Are pine-oak mixed stands in Mediterranean mountains more resilient to drought than their monospecific counterparts?

Are pine-oak mixed stands in Mediterranean mountains more resilient 1 to drought than their monospecific counterparts? 2 Francisco Muñoz-Gálvez1, Asier Herrero2, Esther Pérez-Corona3, Enrique Andivia3 3 1 Terrestrial Ecosystem Ecology and Conservation Research Group, Department of 4 Ecology, Universidad Autónoma de Madrid, Madrid, Spain. 5 2 Forest Ecology and Restoration Group, Department of Life Sciences, University of 6 Alcala, Alcalá de Henares, Spain. 7 3 Department of Biodiversity, Ecology and Evolution, Universidad Complutense de 8 Madrid, Madrid, Spain. 9 This is the accepted manuscript of the article that appeared in final form in Forest Ecology and Management 484: (2021) // Article ID 118955, which has been published in final form at https://doi.org/10.1016/j.foreco.2021.118955. © 2021 Elsevier under CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Abstract 10 Climate change projections point to an increase in the intensity and frequency of extreme 11 drought events with important negative impacts on forest functioning. Predicting these 12 impacts constitutes a crucial challenge for forest managers and for the maintenance of 13 ecosystem services supply. Promoting mixed stands seems a promising strategy for 14 adapting forest ecosystems to ongoing climate change. However, some uncertainty exists 15 regarding whether mixed stands can improve growth resilience to extreme drought events. 16 Here, we aim to assess tree growth response to drought in mixed and monospecific stands 17 of Pinus sylvestris L. and Quercus pyrenaica Willd. in central Spain. We built tree-ring 18 chronologies, and evaluated tree growth sensitivity to water availability and growth 19 resilience components to extreme droughts using linear mixed models. We found 20 contrasting speciesand climate-specific responses to admixture. Q. pyrenaica growth 21 was significantly higher in mixed than in monospecific stands in years without water 22 limitations, while P. sylvestris showed higher growth in mixed stands under dry 23 conditions. Consequently, P. sylvestris and Q. pyrenaica showed higher resistance and 24 recovery to drought in mixed than monospecific stands, respectively. However, Q. 25 pyrenaica was more resistant in monospecific than mixed stands. Our results highlight 26 the importance of water availability and species-specific responses when evaluating 27 admixture effects on drought vulnerability. Overall, we show positive effects of 28 admixture on tree growth and resilience components for both P. sylvestris and Q. 29 pyrenaica, supporting admixture as a management option for adaptation of Mediterranean 30 mountain forests to climate change. 31 Keywords: Climate change, complementarity, forest management, growth stability, 32 Pinus sylvestris, Quercus pyrenaica, tree diversity. 33 34 1. Introduction 35 Climate change is globally altering forest composition, structure and functioning 36 (Allen et al., 2010, 2015; Ruiz-Benito et al., 2017), which ultimately compromises the 37 provision of key ecosystem services to human well-being (Nelson et al., 2013). 38 Mediterranean forests can be especially vulnerable ecosystems due to its high exposition 39 to climate change (Lindner et al., 2010). Over the last century, temperature increased in 40 the Mediterranean basin by 1.4 ºC (Cramer et al., 2018), exacerbating drought impacts on 41 tree growth and triggering drought-induced mortality events (Greenwood et al., 2017; 42 Gazol et al., 2018; Madrigal-González et al., 2018). Climate change scenarios project a 43 worrisome increase of 2-5 ºC along 21st century coupled with a decrease in precipitation 44 of up to 30%, and a higher frequency and intensity of extreme drought events (IPCC, 45 2018; Spinoni et al., 2018). Thus, there is an urgent need to improve our knowledge about 46 forest response to extreme droughts and to adequate management strategies to enhance 47 long-term forest resilience. 48 Admixture, i.e. the increase in tree species diversity, may contribute to stabilize 49 forest functioning and ecosystem services supply in response to disturbances (Gamfeldt 50 et al., 2013). Several studies have showed a positive relationship between tree diversity 51 and forest productivity at different spatial scales (Paquette and Messier, 2010; Ruiz52 Benito et al., 2014; Pretzsch et al., 2015, 2019; Liang et al., 2016; Jactel et al., 2018). 53 Admixture can also stabilize forest productivity and reduce growth sensitivity to climatic 54 variability (del Río et al., 2017). All this body of evidence has led to the promotion of 55 mixed stands in forestry practice worldwide (Bolte et al., 2010). However, some studies 56 have reported a decrease in the magnitude of the positive effect of tree diversity on forest 57 productivity with drought stress (Jactel et al., 2018; Toïgo et al., 2015). Furthermore, 58 there is some uncertainty regarding admixture effects on the stability of forest 59 productivity to extreme drought events (Grossiord, 2019). In this regard, growth 60 resilience, i.e. the capacity of individuals to restore pre-disturbance growth rates after a 61 disturbance (Holling, 1996; Lloret et al., 2011), is an increasingly adopted concept to 62 evaluate forest stability to extreme droughts (Nikinmaa et al., 2020). Despite some 63 evidence of positive effects of admixture on resilience to drought (Lebourgeois et al., 64 2013; Gazol and Camarero, 2016; Steckel et al., 2020), other studies reported speciesor 65 site-specific effects (Pretzsch et al., 2013; Grossiord et al., 2014; Mölder and Leuschner, 66 2014; Merlin et al., 2015; Granda et al., 2018; Jourdan et al., 2019a, 2019b). In addition, 67 few studies have been conducted in Mediterranean areas (Granda et al., 2018), hampering 68 our ability to make a correct assessment of admixture as an appropriate management 69 option for adaptation of Mediterranean forests to climate change. 