.-M., Bal, M.-C., Pèlachs, A., Pérez-Obiol, R., 2012. Holocene treeline changes on the south slope of the Pyrenees: a pedoanthracological analysis. Veg. Hist. Archaeobotany 21, 373–384. Davis, B.A.S., Stevenson, A.C., 2007. The 8.2 ka event and Early–Mid Holocene forests, fires and flooding in the Central Ebro Desert, NE Spain. Quat. Sci. Rev. 26, 1695–1712. De Beaulieu, J.L., Miras, Y., Andrieu-Ponel, V., Guiter, F., 2005. Vegetation dynamics in north-western Mediterranean regions: Instability of the Mediterranean bioclimate. Plant Biosyst. - Int. J. Deal. Asp. Plant Biol. 139, 114–126. - -Cabeza,, J., 2003. Dupré, M., 1992. Palinología. Sociedad Española de Geomorfología. Ejarque, A., Julià, R., Riera, S., Palet, J.M., Orengo, H.A., Miras, Y., Gascón, C., 2009. Tracing the history of highland human management in the eastern Pre-Pyrenees: an interdisciplinary palaeoenvironmental study at the Pradell fen, Spain. The Holocene 19, 1241 –1255. Ejarque, A., Miras, Y., Riera, S., Palet, J.M., Orengo, H.A., 2010. Testing micro-regional variability in the Holocene shaping of high mountain cultural landscapes: a palaeoenvironmental case-study in the eastern Pyrenees. J. Archaeol. Sci. 37, 1468–1479. Fernández, S., Fuentes, N., Carrión, J.S., González-Sampériz, P., Montoya, E., Gil, G., Vega-Toscano, G., Riquelme, J.A., 2007. The Holocene and Upper Pleistocene pollen sequence of Carihuela Cave, southern Spain. Geobios 40, 75–90. Franco Múgica, F., García Antón, M., Maldonado Ruiz, J., Morla Juaristi, C., Sainz Ollero, H., 2001. The Holocene history of Pinus forests in the Spanish Northern Meseta. The Holocene 11, 343–358. Frigola, J., Moreno, A., Cacho, I., Canals, M., Sierro, F.J., Flores, J.A., Grimalt, J.O., Hodell, D.A., Curtis, J.H., 2007. Holocene climate variability in the western Mediterranean region from a deepwater sediment record. Paleoceanography 22. Gaetani, M., Pohl, B., Douville, H., Fontaine, B., 2011. West African Monsoon influence on the summer Euro-Atlantic circulation. Geophys. Res. Lett., L09705 38, n/a–n/a. Giorgi, F., Lionello, P., 2008. Climate change projections for the Mediterranean region. Glob. Planet. Change 63, 90–104. González-Sampériz, P., Valero-Garcés, B.L., Moreno, A., Jalut, G., García-Ruiz, J.M., Martí-Bono, C., Delgado-Huertas, A., Navas, A., Otto, T., Dedoubat, J.J., 2006. Climate variability in the Spanish Pyrenees during the last 30,000 yr revealed by the El Portalet sequence. Quat. Res. 66, 38–52. González-Sampériz, P., Valero-Garcés, B.L., Moreno, A., Morellón, M., Navas, A., Machín, J., DelgadoHuertas, A., 2008. Vegetation changes and hydrological fluctuations in the Central Ebro Basin (NE Spain) since the Late Glacial period: Saline lake records. Palaeogeogr. Palaeoclimatol. Palaeoecol. 259, 157–181. González-Trueba, J.J., Moreno, R.M., Martinez de Pison, E., Serrano, E., 2008. `Little Ice Age’ glaciation and current glaciers in the Iberian Peninsula. The Holocene 18, 551–568. Gottfried, M., Pauli, H., Futschik, A., Akhalkatsi, M., Barančok, P., Benito Alonso, J.L., Coldea, G., Dick, J., Erschbamer, B., Fernández Calzado, M.R., Kazakis, G., Krajči, J., Larsson, P., Mallaun, M., Michelsen, O., Moiseev, D., Moiseev, P., Molau, U., Merzouki, A., Nagy, L., Nakhutsrishvili, G., Pedersen, B., Pelino, G., Puscas, M., Rossi, G., Stanisci, A., Theurillat, J.-P., Tomaselli, M., Villar, L., Vittoz, P., Vogiatzakis, I., Grabherr, G., 2012. Continent-wide response of mountain vegetation to climate change. Nat. Clim. Change 2, 111–115. Guiter, F., Andrieu-Ponel, V., Digerfeldt, G., Reille, M., Beaulieu, J.-L., Ponel, P., 2005. Vegetation history and lake-level changes from the Younger Dryas to the present in Eastern Pyrenees 3. Estanya sequence. Climate at low altitudes 109 (France): pollen, plant macrofossils and lithostratigraphy from Lake Racou (2000 m a.s.l.). Veg. Hist. Archaeobotany 14, 99–118. Harrison, S.P., Digerfeldt, G., 1993. European lakes as palaeohydrological and palaeoclimatic indicators. Quat. Sci. Rev. 12, 233–248. Hoerling, M., Eischeid, J., Perlwitz, J., Quan, X., Zhang, T., Pegion, P., 2012. On the Increased Frequency of Mediterranean Drought. J. Clim. 25, 2146–2161. Jalut, G., Esteban Amat, A., Bonnet, L., Gauquelin, T., Fontugne, M., 2000. Holocene climatic changes in the Western Mediterranean, from south-east France to south-east Spain. Palaeogeogr. Palaeoclimatol. Palaeoecol. 160, 255–290. Kutzbach, J.., Webb, T., 1993. Conceptual basis for understanding late-Quaternay climatess, in: Global Climates since the Last Glacial Maximum. University of Minnesota Press. Lionello, P., 2012. The climate of the Mediterranean Region from the past to the future. Elsevier Science, Burlington. López-Merino, L., Cortizas, A.M., López-Sáez, J.A., 2010. Early agriculture and palaeoenvironmental history in the North of the Iberian Peninsula: a multi-proxy analysis of the Monte Areo mire (Asturias, Spain). J. Archaeol. Sci. 37, 1978–1988. Luterbacher, J., Xoplaki, E., Casty, C., Wanner, H., Pauling, A., Kuttel, M., Rutishauser, T., Bronnimann, S., Fischer, E., Fleitmann, D., 2006. Chapter 1 Mediterranean climate variability over the last centuries: A review, in: Developments in Earth and Environmental Sciences. Elsevier, pp. 27– 148. Magny, M., Combourieu-Nebout, N., de Beaulieu, J.L., Bout-Roumazeilles, V., Colombaroli, D., Desprat, S., Francke, A., Joannin, S., Ortu, E., Peyron, O., Revel, M., Sadori, L., Siani, G., Sicre, M.A., Samartin, S., Simonneau, A., Tinner, W., Vannière, B., Wagner, B., Zanchetta, G., Anselmetti, F., Brugiapaglia, E., Chapron, E., Debret, M., Desmet, M., Didier, J., Essallami, L., Galop, D., Gilli, A., Haas, J.N., Kallel, N., Millet, L., Stock, A., Turon, J.L., Wirth, S., 2013. North-south palaeohydrological contrasts in the central Mediterranean during the Holocene: tentative synthesis and working hypotheses. Clim. Past 9, 2043–2071. Magny, M., Miramont, C., Sivan, O., 2002. Assessment of the impact of climate and anthropogenic factors on Holocene Mediterranean vegetation in Europe on the basis of palaeohydrological records. Palaeogeogr. Palaeoclimatol. Palaeoecol. 186, 47–59. Marcott, S.A., Shakun, J.D., Clark, P.U., Mix, A.C., 2013. A Reconstruction of Regional and Global Temperature for the Past 11,300 Years. Science 339, 1198–1201. Mayewski, P.A., 2004. Holocene climate variability. Quat. Res. 62, 243–255. Meehl, G.A., Stocker, T.F., Collins, W.D., Friedlingstein, P., Gaye, A.T., Gregory, J.M., Kitoh, A., Knutti, R., Murphy, J.M., Nodas, A., Raper, S.C.B., Watterson, I.G., Weaver, A.J., Zhao, Z.C., 2007. Global Climate Projections, in: Solomon, S., Quin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K.B., Tignor, M., Miller, H.L. (Eds.), Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, UK and New York, USA. Miras, Y., Ejarque, A., Orengo, H., Mora, S.R., Palet, J.M., Poiraud, A., 2010. Prehistoric impact on landscape and vegetation at high altitudes: An integrated palaeoecological and archaeological approach in the eastern Pyrenees (Perafita valley, Andorra). Plant Biosyst. - Int. J. Deal. Asp. Plant Biol. 144, 924–939. Miras, Y., Ejarque, A., Riera, S., Palet, J.M., Orengo, H., Eubab, I., 2007. Dynamique holocène de la végétation et occupation des Pyrénées andorranes depuis le Néolithique ancien, d’après l’analyse pollinique de la tourbière de Bosc dels Estanyons (2180 m, Vall del Madriu, Andorre). Comptes Rendus Palevol 6, 291–300. Montserrat-Martí, J., 1992. Evolución glaciar y postglaciar del clima y la vegetación en la vertiente sur del Pirineo: estudio palinológico., Monografías del Instituto Pirenaico de Ecología-CSIC. Zaragoza. Moore, P.D., Webb, J.A., Collinson, M.E., 1991. Pollen Analysis, Second. ed. Blackwell Scientifc Publications. 3. Estanya sequence. Climate at low altitudes 110 Morales-Molino, C., García-Antón, M., 2013. Vegetation and fire history since the last glacial maximum in an inland area of the western Mediterranean Basin (Northern Iberian Plateau, NW Spain). Quat. Res. Morellón, M., Pérez-Sanz, A., Corella, J.P., Büntgen, U., Catalán, J., González-Sampériz, P., GonzálezTrueba, J.J., López-Sáez, J.A., Moreno, A., Pla-Rabes, S., Saz-Sánchez, M. á., Scussolini, P., Serrano, E., Steinhilber, F., Stefanova, V., Vegas-Vilarrúbia, T., Valero-Garcés, B., 2012. A multiproxy perspective on millennium-long climate variability in the Southern Pyrenees. Clim. Past 8, 683–700. Morellón, M., Valero-Garcés, B., Anselmetti, F., Ariztegui, D., Schnellmann, M., Moreno, A., Mata, P., Rico, M., Corella, J.P., 2009a. Late Quaternary deposition and facies model for karstic Lake Estanya (North-eastern Spain). Sedimentology 56, 1505–1534. Morellón, M., Valero-Garcés, B., González-Sampériz, P., Vegas-Vilarrúbia, T., Rubio, E., Rieradevall, M., Delgado-Huertas, A., Mata, P., Romero, Ó., Engstrom, D.R., López-Vicente, M., Navas, A., Soto, J., 2011. Climate changes and human activities recorded in the sediments of Lake Estanya (NE Spain) during the Medieval Warm Period and Little Ice Age. J. Paleolimnol. 46, 423–452. Morellón, M., Valero-Garcés, B., Vegas-Vilarrúbia, T., González-Sampériz, P., Romero, Ó., DelgadoHuertas, A., Mata, P., Moreno, A., Rico, M., Corella, J.P., 2009b. Lateglacial and Holocene palaeohydrology in the western Mediterranean region: The Lake Estanya record (NE Spain). Quat. Sci. Rev. 28, 2582–2599. Moreno, A., López-Merino, L., Leira, M., Marco-Barba, J., González-Sampériz, P., Valero-Garcés, B.L., López-Sáez, J.A., Santos, L., Mata, P., Ito, E., 2011. Revealing the last 13,500 years of environmental history from the multiproxy record of a mountain lake (Lago Enol, northern Iberian Peninsula). J. Paleolimnol. 