70 Admixture positive effects on forest productivity are commonly interpreted on the 71 basis of complementarity, which includes both competition reduction and facilitation 72 mechanisms (Ammer, 2019). On one hand, competition reduction usually occurs through 73 niche partitioning, due to inter-specific differences in physiology, morphology and 74 phenology, leading to disparate resource acquisition strategies (Forrester, 2014; Forrester 75 and Bauhus, 2016). On the other hand, facilitation implies that one species increases the 76 performance of coexisting species (Callaway, 1995). Active hydraulic redistribution 77 (Querejeta et al., 2003; Zapater et al., 2011) and nocturnal water release (Prieto et al., 78 2012) are examples of facilitative mechanisms that result in increased soil moisture. 79 These complementarity effects have been also associated with the biodiversity-stability 80 relationship (Loreau and de Mazancourt, 2013), although other mechanisms such as 81 temporal shifts in species interactions (del Río et al., 2017) and asynchronic species82 specific responses to environmental fluctuations (Morin et al., 2014) have been also 83 proposed. 84 Most of the research on complementarity effects on forest ecosystems have been 85 conducted in forests without severe water limitations, where light-related interactions 86 drive complementarity effects (e.g. Bayer et al., 2013; Pretzsch, 2014; Forrester and 87 Bauhus, 2016). In seasonally dry areas, such as the Mediterranean basin, admixture 88 positive effects rely on reducing competition for water and/or improving water 89 availability (Jucker et al., 2014; Ruiz-Benito et al., 2014). Mediterranean tree species 90 show a wide variety of water use strategies associated with different structural and 91 physiological adaptations, such as stomatal behaviour (isohydric vs. anisohydric species) 92 and rooting strategy (taproot vs superficial) (Zavala et al., 2000; Moreno-Gutiérrez et al., 93 2012; del Castillo et al., 2016; Martín-Gómez et al., 2017). This suggests the existence of 94 complementarity mechanisms among species that would reduce competition for water in 95 mixed forests and thus, enhance resilience to extreme droughts. However, whether 96 admixture modifies tree responses to drought compared to monospecific stands is still 97 under debate (Grossiord, 2019). 98 Mediterranean Iberian forests are excellent models for the evaluation of admixture 99 effects on drought impacts (Vilà-Cabrera et al., 2018). Many of the current coniferous 100 forests of the Iberian Peninsula (mainly Pinus spp.) are the result of large-scale 101 afforestation policies during the 20th century (Vadell et al., 2016). The lack of subsequent 102 forest management has resulted in structurally and functionally homogeneous even-aged 103 dense stands (Villar-Salvador, 2016), which often show low growth rates (Gómez104 Aparicio et al., 2009), high mortality (Sánchez-Salguero et al., 2012), lack of regeneration 105 (Ruiz-Benito et al., 2012), and high vulnerability to fires and pests (Maestre and Cortina, 106 2004). In this context, the admixture of oaks (Quercus spp.) in pine monospecific stands 107 can be considered as a powerful tool to manage Mediterranean forests in the face of 108 climate change (Pausas et al., 2004), by both spreading drought-impact risk among 109 multiple species and beneficial complementarity effects. 110 In this study, we compared tree growth response to drought conditions in mixed 111 and monospecific stands of Scots pine (Pinus sylvestris L.) and Pyrenean oak (Quercus 112 pyrenaica Willd.) in central Spain. These species are frequently mixed in extensive areas 113 of Iberian mountains at the ecotone between monospecific pine and oak stands (Sánchez 114 de Dios et al., 2019). We employed a stand-level “triplet design” coupled with tree-ring 115 data to analyze the effect of water availability on tree growth at an annual scale in both 116 monospecific and mixed stands. Following this approach, we also quantified growth 117 resilience, and associated components (resistance and recovery), to extreme drought 118 events occurred during the last decades using the indices proposed by Lloret et al. (2011). 119 Due to functional differences between target species regarding shade tolerance, leaf habit, 120 rooting depth and water use strategies (del Castillo et al., 2016; Martín-Gómez et al., 121 2017; Moreno-Gutiérrez et al., 2012), we hypothesized lower drought-induced growth 122 reductions and higher resilience to extreme droughts in mixed than monospecific stands 123 due to complementarity effects. 124 2. MATERIAL AND METHODS 125 2.1. Study area 126 The study was conducted in the Sierra de Guadarrama National Park, in the centre 127 of the Iberian Peninsula (40º 50’ 26’’ N; 3º 49’ 34’’ W). Climate is continental 128 Mediterranean, characterized by cold winters and warm and dry summers, with 129 precipitations concentrated in autumn and spring. Mean annual temperature and total 130 precipitation is 11.4ºC and 555.7 mm, respectively (period 1961-2018; data from CRU 131 TS3.10 database, Harris et al. (2014)). P. sylvestris is the dominant tree species at high 132 altitude, co-occurring with Q. pyrenaica at intermediate altitude (1,200-1,600 m a.s.l.), 133 although the later can also be found forming extensive monospecific stands. Understory 134 species vary with altitude, being Pteridium aquilinum ((L.) Kuhn), Genista florida (L.), 135 Ilex aquifolium (L.) and Crataegus monogyna (Jacq.) the most representative species. 136 2.2. Sampling design 137 We selected six independent forest sites on the southern face of the Sierra de 138 Guadarrama at an altitude that ranged from 1,286 to 1,544 m a.s.l. (Table 1). At each site 139 we selected 15 stands of 20 x 20 m (0.04 ha) following a triplet design. Thus, we sampled 140 five monospecific stands of each species (100% of the basal area) and five mixed stands 141 (basal area of the dominant tree species was lower than 70%). Field sampling was carried 142 out from November 2018 to February 2019. We selected one tree in each monospecific 143 stand and two trees (one of each species) in mixed stands (hereinafter focal trees). Focal 144 trees were selected in the centre of the stand, and they were all dominant or co-dominant 145 with no sign of vigour decline (i.e. defoliation or dry branches) or physical damages (e.g. 146 due to snow or herbivory). In mixed stands, the distance between focal trees was lower 147 than 5 m. We recorded the diameter at breast height (DBH) of each target tree and each 148 neighboring tree within a circular plot of 7 m