46, 327–349. Moreno, A., Pérez, A., Frigola, J., Nieto-Moreno, V., Rodrigo-Gámiz, M., Martrat, B., González-Sampériz, P., Morellón, M., Martín-Puertas, C., Corella, J.P., Belmonte, Á., Sancho, C., Cacho, I., Herrera, G., Canals, M., Grimalt, J.O., Jiménez-Espejo, F., Martínez-Ruiz, F., Vegas-Vilarrúbia, T., ValeroGarcés, B.L., 2012. The Medieval Climate Anomaly in the Iberian Peninsula reconstructed from marine and lake records. Quat. Sci. Rev. 43, 16–32. Nikulin, G., KjellströM, E., Hansson, U., Strandberg, G., Ullerstig, A., 2011. Evaluation and future projections of temperature, precipitation and wind extremes over Europe in an ensemble of regional climate simulations. Tellus A 63, 41–55. Ninot, J.M., Carrillo, E., Font, X., Carreras, J., Ferré, A., Masalles, R.M., Soriano, I., Vigo, J., 2007. Altitude zonation in the Pyrenees. A geobotanic interpretation. Phytocoenologia 37, 371–398. Pèlachs, A., Soriano, J.M., Nadal, J., Esteban, A., 2007. Holocene environmental history and human impact in the Pyrenees. Contrib. Sci. 3, 421–429. Pérez-Obiol, R., Bal, M.-C., Pèlachs, A., Cunill, R., Soriano, J.M., 2012. Vegetation dynamics and anthropogenically forced changes in the Estanilles peat bog (southern Pyrenees) during the last seven millennia. Veg. Hist. Archaeobotany 21, 385–396. Pérez-Obiol, R., Jalut, G., Julia, R., Pelachs, A., Iriarte, M.J., Otto, T., Hernandez-Beloqui, B., 2011. MidHolocene vegetation and climatic history of the Iberian Peninsula. The Holocene 21, 75–93. Pérez-Obiol, R., Juliá, R., 1994. Climatic Change on the Iberian Peninsula Recorded in a 30,000-Yr Pollen Record from Lake Banyoles. Quat. Res. 41, 91–98. Pérez-Sanz, A., González-Sampériz, P., Moreno, A., Valero-Garcés, B., Gil-Romera, G., Rieradevall, M., Tarrats, P., Lasheras-Álvarez, L., Morellón, M., Belmonte, A., Sancho, C., Sevilla-Callejo, M., Navas, A., 2013. Holocene climate variability, vegetation dynamics and fire regime in the central Pyrenees: the Basa de la Mora sequence (NE Spain). Quat. Sci. Rev. 73, 149–169. Pla, S., Catalan, J., 2005. Chrysophyte cysts from lake sediments reveal the submillennial winter/spring climate variability in the northwestern Mediterranean region throughout the Holocene. Clim. Dyn. 24, 263–278. Raicich, F., Pinardi, N., Navarra, A., 2003. Teleconnections between Indian monsoon and Sahel rainfall and the Mediterranean. Int. J. Climatol. 23, 173–186. Reimer, P.J., Baillie, M.G.L., Bard, E., Bayliss, A., Beck, J.W., Blackwell, P.G., Bronk Ramsey, C., Buck, C.E., Burr, G.S., Edwards, R.L., Friedrich, M., Grootes, P.M., Guilderson, T.P., Hajdas, I., Heaton, T.J., Hogg, A.G., Hughen, K.A., Kaiser, K.F., Kromer, B., McCormac, F.G., Manning, S.V., Reimer, 3. Estanya sequence. Climate at low altitudes 111 R.W., Richards, D.A., Southon, J.R., Talamo, S., Turney, C.S.M., van der Plicht, J., Weyhenmeyer, C.E., 2009. INTCAL09 and MARINE09 radiocarbon age calibration curves, 050,000 years cal. BP. Radiocarbon 51, 1111–1150. Riera, S., Wansard, G., Julià, R., 2004. 2000-year environmental history of a karstic lake in the Mediterranean Pre-Pyrenees: the Estanya lakes (Spain). CATENA 55, 293–324. Rodwell, M.J., Hoskins, B.J., 2001. Subtropical Anticyclones and Summer Monsoons. J. Clim. 14, 3192– 3211. Rull, V., González-Sampériz, P., Corella, J.P., Morellón, M., Giralt, S., 2011. Vegetation changes in the southern Pyrenean flank during the last millennium in relation to climate and human activities: the Montcortès lacustrine record. J. Paleolimnol. 46, 387–404. Sánchez Goñi, M.F., Hannon, G.E., 1999. High-altitude vegetational pattern on the Iberian Mountain Chain (north-central Spain) during the Holocene. The Holocene 9, 39–57. Stockmarr, J., 1971. Tablets with spores used in absolute pollen analysis. Pollen Spores 13, 614–621. Tarrats, P., Rieradevall, M., González-Sampériz, P., Pérez-Sanz, A., Valero-Garcés, B., Moreno, A., 2014. Relating actual with subfossil chironomid assemblages. Holocene habitat changes and paleoenvironmental reconstruction of Basa de la Mora Lake (Central Pyrenees). Presented at the Erupean Geoscience Union, Vienna. Valero-Garcés, B.L., Navas, A., Machin, J., Stevenson, T., Davis, B., 2000. Responses of a Saline Lake Ecosystem in a Semiarid Region to Irrigation and Climate Variability. The History of Salada Chiprana, Central Ebro Basin, Spain. Ambio 29, 344–350. Vegas, J., Ruiz-Zapata, B., Ortiz, J.E., Galán, L., Torres, T., García-Cortés, Á., Gil-García, M.J., PérezGonzález, A., Gallardo-Millán, J.L., 2010. Identification of arid phases during the last 50 cal. ka BP from the Fuentillejo maar-lacustrine record (Campo de Calatrava Volcanic Field, Spain). J. Quat. Sci. 25, 1051–1062. Vegas-Vilarrúbia, T., González-Sampériz, P., Morellón, M., Gil-Romera, G., Pérez-Sanz, A., ValeroGarcés, B., 2013. Diatom and vegetation responses to Late Glacial and Early Holocene climate changes at Lake Estanya (Southern Pyrenees, NE Spain). Palaeogeogr. Palaeoclimatol. Palaeoecol. 392, 335–349. Xoplaki, E., González-Rouco, F., Luter, J., Wanner, H., 2003. Mediterranean summer air temperature variability and its connection to the large-scale atmospheric circulation and SSTs. Clim. Dyn. 20, 723–739. 3. Estanya sequence. Climate at low altitudes 112 APPENDIX Figure A3.1. Arboreal pollen from Lake Estaña sequence. 3. Estanya sequence. Climate at low altitudes 113 Figure A3.2. Non-Arboreal pollen from Lake Estaña sequence. 4. Climate models 120 reflection of generally more humid conditions that would suppress fires in the already forested lowlands but allow them to increase as forests expanded into higher elevation areas (Vannière et al., 2011). The changes in climate were spatially complex (Roberts et al., 2011), but pollen-based climate reconstructions (e.g. Cheddadi et al., 1997; Davis et al., 2003; Bartlein et al., 2011) show that most of the Mediterranean region was characterized by an increase in plant-available moisture. These regional palaeo-climate data from the Mediterranean area during the Mid-Holocene are in agreement, as we have described before, with the results obtained in the present PhD dissertation from the BSM and EST sequences (chapters 2 and 3 respectively). According to the palaeo-environmental reconstructions accomplished in the previous chapters, the MidHolocene in the Pyrenees was a period of particularly mild climate conditions characterized by warm winters and humid conditions that lead to the establishment of a Mediterraneanforest type dominated by semi-deciduous Quercus in the lowlands (EST sequence, chapter 3), and a well-developed deciduous forest dominated by Betula, Corylus and deciduous Quercus in the highlands (BSM sequence, chapter 2), reaching up to the treeline (fig.4.1). Figure 4.1. Vegetation and lake level reconstructions for lakes Estanya and Basa de la Mora. The Mid Holocene is highlighted in yellow. The lake level reconstruction for Lake Estaña has been published by Morellón et al. (2009). 4. Climate models 121 Such changes in precipitation and temperature compared to today require large-scale shifts in atmospheric and oceanic patterns. Nevertheless, though palaeo-climate data allow us to know the evolution of the climate in the past, the mechanisms beyond those changes cannot be easily deduced. To this purpose, climate models attempt to simulate past climates mechanisms based on prescribed Earth’s physico-chemical parameters such as solar insolation, orbital configuration, atmosphere composition, ocean temperature or ice-sea extent among others. Considering climate features in the Mediterranean region during the Mid-Holocene are relatively well-know at a regional scale, this period provides an opportunity to examine climate-model performance. Systematic comparisons with observations have shown that global climate models are unable to reproduce the observed MH patterns of rainfall changes in the Mediterranean. In particular, they do not show a sufficiently large increase in summer rainfall to explain the shift towards deciduous vegetation. This was identified as a problem in atmosphere-only simulations of the mid-Holocene made during the first phase of the Palaeoclimate Modelling Intercomparison Project (PMIP1: see e.g. Masson et al., 1999; Guiot et al., 1999; Bonfils et al., 2004). Coupled ocean-atmosphere simulations made during PMIP2 were able to simulate the types of climate changes seen in the Mediterranean, but the geographic placement of these climate types, the spatial extent and the magnitude of the changes were not well captured (Brewer et al., 2007). In particular, the simulated changes in precipitation are small and insufficient to explain the observed expansion of deciduous forests in the region. The Mediterranean climate involves a complex interaction between different processes acting at several different spatio-temporal scales (Xoplaki et al., 2003; Luterbacher et al., 2006; CLIVAR, 2010; Lionello, 2012). However, interannual variability in Mediterranean summer precipitation is linked to variability in the strength of the Afro-Asian monsoon system (Rodwell and Hoskins, 2001; Raicich et al. 2003; Gaetani et al 2011). Analyses of climate model simulations of the present day suggest that Mediterranean summer precipitation is suppressed during years when the Afro-Asian monsoon system is strong. This results from intensification of the Hadley cell and enhanced subsidence in the subtropics (i.e. strengthening of the Azores High), leading to high pressure over the eastern Mediterranean which results in decreased rainfall (Gaetani et al., 2011). However, when monsoon intensification is accompanied by northward movement of the intertropical convergence