radius measured from the focal tree. We 149 calculated the basal area of the circular plot and the Lorimer’s distance-independent 150 competition index (LCI) (Lorimer, 1983) as: 151 𝐿𝐶𝐼%= ' 𝐷𝐵𝐻+𝐷𝐵𝐻% ⁄ - +./ 152 where DBHi is the diameter at breast height of the target tree i, and DBHj is the diameter 153 at breast height of the neighboring tree j. 154 2.3. Dendroecological methods 155 We extracted one wood core per target tree at breast height using a Pressler 156 increment borer (5 mm; Haglöf, Sweden). Wood cores were processed following standard 157 dendrochronological methods (Fritts, 1976). First, wood cores were air-dried and glued 158 on wooden supports. Then, they were sanded using sandpapers of progressively finer 159 grains to maximize the visibility of the tree rings. Tree growth series were visually cross160 dated using pointer years (Yamaguchi, 1991). Wood cores were scanned at 1,200 dpi 161 resolution (EPSON® Perfection v800) and tree-ring widths were measured to the nearest 162 0.01 mm using ImageJ® (Schneider et al., 2012). 163 Ring-width series were converted to basal area increment (BAI) assuming stem 164 growth is approximately concentric: 165 𝐵𝐴𝐼 =1𝜋(𝑟56−1𝑟58/6) 166 where rt and rt-1 are the stem radius at the end and at the beginning of a given annual ring, 167 respectively. BAI reflects whole tree growth better than the one-dimensional growth of 168 tree ring width (Biondi and Qeadan, 2008). We also quantified annual tree size as the 169 basal area of the tree for the whole BAI series, representing the increase in size with 170 ageing. Even though the oldest tree was dated to 1905, we selected as study period 1961171 2018 for the robustness of statistical analysis (70% of the target trees in 1961). 172 2.4. Identification of extreme drought events 173 We used the CRU TS3.10 database (Harris et al., 2014) to characterized the 174 climate of the study sites for the period 1961-2018. Annual mean temperature and annual 175 precipitation from this database were highly correlated (r = 0.89 and 0.74, respectively 176 using common years) with data from the nearest meteorological station (Navacerrada, 25 177 km far from the closest site and at 1,894 m a.s.l). Water availability (P-PET) was 178 characterized as the difference between annual precipitation and potential 179 evapotranspiration (PET) following Bigler et al. (2006). PET was calculated following 180 Thornthwaite (1948). We calculated P-PET from October of the previous year to 181 September of the present year to account for the influence of previous year conditions on 182 the current growing season (Madrigal-González et al., 2017a). 183 Drought events were identified as extremely dry years with a significant reduction 184 on tree growth (Schweingruber et al., 1990). Specifically, we selected as extreme 185 droughts those years where P-PET was below the 15th percentile of the time series (i.e. P186 PET values under -251.4 mm) and where at least 60% of the sampled trees showed a BAI 187 reduction of at least 20% relative to the BAI average in the three preceding years. We 188 selected 1986, 1995, 2005, 2012 and 2017 as extreme drought events (Fig. S1). 189 2.5. Resistance, resilience and recovery to drought events 190 To evaluate growth responses to selected drought events, we calculated growth 191 resistance, resilience and recovery indices following Lloret et al. (2011): 192 𝑅𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒1 =1𝐷𝑟 𝑃𝑟𝑒𝐷𝑟 ⁄ 193 𝑅𝑒𝑠𝑖𝑙𝑖𝑒𝑛𝑐𝑒1 =1𝑃𝑜𝑠𝑡𝐷𝑟 𝑃𝑟𝑒𝐷𝑟 ⁄ 194 𝑅𝑒𝑐𝑜𝑣𝑒𝑟𝑦1=1𝑃𝑜𝑠𝑡𝐷𝑟 𝐷𝑟 ⁄ 195 where Dr is the BAI the year of the drought event and PreDr and PostDr the mean BAI 196 for three years before and after the drought event, respectively. We only calculated 197 resistance index in 2017 since growth series finished in 2018. We also characterized water 198 availability differences between evaluated periods as: 199 𝑃𝑃𝐸𝑇IJK%K5L-MJ 1=1𝑃𝑃𝐸𝑇NI −𝑃𝑟𝑒𝑃𝑃𝐸𝑇 200 𝑃𝑃𝐸𝑇IJK%O%J-MJ 1= 1𝑃𝑜𝑠𝑡𝑃𝑃𝐸𝑇−𝑃𝑟𝑒𝑃𝑃𝐸𝑇 201 ± 0.04, respectively), whereas P. sylvestris showed similar recovery in both stand types 297 (1.28 ± 0.04 and 1.39 ± 0.04) (Fig. 3b, Table S5). In addition, growth recovery was also 298 affected by differences in water availability after drought (P-PETrec) and by the 299 interaction species × drought intensity (P-PETdr). Both species showed greater recovery 300 with increasing P-PETrec and decreasing P-PETdr, respectively (Table S6). However, the 301 increase in recovery with decreasing drought intensity was higher in Q. pyrenaica than in 302 P. sylvestris (estimated model slopes ± SE, 0.35 ± 0.05 and 0.14 ± 0.05, respectively; Fig 303 4a, Table S6). 304 Growth resilience was also affected by water availability differences between the 305 postand the pre-drought periods (P-PETres) and by the interaction species × drought 306 intensity (P-PETdr) (Table S7). However, we did not find any effect of stand type on 307 growth resilience (Fig. 3c). Growth resilience was negatively related to P-PETres 308 (estimated model slope ± SE, -0.20 ± 0.06). Analogously to growth recovery, resilience 309 increased with decreasing drought intensity, especially in Q. pyrenaica (estimated model 310 slopes ± SE, 0.46 ± 0.06 and 0.002 ± 0.06, respectively) (Fig 4b). Growth resilience was 311 also affected by the interaction species × tree size. Growth resilience decreased with tree 312 size in P. sylvestris, but no effect was found in Q. pyrenaica (estimated model slopes ± 313 SE, 0.07 ± 0.02 and 0.02 ± 0.02, respectively; Table S8). 314 315 Figure 3: Boxplots of growth resistance (a), recovery (b) and resilience (c) for study 316 species and stand type. For each boxplot, the P-value of the comparison between stand 317 types is showed. Asterisks show significant interactions (P < 0.05). Boxes are 95% and 318 5% percentile values, whiskers depict maximum and minimum values and the solid lines 319 indicate the median. 320 321 322 Figure 4: Model prediction (± 95% confidence interval) for growth recovery (a) and 323 resilience (b) for each study species in response to drought intensity (P-PETdr). 324 325 326 4. DISCUSSION 327 4.1. Admixture effects on tree growth and resilience components 328 Our results show beneficial effects of admixture on tree growth for the widely distributed 329 sub-Mediterranean Q. pyrenaica and boreal P. sylvestris in drought-limited 330 Mediterranean mountains. Admixture positive effects on tree growth response to water 331 availability were species-specific, which resulted in contrasting effects on the different 332 components of growth resilience to extreme drought events. P. sylvestris showed higher 333 growth in mixed than in monospecific stands in years with low water availability, which 334 also resulted in a higher growth resistance to extreme droughts in mixed stands. On the 335 contrary, growth differences between mixed and monospecific stands increased with 336 increasing water availability for Q. pyrenaica, showing higher recovery after extreme 337 drought events in mixed stands. On the one hand, these results can help to clarify the 338 ongoing debate on the relationship between tree diversity and resilience to drought in 339 forest ecosystems, pointing to the importance of speciesand climate-specific effects. On 340 the other hand, our results have key implications for forest management, suggesting 341 P.sylvestris-Q. pyrenaica mixed stands as an adaptation solution for mid-elevation forests 342 in the Iberian Peninsula under increased aridity. 343 The positive effect of admixture on tree growth can be explained by 344 complementarity due to both mechanisms, facilitation and competition reduction (Loreau 345 and Hector., 2001; Callaway, 2007; Brooker et al., 2008). According to the stress gradient 346 hypothesis (Bertness and Callaway, 1994), positive interactions among species are more 347 common in areas with high environmental stress, which is the case of drought-limited 348 forests in Mediterranean mountains (e.g. Gómez-Aparicio et al., 2004). On the one hand, 349 competition reduction relies on inter-specific differences in resource acquisition 350 strategies. Study species show remarkable differences in shade tolerance, leaf phenology, 351 water-use strategy, and root structure (Niinemets and Valladares, 2006; Poyatos et al., 352 2008; Río and Sterba, 2009). Among them, vertical rooting stratification is a key 353 complementarity mechanism in drought-limited forest ecosystems (Grossiord, 2019). In 354 this regard, some studies in mixed stands showed that conifers have access to shallower 355 water resources while oak species can access deeper ones due to a more extensive and 356 deep root system (Poyatos et al., 2008; del Castillo et al., 2016; Martín-Gómez et al., 357 2017). On the other hand, facilitation could occur through hydraulic lift by oak species 358 under moderate and severe drought conditions, which increases water availability in the 359 upper soil horizons for the admixed species (Querejeta et al., 2003; Zapater et al., 2011). 360 The higher growth during dry years and resistance to extreme droughts for P. 361 sylvestris in mixed than monospecific stands might be driven mainly by water-related 362 facilitation mechanisms. Hydraulic lift can increase not only water availability for P. 363 sylvestris, but also root growth and functioning as well as nutrient availability due to 364 positive effects of increasing humidity on organic matter decomposition and 365 mineralization (Rothe and Binkley, 2001; Richards et al., 2010; Prieto et al., 2012, del 366 Castillo et al., 2016). Competition reduction in response to low water availability seems 367 not to play a prevailing role in our study since admixture showed neutral effects on Q. 368 pyrenaica growth in dry years. In fact, Q. pyrenaica showed higher resistance to extreme 369 droughts in monospecific than in mixed stands. Accordingly, Steckel et al., (2020) 370 showed a reduction of admixture positive effects on Q. robur and Q. petraea response to 371 drought in driest sites when co-occuring with P. sylvestris. 372 Interestingly, admixture positive effects on Q. pyrenaica growth emerged with 373 increasing water availability. Reduced inter-specific competition coupled to higher 374 aboveand below-ground competitive capacity could allow Q. pyrenaica to maximize 375 light and nutrient capture in mixed stands under non-limited water conditions 376 (Longuetaud et al., 2013; Madrigal-González et al., 2016), which might also explain the 377 higher recovery of Q. pyrenaica after extreme droughts in mixed than in monospecific 378 stands. In addition, functional differences in leaf traits between study species can also 379 improve mineralization and decomposition processes, ultimately increasing nutrient 380 availability in mixed stands (Rothe and Binkley, 2001; Andivia et al., 2016; Santonja et 381 al., 2017). Despite P. sylvestris is also likely to benefit from increased nutrient 382 availability, competition for light could offset positive admixture effects under moderate 383 to high water availiability conditions due to larger leaf areas. In this context, Q. pyrenaica 384 could be favoured over P. sylvestris due to its broad-leaved habit and higher tolerance to 385 shade (Zavala et al., 2000; Niinemets and Valladares, 2006). 386 4.2. Growth resilience components 387 To analyze growth resilience components, we simultaneously considered all extreme 388 drought events occurred during recent decades (1986-2018), which represents a more 389 realistic approach to assess overall growth response to drought than analyzing each event 390 separately. In fact, under ongoing climate change, trees are exposed to recurrent extreme 391 drought events (Spinoni et al., 2018), which has been proved to reduce the resilience 392 capacity of forest tree species (Andivia et al., 2020; Serra-Maluquer et al., 2018). 393 However, by doing so, we did not account for specific details about drought onset and 394 duration of each event, which could influence species drought sensitivity (Hoffmann et 395 al., 2018). To partly avoid this, we followed recent recommendations to quantify growth 396 resilience (DeSoto et al., 2020; Schwarz et al., 2020). Specifically, we evaluated growth 397 resilience components after controlling for among-events differences in drought intensity 398 (PPETdr) and water availability differences between growth periods (PPETre, PPETres, 399 PPETrec). 