zone, as model simulations indicate occurred in the mid-Holocene (Braconnot et al., 2007a; Marzin and Braconnot, 2009), the Azores high is also displaced northeastward and weakened (e.g. Harrison et al., 1992). This has been shown to have a significant impact on precipitation in the eastern North America (Forman et al., 1995; van Soelen et al., 2012) and could potentially lead to increased summer rainfall in the Mediterranean region. The PMIP2 simulations show a significant enhancement and northward expansion of the African monsoon during the Mid-Holocene in response to changes in insolation forcing (Braconnot et al. 2007a). However, comparisons with pollen-based estimates of the change 4. Climate models 122 in mean annual precipitation (Joussaume et al., 1999; Bartlein et al., 2011) show that the models underestimate the increase in precipitation by between 20 and 50% (Braconnot et al. 2007a; Braconnot et al., 2012). Most models fail to produce a sufficient northward expansion of the monsoon. This underestimation of monsoon expansion is also present in the CMIP5 MH simulations (see e.g. Harrison et al., 2013). It is possible that this bias in the simulation of the African monsoon is linked to the failure to simulate the MH Mediterranean climate accurately, since larger shifts in the position of the monsoon are produced by models incorporating land-surface feedbacks and/or with higher spatial resolution (Levis et al., 2004; Wohlfahrt et al., 2004; Bosmans et al., 2012). MH model simulations, made with the same models that are used for future projections, have been made as part of the fifth phase of the Coupled Model Intercomparison Project (CMIP5: Taylor et al., 2012) and are being analysed as part of the third phase of the Palaeoclimate Modelling Intercomparison Project (PMIP3: Bracconnot et al., 2012). Kelley et al. (2012) have shown that the simulation of the seasonal cycle of precipitation in the Mediterranean region under modern conditions is reasonable, although as in earlier versions of the models the amplitude of the cycle is more muted than observed with too little rain in winter and too much rain in summer (Brands et al., 2013). However, evaluation of CMIP5 model performance against modern observations suggests that some aspects of the simulation of the Afro-Asian monsoons (see e.g. Monerie et al., 2012; Roehrig et al., 2013; Sperber et al., 2012) are improved compared to earlier versions of the models, although preliminary assessments of the CMIP5 model indicate that improvements in the modern simulations do not translate into improvements in the simulation of the MH monsoon climate (Harrison et al., 2013), and thus, given the dynamic links between the monsoon and Mediterranean precipitation, in MH Mediterranean climate changes. 4.1.1. Objectives In this chapter, we examine the performance of the CMIP5 models for modern and MH climates, and compare the simulated climates with modern and palaeo-observations from Bartlein et al., (2011) data base. This allows us to assess whether biases in the control simulations influence the MH simulations and to investigate whether regional biases in the simulation of MH monsoon changes influence model performance in the Mediterranean. 4.2. METHODS We present analyses of the pre-Industrial (piControl) and MH (midHolocene) made by 12 coupled ocean-atmosphere models from the fifth phase of the Coupled Climate Modelling Intercomparison Project (CMIP5). In order to investigate whether biases in the control simulation influence the realism of the midHolocene climates, we first evaluate the piControl 4. Climate models 123 simulation. We use modern observations from the CRU TS3.1 data set, in the absence of climate reconstructions from northern African for the piControl interval. The piControl simulation is driven by boundary conditions appropriate for 1850 AD, but comparisons with a subset of transient historical simulations show that the spatial patterns and magnitudes of seasonal climates are very similar. In order to evaluate whether models capture the spatial expression of specific seasonal patterns, we define a number of climate types using the modern observations and apply these definitions to delimit these climate types in the piControl and midHolocene simulations. We evaluate the midHolocene simulations using quantitative climate reconstructions derived from a global data base of pollen records which still not include the BSM and EST data presented in this Thesis. Although there are many kinds of palaeorecord that indicate that northern Africa and the circum-Mediterranean region were wetter during the mid-Holocene, including e.g. lake-level and archaeological records, these other sources of information do not provide quantitative estimates of the change in precipitation. Comparisons of simulated and observed climates are based on the simulated precipitation both within climate zones and within geographic zones. 