400 Decreasing drought intensity and differences in water availability between post401 and drought periods had a positive effect on growth recovery of both species. This agrees 402 with other studies pointing to the importance of site climatic conditions on growth 403 recovery capacity (Gazol et al., 2017; Steckel et al., 2020). Decreasing drought intensity 404 also increased resilience for Q. pyrenaica, but not for P. sylvestris. This, and the higher 405 effect of decreasing drought intensity on Q. pyrenaica recovery, might be partly explained 406 by the anisohydric behavior of oaks, i.e. the preservation of transpiration rates at low 407 water potential (Fernández-De-Uña et al. 2017, Martín-Gómez et al. 2017). Thus, Q. 408 pyrenaica might take advantage of increased water availability, and faster refilling of soil 409 water reserves with decreasing drought intensity, to maximize post-drought tree growth, 410 and thus recovery and resilience. In fact, oak species at dry sites show positive drought 411 legacies (Anderegg et al., 2015). On the contrary, the isohydric behavior of P. sylvestris 412 (i.e. tight stomatal control under drought conditions; Irvine et al., 1998), seems to respond 413 to specific drought threshold through prolonged stomata closure, reducing photosynthesis 414 and depleting carbohydrate reserves, which might ultimately impair post-drought growth. 415 Finally, the negative effect of differences in water availability between postand pre416 drought periods on resilience may reflect a negative legacy of moderate dry conditions 417 during pre-disturbance period on post-drought growth. We also considered tree size when 418 evaluating growth resilience components, since size strongly influences tree growth 419 dynamics and thus resilience capacity (Andivia et al., 2020). Tree size was negatively 420 related to growth resilience for P. sylvestris, which contrasts to previous studies with this 421 species using different size categories (Merlin et al., 2015). Larger trees can be more 422 exposed to drought due to their greater foliar biomass and the dominant position in the 423 stand (Martín-Benito et al., 2008), which might increase water demand and respiration 424 costs affecting post-drought recovery. 425 Contrary to our hypothesis, we did not find differences in species resilience 426 capacity mediated by stand type. These results can be related to reported trade-offs 427 between growth resistance and recovery (Hodgson et al., 2015; Hoffmann et al., 2018). 428 This trade-off might explain the observed higher recovery in mixed stands and higher 429 resistance in monospecific ones for Q. pyrenaica. The lack of this trade-off for P. 430 sylvestris, which showed higher resistance in mixed than monospecific stands but similar 431 recovery, could be due to the above-mentioned strong negative drought legacy effects 432 reported for conifer species (Anderegg et al., 2015), which could impair post-drought 433 recovery in both type of stands. 434 4.3. Implications for forest management 435 Our results showed positive effects of admixture on tree growth and resilience 436 components for both P. sylvestris and Q. pyrenaica. The positive effect of admixture on 437 P. sylvestris growth response during dry years and resistance to extreme droughts can be 438 critical at the southern distribution limit of the species under increased aridity conditions. 439 In addition, enhancement of Q. pyrenaica tree growth in years without water limitations, 440 suggests that increasing tree diversity can also contribute to increase the productivity of 441 Mediterranean mountain forests (Río and Sterba, 2009). On the other hand, we found a 442 negative effect of competition on tree growth irrespectively of species identity and the 443 type of stand analysed, which suggests that thinning is a key tool to improve forest 444 resilience and response to drought, in agreement with previous studies (Kohler et al., 445 2010; Sohn et al. 2016). This is especially relevant for monospecific conifer stands in the 446 Iberian peninsula, since lack of forest management during the last decades have led to 447 dense stands with a high vulnerability to drought, fires and pests (Maestre and Cortina, 448 2004; Gómez-Aparicio et al., 2011; Sánchez-Salguero et al., 2012). However, drought 449 impacts on forest dynamics rely not only on tree growth responses but also on recruitment 450 and mortality (Allen et al., 2010; Madrigal-González et al., 2017b). Thus, further studies 451 should also evaluate admixture effects on other demographic rates to provide a 452 comprehensive view of the response of mixed stand to drought events (Andivia et al., 453 2020; Madrigal-González et al., 2017b). This is of pivotal importance to properly evaluate 454 tree diversity effects on forest resilience, but also to design forest management strategies 455 oriented to guarantee the long-term persistence of mixed stands. 456 457 5. CONCLUSIONS 458 Our results suggest that mixed stands of P. sylvestris and Q. pyrenaica are less vulnerable 459 to drought than their monospecific counterparts, corroborating positive complementarity 460 effects between contrasting functional species. Thus, this study contributes to the growing 461 body of evidences supporting admixture as a management option for adaptation of forests 462 to climate change. Promoting mixed stands of pine and oak species may contribute to 463 increase forest productivity while reducing vulnerability to climatic disturbances. 464 However, positive effects on tree growth and resilience were species-specific and 465 contingent upon water availability, which suggests that further studies should include 466 more species combination and the whole environmental gradient over the natural area 467 where species co-occur. 468 469 Acknowledgements 470 We acknowledge funding support by ADAPTAMIX (Spanish Ministry of Science and 471 Innovation, PID2019-110470RA-I00) and REMEDINAL-TE (Regional Government of 472 Madrid, S2018/EMT-4338) grants, and by Complutense University of Madrid and Banco 473 Santander (GR105/18). EA was supported by a Postdoctoral grant funded by the 474 Complutense University of Madrid (CT39/17) and AH by the University of Alcalá Own 475 Research Programme’s 2019/20 Postdoctoral Grant. We thank Raquel González, Julen 476 Astigarraga and Eva Monserrat for their help during field work. 477 478 Declaration of interests: None 479 480 REFERENCES 481 Allen, C.D., Breshears, D.D., McDowell, N.G., 2015. On underestimation of global 482 vulnerability to tree mortality and forest die-off from hotter drought in the 483 Anthropocene. Ecosphere 6, 1–55. https://doi.org/10.1890/ES15-00203.1 484 Allen, C.D., Macalady, A.K., Chenchouni, H., Bachelet, D., McDowell, N., Vennetier, 485 M., Kitzberger, T., Rigling, A., Breshears, D.D., Hogg, E.H. (Ted., Gonzalez, P., 486 Fensham, R., Zhang, Z., Castro, J., Demidova, N., Lim, J.H., Allard, G., Running, 487 S.W., Semerci, A., Cobb, N., 2010. A global overview of drought and heat-induced 488 tree mortality reveals emerging climate change risks for forests. For. Ecol. Manage. 489 259, 660–684. https://doi.org/10.1016/j.foreco.2009.09.001 490 Ammer, C., 2019. Diversity and forest productivity in a changing climate. New Phytol. 491 221, 50–66. https://doi.org/10.1111/nph.15263 492 Anderegg, W.R.L., Schwalm, C., Biondi, F., Camarero, J.J., Koch, G., Litvak, M., Ogle, 493 K., Shaw, J.D., Shevliakova, E., Williams, A.P., Wolf, A., Ziaco, E., Pacala, S., 494 2015. Pervasive drought legacies in forest ecosystems and ther implications for 495 carbon cycle models. Science (80-. ). 349, 528–531. 496 https://doi.org/10.1017/CBO9781107415324.004 497 Andivia, E., Rolo, V., Jonard, M., Formánek, P., Ponette, Q., 2016. Tree species identity 498 mediates mechanisms of top soil carbon sequestration in a Norway spruce and 499 European beech mixed forest. Ann. For. Sci. 73, 437–447. 500 Jourdan, Marion, Kunstler, G., Morin, X., 2019. How neighbourhood interactions control 641 the temporal stability and resilience to drought of trees in mountain forests. J. Ecol. 642 1–12. https://doi.org/10.1111/1365-2745.13294 643 Jourdan, M., Lebourgeois, F., Morin, X., 2019. The effect of tree diversity on the 644 resistance and recovery of forest stands in the French Alps may depend on species 645 differences in hydraulic features. For. Ecol. Manage. 450, 1–41. 646 https://doi.org/10.1016/j.foreco.2019.117486 647 Jucker, T., Bouriaud, O., Avacaritei, D., Dǎnilǎ, I., Duduman, G., Valladares, F., Coomes, 648 D.A., 2014. Competition for light and water play contrasting roles in driving 649 diversity-productivity relationships in Iberian forests. J. Ecol. 102, 1202–1213. 650 https://doi.org/10.1111/1365-2745.12276 651 Kohler, M., Sohn, J., Nägele, G., Bauhus, J., 2010. Can drought tolerance of Norway 652 spruce (Picea abies (L.) Karst.) be increased through thinning? Eur. J. For. Res. 129, 653 1109–1118. https://doi.org/10.1007/s10342-010-0397-9 654 Lebourgeois, F., Gomez, N., Pinto, P., Mérian, P., 2013. Mixed stands reduce Abies alba 655 tree-ring sensitivity to summer drought in the Vosges mountains, western Europe. 656 For. Ecol. Manage. 303, 61–71. https://doi.org/10.1016/j.foreco.2013.04.003 657 Liang, J., Crowther, T.W., Picard, N., Wiser, S., Zhou, M., Alberti, G., Schulze, E.D., 658 McGuire, A.D., Bozzato, F., Pretzsch, H., De-Miguel, S., Paquette, A., Hérault, B., 659 Scherer-Lorenzen, M., Barrett, C.B., Glick, H.B., Hengeveld, G.M., Nabuurs, G.J., 660 Pfautsch, S., Viana, H., Vibrans, A.C., Ammer, C., Schall, P., Verbyla, D., 661 Tchebakova, N., Fischer, M., Watson, J. V., Chen, H.Y.H., Lei, X., Schelhaas, M.J., 662 Lu, H., Gianelle, D., Parfenova, E.I., Salas, C., Lee, E., Lee, B., Kim, H.S., 663 Bruelheide, H., Coomes, D.A., Piotto, D., Sunderland, T., Schmid, B., Gourlet664 Fleury, S., Sonké, B., Tavani, R., Zhu, J., Brandl, S., Vayreda, J., Kitahara, F., 665 Searle, E.B., Neldner, V.J., Ngugi, M.R., Baraloto, C., Frizzera, L., Bałazy, R., 666 Oleksyn, J., Zawiła-Niedźwiecki, T., Bouriaud, O., Bussotti, F., Finér, L., 667 Jaroszewicz, B., Jucker, T., Valladares, F., Jagodzinski, A.M., Peri, P.L., Gonmadje, 668 C., Marthy, W., O’Brien, T., Martin, E.H., Marshall, A.R., Rovero, F., Bitariho, R., 669 Niklaus, P.A., Alvarez-Loayza, P., Chamuya, N., Valencia, R., Mortier, F., Wortel, 670 V., Engone-Obiang, N.L., Ferreira, L. V., Odeke, D.E., Vasquez, R.M., Lewis, S.L., 671 Reich, P.B., 2016. Positive biodiversity-productivity relationship predominant in 672 global forests. Science (80-. ). 354. https://doi.org/10.1126/science.aaf8957 673 Lindner, M., Maroschek, M., Netherer, S., Kremer, A., Barbati, A., Garcia-Gonzalo, J., 674 Seidl, R., Delzon, S., Corona, P., Kolström, M., Lexer, M.J., Marchetti, M., 2010. 675 Climate change impacts, adaptive capacity, and vulnerability of European forest 676 ecosystems. For. Ecol. Manage. 259, 698–709. 677 https://doi.org/10.1016/j.foreco.2009.09.023 678 Lloret, F., Keeling, E.G., Sala, A., 2011. Components of tree resilience: Effects of 679 successive low-growth episodes in old ponderosa pine forests. Oikos 120, 1909– 680 1920. https://doi.org/10.1111/j.1600-0706.2011.19372.x 681 Longuetaud, F., Piboule, A., Wernsdörfer, H., Collet, C., 2013. Crown plasticity reduces 682 inter-tree competition in a mixed broadleaved forest. Eur. J. For. Res. 132, 621–634. 683 https://doi.org/10.1007/s10342-013-0699-9 684 Loreau, M., de Mazancourt, C., 2013. Biodiversity and ecosystem stability: A synthesis 685 of underlying mechanisms. Ecol. Lett. 16, 106–115. 686 https://doi.org/10.1111/ele.12073 687 Loreau M., Hector. A., 2001. Partitioning selection and complementarity in biodiversity 688 experiments. Nature 412, 72–76. 689 Lorimer, C.G., 1983. Tests of age-independent competition indices for individual trees in 690 natural hardwood stands. For. Ecol. Manage. 6, 343–360. 691 Madrigal-González, J., Andivia, E., Zavala, M.A., Stoffel, M., Calatayud, J., Sánchez692 Salguero, R., Ballesteros-Cánovas, J., 2018. Disentangling the relative role of 693 climate change on tree growth in an extreme Mediterranean environment. Sci. Total 694 Environ. 642, 619–628. https://doi.org/10.1016/j.scitotenv.2018.06.064 695 Madrigal-González, J., Ballesteros-Cánovas, J.A., Herrero, A., Ruiz-Benito, P., Stoffel, 696 M., Lucas-Borja, M.E., Andivia, E., Sancho-García, C., Zavala, M.A., 2017a. Forest 697 productivity in southwestern Europe is controlled by coupled North Atlantic and 698 Atlantic Multidecadal Oscillations. Nat. Commun. 8, 1–8. 699 https://doi.org/10.1038/s41467-017-02319-0 700 Madrigal-González, J., Herrero, A., Ruiz-Benito, P., Zavala, M.A., 2017b. Resilience to 701 drought in a dry forest: Insights from demographic rates. For. Ecol. Manage. 389, 702 167–175. https://doi.org/10.1016/j.foreco.2016.12.012 703 Madrigal-González, J., Ruiz-benito, P., Ratcliffe, S., Calatayud, J., 2016. 