4.2.1. Data sources: CMIP5 simulations We examine precipitation changes between a MH (midHolocene, 6000 yr B.P) equilibrium simulation and a control simulation representing pre-industrial conditions (piControl) using 12 models from the fifth phase of the Coupled Modelling Intercomparison Project (CMIP5). Both the midHolocene and piControl are equilibrium simulations. We use the midHolocene and piControl simulations in the CMIP5 (http://cmip-pcmdi.llnl.gov/cmip5/data_portal.html) archive as of 15th August 2012 (table 4.1). Seven of these simulations are made with oceanatmosphere (OA) models, and the other 5 models include an interactive carbon cycle (OAC). The piControl simulation has boundary conditions (insolation, greenhouse gas concentrations) appropriate for 1850 CE. The midHolocene experiment shows the response to changes in the seasonal and latitudinal distribution in insolation 6000 years ago; greenhouse gas concentrations are set at piControl levels (for details of the experimental protocol see Taylor et al., 2012; Braconnot et al., 2012). To assess whether the piControl state differs from recent observed climates, we used outputs from a historical simulation (historical: 1850 to 2005 CE) available for 6 of the models. The historical simulation is forced by time-varying changes in solar, volcanic, and greenhouse gases (Taylor et al., 2012; Braconnot et al., 2012). The output from each model was interpolated to a common grid (0.5 ) using bilinear interpolation to facilitate comparisons and the calculation of zonal averages. Long-term mean monthly, seasonal, and annual precipitation values were obtained by averaging the last 100 years of the piControl and midHolocene simulations, except in the case of HADGEM2-CC where only 35 years of midHolocene simulated outputs are available. Long-term means of 4. Climate models 124 the six historical simulations were obtained by averaging the last 30 years of each simulation. All averages were areally-weighted (by the area of the model grid cells). 4.2.2. Data sources: Modern and mid-Holocene climate data Observations of the modern climate are taken from the CRU TS3.1 data set (Harris et al., 2013), which provides monthly precipitation values on a 0.5º grid for the interval 1850 to 2006. We have created a monthly precipitation climatology using data from January 1961 through to December 1990. Zonal averages are constructed by areally-weighting the gridded values. Bartlein et al. (2011) provide quantitative reconstructions of mean annual precipitation (MAP), expressed as anomalies from the present, from a global data base of pollen and plant macrofossil records. The original site-based reconstructions were averaged to provide gridded values on a 2x2º grid, and differences between the site reconstructions within each grid were used to provide an estimate of reconstruction uncertainty (as a pooled estimate of the standard error). The data set provides mid-Holocene estimates of MAP anomalies for 62 cells (out of a possible 397 cells) within the area of interest (latitude: 0ºN-45ºN, longitude: 20ºW-30ºE). Model name Type Resolution (number of gridcells: latitude, longitude) Year length Simulations Atmosph Ocean Sea Ice Mid Holocene PI Control hist BCC-CSM1-1 OAC 64, 128 232, 360 232, 360 365 X X CCSM4 OA 192, 288 320384 320384 365 X X X CNRM-CM5 OA 128, 256 292, 362 292, 362 365-366 X X CSIRO-Mk3-6-0 OA 96, 192 189, 192 96, 192 365 X X CSIRO-Mk3L-1-2 OA 56, 64 128, 225 56, 64 365 X X GISS-E2-R OA 90, 144 90, 144 90, 144 365 X X X HadGEM2-CC OAC 145, 192 216, 360 216, 360 360 X X HadGEM2-ES OAC 145, 192 216, 360 216, 360 360 X X IPSL-CM5A-LR OAC 96, 96 149, 182 149, 182 365 X X X MIROC-ESM OAC 64, 128 192, 256 192, 256 365 X X X MPI-ESM-P OA 96, 192 220, 256 220, 256 365-366 X X X MRI-CGCM3 OA 160, 320 360, 368 360, 368 365 X X X Table 4.1. Characteristics of the CMIP5 models used in these analyses. 4. Climate models 125 4.2.3. Definition of climate regions Precipitation regimes can be characterized by a combination of the form of the seasonal cycle, seasonal concentration, and magnitude. We determined these characteristics of modern precipitation (using the CRU TS3.1 data set) for zonally averaged 5º latitude bands between 0 and 45ºN. The seasonal cycle of precipitation in each 5º latitude band was characterized according to the number of distinct rainfall peaks present in the 12-month precipitation climatology, using the R package “pastecs” to determine whether there was a significant ‘pit’ or ‘peak’ in any month. A pit or peak is considered significant if the probability of turning points occurring in a random series is <0.05, given by: () where n is the number of observations at time t (Ibanez, 