704 Complementarity effects on tree growth are contingent on tree size and climatic 705 conditions across Europe. Sci. Rep. 6, 11. https://doi.org/10.1038/srep32233 706 Maestre, F.T., Cortina, J., 2004. Are Pinus halepensis plantations useful as a restoration 707 tool in semiarid Mediterranean areas? For. Ecol. Manage. 198, 303–317. 708 https://doi.org/10.1016/j.foreco.2004.05.040 709 Martín-Benito, D., Cherubini, P., Del Río, M., Cañellas, I., 2008. Growth response to 710 climate and drought in Pinus nigra Arn. trees of different crown classes. Trees - 711 Struct. Funct. 22, 363–373. https://doi.org/10.1007/s00468-007-0191-6 712 Martín-Gómez, P., Aguilera, M., Pemán, J., Gil-Pelegrín, E., Ferrio, J.P., 2017. 713 Contrasting ecophysiological strategies related to drought: the case of a mixed stand 714 of Scots pine (Pinus sylvestris) and a submediterranean oak (Quercus subpyrenaica). 715 Tree Physiol. 37, 1478–1492. https://doi.org/10.1093/treephys/tpx101 716 McDowell, N., Pockman, W.T., Allen, C.D., Breshears, D.D., Cobb, N., Kolb, T., Plaut, 717 J., Sperry, J., West, A., Williams, D.G., Yepez, E.A., 2008. Mechanisms of plant 718 survival and mortality during drought: Why do some plants survive while others 719 succumb to drought? New Phytol. https://doi.org/10.1111/j.1469720 8137.2008.02436.x 721 Merlin, M., Perot, T., Perret, S., Korboulewsky, N., Vallet, P., 2015. Effects of stand 722 composition and tree size on resistance and resilience to drought in sessile oak and 723 Scots pine. For. Ecol. Manage. 339, 22–33. 724 https://doi.org/10.1016/j.foreco.2014.11.032 725 Mölder, I., Leuschner, C., 2014. European beech grows better and is less drought sensitive 726 in mixed than in pure stands: Tree neighbourhood effects on radial increment. Trees 727 - Struct. Funct. 28, 777–792. https://doi.org/10.1007/s00468-014-0991-4 728 Moreno-Gutiérrez, C., Dawson, T.E., Nicolás, E., Querejeta, J.I., 2012. Isotopes reveal 729 contrasting water use strategies among coexisting plant species in a mediterranean 730 ecosystem. New Phytol. 196, 489–496. https://doi.org/10.1111/j.1469731 8137.2012.04276.x 732 Morin, X., Fahse, L., de Mazancourt, C., Scherer-Lorenzen, M., Bugmann, H., 2014. 733 Temporal stability in forest productivity increases with tree diversity due to 734 asynchrony in species dynamics. Ecol. Lett. 17, 1526–1535. 735 https://doi.org/10.1111/ele.12357 736 Nelson, E.J., Kareiva, P., Ruckelshaus, M., Arkema, K., Geller, G., Girvetz, E., Goodrich, 737 D., Matzek, V., Pinsky, M., Reid, W., Saunders, M., Semmens, D., Tallis, H., 2013. 738 Climate change’s impact on key ecosystem services and the human well-being they 739 support in the US. Front. Ecol. Environ. 11, 483–493. 740 https://doi.org/10.1890/120312 741 Niinemets, Ü., Valladares, F., 2006. Tolerance to shade, drought, and waterlogging of 742 temperate northern hemisphere trees and shrubs. Ecol. Monogr. 76, 521–547. 743 https://doi.org/10.1890/0012-9615(2006)076[0521:TTSDAW]2.0.CO;2 744 Nikinmaa, L., Lindner, M., Cantarello, E., Jump, A.S., Siedl, R., Winkel, G., Muys, B., 745 2020. Review the use of resilience concepts in forest sciences. Curr. For. Reports. 746 Paquette, A., Messier, C., 2010. The effect of biodiversity on tree productivity: from 747 temperate to boreal forests. Glob. Ecol. Biogeogr. 20, 170–180. 748 https://doi.org/10.1111/j.1466-8238.2010.00592.x 749 Pausas, J.G., Blade, C., Valdecantos, A., Seva, J.P., Fuentes, D., Alloza, J.A., Vilagrosa, 750 A., Bautista, S., Cortina, J., Vallejo, R., 2004. Pines and oaks in the restoration of 751 Mediterranean landscapes of Spain: New perspectives for an old practice - a review. 752 Plant Ecol. 171, 209–220. 753 Pinheiro, J., Bates, D., DebRoy, S., Sarkar, D., R., 2018. Linear and Nonlinear Mixed 754 Effects Models. 755 Poyatos, R., Llorens, P., Piñol, J., Rubio, C., 2008. Response of Scots pine (Pinus 756 sylvestris L.) and pubescent oak (Quercus pubescens Willd.) to soil and atmospheric 757 water deficits under Mediterranean mountain climate. Ann. For. Sci. 65. 758 https://doi.org/10.1051/forest:2008003 759 Pretzsch, H., 2014. Canopy space filling and tree crown morphology in mixed-species 760 stands compared with monocultures. For. Ecol. Manage. 327, 251–264. 761 https://doi.org/10.1016/j.foreco.2014.04.027 762 Pretzsch, H., del Río, M., Ammer, C., Avdagic, A., Barbeito, I., Bielak, K., Brazaitis, G., 763 Coll, L., Dirnberger, G., Drössler, L., Fabrika, M., Forrester, D.I., Godvod, K., 764 Heym, M., Hurt, V., Kurylyak, V., Löf, M., Lombardi, F., Matović, B., Mohren, F., 765 Motta, R., den Ouden, J., Pach, M., Ponette, Q., Schütze, G., Schweig, J., 766 Skrzyszewski, J., Sramek, V., Sterba, H., Stojanović, D., Svoboda, M., 767 Vanhellemont, M., Verheyen, K., Wellhausen, K., Zlatanov, T., Bravo-Oviedo, A., 768 2015. Growth and yield of mixed versus pure stands of Scots pine (Pinus sylvestris 769 L.) and European beech (Fagus sylvatica L.) analysed along a productivity gradient 770 through Europe. Eur. J. For. Res. 134, 927–947. https://doi.org/10.1007/s10342771 015-0900-4 772 Pretzsch, H., Schütze, G., Uhl, E., 2013. Resistance of European tree species to drought 773 stress in mixed versus pure forests: Evidence of stress release by inter-specific 774 facilitation. Plant Biol. 15, 483–495. https://doi.org/10.1111/j.1438775 8677.2012.00670.x 776 Pretzsch, H., Steckel, M., Heym, M., Biber, P., Ammer, C., Ehbrecht, M., Bielak, K., 777 Bravo, F., Ordóñez, C., Collet, C., Vast, F., Drössler, L., Brazaitis, G., Godvod, K., 778 Jansons, A., de-Dios-García, J., Löf, M., Aldea, J., Korboulewsky, N., Reventlow, 779 D.O.J., Nothdurft, A., Engel, M., Pach, M., Skrzyszewski, J., Pardos, M., Ponette, 780 Q., Sitko, R., Fabrika, M., Svoboda, M., Černý, J., Wolff, B., Ruíz-Peinado, R., del 781 Río, M., 2019. Stand growth and structure of mixed-species and monospecific stands 782 of Scots pine (Pinus sylvestris L.) and oak (Q. robur L., Quercus petraea (Matt.) 783 Liebl.) analysed along a productivity gradient through Europe. Eur. J. For. Res. 784 https://doi.org/10.1007/s10342-019-01233-y 785 Prieto, I., Armas, C., Pugnaire, F.I., 2012. Water release through plant roots: New insights 786 into its consequences at the plant and ecosystem level. New Phytol. 193, 830–841. 