1982). We calculated the total precipitation in each season (spring: March, April, May; summer: June, July, August; autumn: September, October, November; winter: December, January, February) and for the whole year. A measure of seasonal concentration was calculated following Kelley et al. (2013), where the magnitude of precipitation in each month is represented by the length of a vector in the complex plane and the direction of the vector represents the timing (with January set to 0º). The length of the mean vector divided by the annual precipitation provides an index of seasonal concentration (C), where C is 1 when the precipitation is concentrated in a single month and 0 when it is evenly distributed throughout the year. We applied these definitions to determine the position of different precipitation regimes in the piControl and midHolocene simulations. Comparison of the observed limits and those identified in the piControl allows us to examine (a) whether the models produce these distinctive precipitation regimes and (b) how well they simulate their placement independently of whether they simulate the correct magnitude of precipitation. Comparison of the piControl and midHolocene limits allows us to characterize shifts in precipitation regimes, again independent of changes in precipitation magnitude. 4.2.4. Analyses of the Model Simulations We evaluate model performance for piControl simulation in two steps. First we examine whether the models reproduce the spatial extent of different precipitation regimes, and then we examine whether they reproduce the magnitude of total annual and of seasonal precipitation. Long-term means for the period 1961-1990 from the CRU TS3.1 data set (Harris et al., 2013) are compared with long-term averages for the last 100 years of the piControl. The standard deviation (SD) of the observations provides a measure of the significance of the difference between observations and simulations. We examine the 4. Climate models 126 differences between simulated and observed climate for the latitude band corresponding to a given precipitation regime in the observations. We also compare the differences in the amount of precipitation for the geographic region identified as falling within a specific precipitation regime in each model, which may be less/more extensive than the region identified in the observations. We also examine the change in precipitation in the mid-Holocene in two steps. First we identify the spatial extent of each precipitation regime in the midHolocene simulations and compare this with the spatial extent shown in the piControl simulation of the same model. This allows us to identify whether there have been shifts in the precipitation regimes. We then examine the magnitude of the precipitation change in the latitude band characterized by a specific regime in both the piControl and the midHolocene simulations for each model. This allows us to identify whether there has been a change in precipitation in situ. We use the standard deviation of the piControl simulation for each model to determine whether the change between midHolocene and piControl is significant. We examine whether the biases in simulated precipitation (both the bias in spatial extent of a given precipitation regime and the bias in the magnitude of the simulated precipitation) influence the simulated change in precipitation between piControl and midHolocene. The bias and anomaly values have been obtained firstly for discrete geographical zones (the zones characterized by different rainfall regimes today, as defined from the CRU data set) and secondly for the model-defined regions of these different rainfall regimes (e.g. the region where the simulated rainfall is of the monsoon type). We use linear regression to examine the relationship between precipitation biases and anomalies for all models, and for the OA and OAC classes of models. The realism of the simulated change in precipitation (midHolocene-piControl) is assessed by comparing with reconstructions of mean annual precipitation (MAP) from the Bartlein et al. (2011) data set. Comparisons are made by averaging the simulated precipitation for the grid cells where there are observations within each 5º latitude band. There are sufficient data in most of the 5º latitude bands to make robust comparisons. 4.3. RESULTS 4.3.1. Modern observed climate The modern climate of the region can be divided into four distinct latitudinal zones, differentiated by marked differences in the seasonal distribution and amount of rainfall (fig. 4.2). In the south, the equatorial band is characterized by high rainfall (~1800 mm) throughout the year (fig. 4.3) but with peaks in precipitation in spring (~ 460 mm) and autumn (~600 mm) and less rainfall in summer. This pattern reflects the seasonal migration 4. Climate models 127 north and south of the inter-tropical convergence zone. The “double-peak” rainfall pattern (hereafter DP) occurs between 0 to 5ºN. The region further north (5-20 ºN) is characterized by summer monsoonal rainfall and dry winters. The amount of rainfall declines progressively from ca 650 mm in summer (June, July, August) in the south to less than 100 mm in the north. The desert area (20-30ºN) is characterized by low rainfall (<100 mm/yr). There is no pronounced seasonal differentiation in rainfall in the desert, although southern regions tend to have slightly more rain in summer than winter and northern regions slightly more rainfall in winter than summer. The Mediterranean zone (30-45ºN) is characterized by higher rainfall, increasing from 200 mm/year in the south band to 780 mm/year in the north. The rainfall is concentrated in the winter half-year, with a pronounced summer drought. Figure 4.2: Observed seasonal cycle of precipitation in each of the defined climate zones, using the CRU T3.1 data set (Harris et al., 2013). The mean precipitation each month (mm) is shown by the black line, with the standard deviation shown by the bars. The grey shading shows the maximum and minimum rainfall experienced within the observation period (1961 to 1990). Note that the scale for the desert region differs from that used for the other regions. Months are numbered consecutively from January (1) through to December (12). 4. Climate models 128 4.3.2. PiControl simulations These four rainfall regimes can generally be identified in the piControl simulations, although two of the models (CNRM-CM5, MRI-CGCM3) fail to reproduce the DP pattern in the equatorial zone. However, several models represent the spatial extent of the regimes poorly. Thus 5 out of the 12 models show monsoon penetration further north than observed (fig. 4.4a). Most models place the northern limit of the desert correctly, but two models (CSIROFigure 4.3: Observed and simulated modern and palaeo-precipitation patterns. The total summer (June, July, August) and winter (December, January, February) precipitation from the CRU T3.1 data set (Harris et al., 2013) are compared to ensemble averages of the piControl outputs of the 12 CMIP5 models. The simulated change in precipitation between the Mid-Holocene and piControl simulations (midHolocene-piControl) is shown based on the ensemble average of the midHolocene outputs of the 12 CMIP5 models. The observed anomalies in mean annual precipitation (MAP) between the midHolocene and the present day are average values for 2x2º grids from the Bartlein et al. (2011) data set. 4. Climate models 129 Mk3L-1-2, IPSL-CM5A-LR) show the area of low rainfall and low rainfall seasonality extending further north than observed. Season Anomaly BCCCSM1.1 CCSM4 CNRMCM5 CSIROMk3-6-0 CSIROMk3L-1-2 GISS-E2R Mediterranean annual 15.0 24.4 32.6 -21.2 8.4 75.7 spring 13.9 3.7 16.1 13.0 4.2 28.4 summer -4.3 2.7 4.7 -10.7 -2.4 20.5 autumn -0.4 10.5 14.0 -21.6 6.6 20.3 winter 5.8 7.4 -2.3 -1.9 0.0 6.5 Desert annual 7.7 29.5 32.7 -18.5 4.6 22.8 spring 0.1 0.6 3.6 0.8 1.3 10.6 summer 0.8 14.3 9.6 -7.4 3.2 7.7 autumn 2.8 13.9 18.5 -11.5 0.9 3.9 winter 4.1 0.6 1.0 -0.5 -0.8 0.7 Monsoon annual 47.4 148.6 155.8 -53.5 47.2 116.1 spring -11.2 -2.6 -7.4 -2.6 -2.7 -7.5 summer 33.6 80.4 97.2 -20.3 21.5 91.6 autumn 29.8 76.6 78.7 -28.3 32.9 41.4 winter -4.7 -5.9 -12.6 -2.3 -4.5 -9.4 DP annual -76.7 -13.3 - -208.8 -36.4 2.8 spring -41.3 0.5 - -12.7 -0.6 -56.2 summer 1.1 43.8 - -68.1 33.9 87.1 autumn 29.1 53.1 - -106.8 63.2 71.0 winter -65.5 -110.7 - -21.1 -132.9 -99.1 Season Anomaly HadGEM2 -CC HadGEM2 -ES IPSLCM5A-LR MIROCESM MPI-ESMP MRICGCM3 Mediterranean annual 30.9 9.2 40.1 30.8 14.3 15.4 spring 14.5 13.0 22.2 11.6 17.6 10.4 summer 40.6 -8.8 14.0 0.8 3.5 -2.8 autumn -40.9 2.9 8.0 6.0 -3.9 -5.9 winter 16.7 2.1 -4.1 12.4 -2.9 13.8 Desert annual 4.5 4.4 6.8 26.1 14.3 7.7 spring 0.6 2.1 1.8 4.2 2.1 0.1 summer 3.4 4.2 2.8 12.3 6.7 0.8 autumn 2.1 -1.3 2.3 10.1 5.1 2.8 winter -1.7 -0.5 -0.1 -0.4 0.4 4.1 Monsoon annual 207.6 210.4 202.4 182.1 219.9 88.5 spring -39.1 19.3 6.0 -19.7 17.5 -1.9 summer 15.4 122.8 110.0 142.4 132.8 68.3 autumn 233.6 72.5 89.6 72.2 70.2 23.7 winter -2.2 -4.3 -3.2 -12.7 -0.7 -1.5 DP annual 123.0 142.0 - -244.4 14.3 - spring -100.9 -1.7 - -87.2 -32.0 - summer -1.8 8.6 - 29.3 38.7 - autumn 154.9 168.9 - -9.6 55.6 - winter 70.7 -33.9 - -176.9 -48.1 - Table 4.2. Summary of area-averaged climate anomalies (midHolocene minus piControl) for individual models for individual seasons and for mean annual precipitation. Bold font indicates values that are significantly different from the interannual variability of the modern observations. The seasons are spring: March, April, May; summer: June, July, August; autumn: September, October, November; winter: December, January, February.