787 https://doi.org/10.1111/j.1469-8137.2011.04039.x 788 Querejeta, J.I., Egerton-Warburton, L.M., Allen, M.F., 2003. Direct nocturnal water 789 transfer from oaks to their mycorrhizal symbionts during severe soil drying. 790 Oecologia 134, 55–64. https://doi.org/10.1007/s00442-002-1078-2 791 Richards, A.E., Forrester, D.I., Bauhus, J., Scherer-Lorenzen, M., 2010. The influence of 792 mixed tree plantations on the nutrition of individual species: A review. Tree Physiol. 793 30, 1192–1208. https://doi.org/10.1093/treephys/tpq035 794 Richards, F.J., 1959. A flexible growth function for empirical use. J. Exp. Bot. 10, 290– 795 301. https://doi.org/10.1093/jxb/10.2.290 796 Río, M. Del, Sterba, H., 2009. Comparing volume growth in pure and mixed stands of 797 Pinus sylvestris and Quercus pyrenaica. Ann. For. Sci. 66. 798 https://doi.org/10.1051/forest/2009035 799 Rothe, A., Binkley, D., 2001. Nutritional interactions in mixed species forests: A 800 synthesis. Can. J. For. Res. 31, 1855–1870. https://doi.org/10.1139/x01-120 801 Ruiz-Benito, P., Gómez-Aparicio, L., Paquette, A., Messier, C., Kattge, J., Zavala, M.A., 802 2014. Diversity increases carbon storage and tree productivity in Spanish forests. 803 Glob. Ecol. Biogeogr. 23, 311–322. https://doi.org/10.1111/geb.12126 804 Ruiz-Benito, P., Gómez-Aparicio, L., Zavala, M.A., 2012. Large-scale assessment of 805 regeneration and diversity in Mediterranean planted pine forests along ecological 806 gradients. Divers. Distrib. 18, 1092–1106. https://doi.org/10.1111/j.1472807 4642.2012.00901.x 808 Ruiz-Benito, P., Ratcliffe, S., Zavala, M.A., Martínez-Vilalta, J., Vilà-Cabrera, A., Lloret, 809 F., Madrigal-González, J., Wirth, C., Greenwood, S., Kändler, G., Lehtonen, A., 810 Kattge, J., Dahlgren, J., Jump, A.S., 2017. Climateand successional-related 811 changes in functional composition of European forests are strongly driven by tree 812 mortality. Glob. Chang. Biol. 23, 4162–4176. https://doi.org/10.1111/gcb.13728 813 Sánchez-Salguero, R., Navarro-Cerrillo, R.M., Swetnam, T.W., Zavala, M.A., 2012. Is 814 drought the main decline factor at the rear edge of Europe? The case of southern 815 Iberian pine plantations. For. Ecol. Manage. 271, 158–169. 816 https://doi.org/10.1016/j.foreco.2012.01.040 817 Santonja, M., Fernandez, C., Proffit, M., Gers, C., Reiter, I.M., Cramer, W., Baldy, V., 818 2017. Plant litter mixture partly mitigates the negative effects of extended drought 819 on soil biota and litter decomposition in a Mediterranean oak forest. 820 Schneider, C.A., Rasband, W.S., Eliceiri, K.W., 2012. NIH Image to ImageJ: 25 years of 821 image analysis. Nat. Methods. https://doi.org/10.1038/nmeth.2089 822 Schwarz, J.A., Skiadaresis, G., Kohler, M., Kunz, J., Schnabel, F., Vitali, V., Bauhus, J., 823 2020. Quantifying growth responses of trees to drought - a critique of the Lloret824 indicators and recommendations for future studies. Curr. For. Reports. 825 https://doi.org/https://doi.org/10.32942/osf.io/5ke4f 826 Schweingruber, F.H., Eckstein, D., Serre-Bachet, F., Bräker, O.U., 1990. Identification, 827 presentation and interpretation of event years and pointer years in 828 dendrochronology. Dendrochronologia 8, 9–38. 829 Serra-Maluquer, X., Mencuccini, M., Martínez-Vilalta, J., 2018. Changes in tree 830 resistance, recovery and resilience across three successive extreme droughts in the 831 northeast Iberian Peninsula. Oecologia 187, 343–354. 832 https://doi.org/10.1007/s00442-018-4118-2 833 Sohn, J.A., Hartig, F., Kohler, M., Huss, J., Bauhus, J., 2016. Heavy and frequent thinning 834 promotes drought adaptation in Pinus sylvestris forests. Ecol. Appl. 26, 2190–2205. 835 https://doi.org/10.1002/eap.1373 836 Spinoni, J., Vogt, J. V., Naumann, G., Barbosa, P., Dosio, A., 2018. Will drought events 837 become more frequent and severe in Europe? Int. J. Climatol. 38, 1718–1736. 838 https://doi.org/10.1002/joc.5291 839 Steckel, M., del Río, M., Heym, M., Aldea, J., Bielak, K., Brazaitis, G., Černý, J., Coll, 840 L., Collet, C., Ehbrecht, M., Jansons, A., Nothdurft, A., Pach, M., Pardos, M., 841 Ponette, Q., Reventlow, D.O.J., Sitko, R., Svoboda, M., Vallet, P., Wolff, B., 842 Pretzsch, H., 2020. Species mixing reduces drought susceptibility of Scots pine 843 (Pinus sylvestris L.) and oak (Quercus robur L., Quercus petraea (Matt.) Liebl.) – 844 Site water supply and fertility modify the mixing effect. For. Ecol. Manage. 461, 845 117908. https://doi.org/10.1016/j.foreco.2020.117908 846 Team, R.C., 2018. R: A language and environment for statistical computing. 847 Thornthwaite, C.W., 1948. An Approach toward a Rational Classification of Climate. 848 Geogr. Rev. 38, 55–94. 849 Toïgo, M., Vallet, P., Perot, T., Bontemps, J.-D., Piedallu, C., Courbaud, B., 2015. 850 Overyielding in mixed forests decreases with site productivity. J. Ecol. 103, 502– 851 512. https://doi.org/10.1111/1365-2745.12353 852 Vadell, E., de-Miguel, S., Pemán, J., 2016. Large-scale reforestation and afforestation 853 policy in Spain: A historical review of its underlying ecological, socioeconomic and 854 political dynamics. Land use policy 55, 37–48. 855 https://doi.org/10.1016/j.landusepol.2016.03.017 856 Vilà-Cabrera, A., Coll, L., Martínez-Vilalta, J., Retana, J., 2018. Forest management for 857 adaptation to climate change in the Mediterranean basin: A synthesis of evidence. 858 For. Ecol. Manage. 407, 16–22. https://doi.org/10.1016/j.foreco.2017.10.021 859 Villar-Salvador, P., 2016. Restoration of Spanish pine plantations: A main challenge for 860 the 21st century. Reforesta 53–66. https://doi.org/10.21750/refor.1.04.4 861 Yamaguchi, D.K., 1991. A simple method for cross-dating increment cores from living 862 trees. Can. J. For. Res. 21, 414–416. 863 Zapater, M., Hossann, C., Bréda, N., Bréchet, C., Bonal, D., Granier, A., 2011. Evidence 864 of hydraulic lift in a young beech and oak mixed forest using 18O soil water 865 labelling. Trees - Struct. Funct. 25, 885–894. https://doi.org/10.1007/s00468-011866 0563-9 867 Zavala, M.A., Espelta, J.M., Retana, J., 2000. Constraints and trade-offs in Mediterranean 868 plant communities: the case of mixed holm oak (Quercus ilex L)-Aleppo pine (Pinus 869 halepensis Mill.) forests. Bot. Rev. 66, 119–149. 870 Zuur, A.F., Ieno, E.N., Walker, N., Saveliev, A.A., Smith, G.M., 2009. Mixed effects 871 models and extensions in ecology with R. Springer. 872 873