Long-term implications of traditional forest regulation methods applied to Maritime pine (Pinus pinaster Ait.) forests in central Spain: a century of management plans

Standard Article - doi: 10.3832/ifor0526-003 ©iForest – Biogeosciences and Forestry Introduction Operational forestry is limited by social, ecological, economic, and legal constraints. In Spain, as in many countries around the world, forestry science originated in the Central Europe of the late eighteenth cen tury, when the main concern was wood/fuel supply and game protection. As is well known, forestry possesses a long-term nature arising from long tree life-spans that, in tem perate zones, can cover several human ge nerations. In this context, historical analysis of forest organization can be used to develop criteria and indicators to evaluate the ma nagement processes in different forest areas effectively. In spite of its limitations, this kind of retrospective studies is acknow ledged in adaptive management as a source of valuable information for shaping forest management practice. Historical studies are an attempt to demonstrate changes in ma nagement indicators, including the number of trees per hectare or wood volume by using certain variables or parameters. By doing this, we can study the long-term sustainabi lity of the management methods used. As forest management has a multidimensional impact on forests and forest communities, the best strategy in analysing management effects is to include the largest possible num ber of indicators provided by the forests management plans. Over the last decade, dif ferent analyses have been performed on changes in organized forests to observe the response to various management models (Charnley et al. 2008). The greatest diffi culties in carrying out these types of studies are the limited amount of reliable document sources (mainly management plans), because document structure changes as legal regula tions evolve. The numerical data are there fore difficult to compare, and only general trends can be defined. The limited span of this kind of studies is not strange, conside ring the difficulty of obtaining long-term, re liable, comparable data sets. In southern Europe, Spain is the only country where forest management plans have been applied effectively and indicators reflecting the silvi culture applied in the past can be analysed thoroughly. It is important to emphasize that, in most cases, the organization of Spanish forests has not included economic variables in forest management as clearly stated ob jectives. Revenue and expense information is rarely included, and it is very difficult to find the information needed to perform a proper economic evaluation. Although retrospective evaluations of forest management results are generally performed on individual forests, we can gain insight on a landscape approach over a long-term period by studying opera tional forestry documents. Forest model, permanent plot, and tempo rary plot approaches have been defined as useful tools for evaluating forest manage ment practices. All these methods have dif ferent limitations (mainly spatial and tem poral) and conclusions drawn from such studies can be limited by the monitoring design. As usual in forestry research, one of the main concerns is the limited time span analysed and the limited spatial scope of the study. Studies on Pinus pinaster management ef fects are scarce despite its being one of the most representative conifers in the Mediter ranean basin, where it is widely distributed, occupying around four million ha (Ribeiro et al. 2001). The species grows in a wide range of ecological conditions: from pure Mediter ranean climate conditions such as those found in eastern Spain, Italy, and southern France, to the continental climate of inland Spain or the Atlantic climate of western France, Portugal, and north-western Spain. © SISEF http://www.sisef.it/iforest/ 33 iForest (2010) 3: 33-38 (1) Dept. de Producción Vegetal y Recursos Forestales, E.T.S. de Ingenierías Agrarias, Universidad de Valladolid, Avda. de Madrid, 44 E-34004 Palencia (Spain); (2) Sustainable Forest Management Research Institute UVa-INIA (Spain); (3) Departamento de Estadística e Investigación Operativa. Universidad de Valladolid (Spain); (4) Universidad Nacional de Colombia, Medellín (Colombia) @ @ Felipe Bravo ([email protected]) Received: Nov 03, 2009 - Accepted: Dec 18, 2009 Citation: Bravo F, Osorio LF, Pando V, Del Peso C, 2010. Long-term implications of traditional forest regulation methods applied to Maritime pine (Pinus pinaster Ait.) forests in central Spain: a century of management plans. iForest 3: 33-38 [online: 2010-03-02] URL: http://www.sisef.it/iforest/doi.php? doi=10.3832/ifor0526-003 Fig. 1 - Location of the study area (southern Iberian Range). Long-term implications of traditional forest regulation methods applied to Maritime pine (Pinus pinaster Ait.) forests in central Spain: a century of management plans Bravo F (1-2), Osorio LF (1-4), Pando V (2-3), Del Peso C (1-2) Pinus pinaster stands in the northern Iberian range (Spain) have been under forest management plans for more than a century. The main goals have been sustainable yield and conservation and enhancement of forest conditions. In the study area, forest management plan implementation started to be effec5 tive in the late 19th and early 20th centuries and the plans have been evaluated every ten years. In these forests, a permanent forest compartment method has been applied as the management system. Different regeneration methods have been applied (uniform shelterwood and clear cutting in Soria, selective and uniform shelterwood and continuous cover forestry in Teruel). To analyse the evolution of each forest group, five management indicators have been tested in this study: number of merchantable trees, standing volume, resin produc5 tion, allowable yield, and harvested volume. Forest situation improvement and maintenance of forest integrity are not only a consequence of the actions planed by foresters, they are also influenced by socio-economic transforma5 tions that have occurred during the last century such as the weakness of the resin market, decreasing demand for fire wood, restriction of pasture activi5 ties, and increasing recreation use. Keywords: Forest regulation, Indicators, Standing volume, Harvest volume, Pinus pinaster Ait. Bravo F et al. - iForest 3: 33-38 Existing studies have analysed silviculture by developing yield and growth models (Bravo-Oviedo et al. 2006 and BravoOviedo et al. 2007, Pascual et al. 2007), re generation (Herrero et al. 2007), genetics (Vendramin et al. 1998, González-Martínez et al. 2001, De-Lucas et al. 2008, De-Lucas et al. 2009), and economics (Bravo et al. 2008). However, there are no studies on practical long-term forestry effects in Medi terranean Maritime pine forests. Effective forestry application requires un derstanding the long-term effects of different management strategies. Although an exten sive permanent plot network can be the most useful (and the most expensive) way to ob tain responses to some questions on longterm effects of silviculture by monitoring ac tual forestry actions, valuable information useful in guiding forest management can be obtained. In this study, our main objective was to evaluate the long-term implications of traditional forest management practices upon forest sustainability of Maritime pine (Pinus pinaster Ait.) stands in the Southern Iberian Range (Central Spain). This analysis was performed on three groups of forests that have similar characteristics, through five dif ferent management indicators including the number of merchantable trees, standing volume, resin production, and allowable and actual harvest. The proposed methodology can serve as a model for performing similar studies in other forest ecosystems under an adaptive management approach. Materials and methods General forest characteristics The organized Pinus pinaster forests stu died are concentrated into three groups, two in the province of Soria (Almazán and Burgo de Osma forests) and one in the province of Teruel (Albarracín forest) in the southern Iberian Range in Central Spain (Fig. 1). Covering an area of 17 171 hectares, these three groups were first organized in 1899, 1903, and 1907 (Tab. 1). Management revi sions are generally performed every ten years, but in some cases they have not been carried out because of situations of public or der (i.e., the Spanish civil war) or budget re strictions. Teruel forests have only been evaluated since 1934 due to the loss of the oldest documents on most of its forests. All of these forests are organized using the per manent forest compartment method as de scribed by Davis et al. (2001). Regeneration methods for the forests in Soria have shifted from shelterwood (in which the main activity was resin tapping) to seed tree harvest (where the main objective has shifted to wood production). In Teruel forests, the re generation method has shifted from a selec tion system to shelterwood and, finally, to high grading, where the priorities are recre ational use and fire prevention. To provide insight on long-term forest management impact, a multidimensional ap proach based on five indicators was used: the number of merchantable trees, defined as the number of trees with a diameter at breast height (DBH) greater than or equal to 20 centimetres; standing volume, expressed in cubic meters per hectare; resin production, in kilograms of resin per tree; and both allow able and actual harvest volumes, expressed in cubic meters per hectare. The main objec tives pursued with these organizations throughout the region were to increase stan ding volumes and perform interventions that would make it possible to improve natural regeneration conditions, while increasing and sustaining incomes for local populations. Data analysis Univariate analysis of variance was per formed to test the null hypothesis of equality of mean effects of the various revisions. Multivariate analysis tested the null hypo thesis of equal effect of the revisions of in teraction between the factors in the three variables (standing volumes and allowable and actual harvest volumes) being studied and the repetition variables (Davis 2002, Quinn & Keough 2002). iForest (2010) 3: 33-38 34 © SISEF http://www.sisef.it/iforest/ Tab. 1 - General characteristics of forests: province of Soria and Teruel. Group Forests number Name Municipality Area (ha) Organiza! tion Year Number of effective evaluations Rotation age Almazán 64 Pinar de Matamala Matamala 1615.00 1899 9 80 65 Pinar de Matute Matamala 1114.34 1899 9 80 186 Pinar y Marojal Tardelcuende 629.33 1899 9 80 185 Manadizo y San Gregorio Tardelcuende 2641.95 1899 9 80 Burgo de Osma 85 Pinar de Fuenterrey Quintana de Gormaz 1427.00 1903 7 80 103 Pinar de Valdenebro Valdenebro 1027.00 1903 7 80 55 Pinar de Bayubas Bayubas 1490.85 1903 7 80 57 La Mata y Pinarejo Berlanga de Duero 464.37 1903 7 80 Albarracín 4 Dehesas Nuevas de Bezas Albarracín 535.00 1907 4 120 6 Muela Mediana Gea Albarracín 2198.20 1907 4 120 8 Patio Arriba Rey Don Jaime Albarracín 1012.00 1907 4 120 7 Ortezuelo Gea Albarracín 1495.00 1907 4 120 14 El Pinar Bezas 1521.00 1907 4 120 Tab. 2 - Univariate analysis of variance by merchantable trees. Group Forest Sum of squares Degrees of Freedom Mean Square p-value Almazán Forest 192795 3 64273 0.005 Revision 28510 8 3563.8 Error 22550 24 939.6 Burgo de Osma Forest 24529 3 8176.4 0.000 Revision 91830 6 15305.6 Error 13571 18 753.9 Albarracín Forest 147145.3 5 25449.1 0.037 Revision 2347.3 3 782.4 Error 3230.0 15 215.3 Long-term implications of traditional forest regulation methods in Spain Merchantable trees, standing volume, and resin production Univariate analysis of variance was used to evaluate the effect that organizations and harvest treatments had on the forests, by ob serving the changes in certain variables such as the number of merchantable trees (tree/ha), standing volume (m3/ha), and resin production (kg/tree) in the groups of Al mazán, Burgo de Osma and Albarracín. In this case, two parameters of variance were performed for the independent variable: the first reflects the general variability of the measurements between the forests, and the other represents the variability of in-forest measurements that is attributed to the effects of the organizations and successive revi sions. These methods are characterized by taking measurements of one single subject over time or under different conditions, re quiring fewer subjects than a completely ran domized design, and allowing for elimina tion of residual variation due to the diffe rences among subjects (because the same ones are used). Univariate analyses of vari ance also make it possible to observe the dis tribution and difference among the means for each group of forests compared to the vari able studied in the medium and long term. Therefore, in the simplest case of studying a factor that acts on several levels, the follo wing model was used for the work (eqn. 1): where μ is the effect of the general mean, αi is the random effect of the forest i with dis tribution N (0,σ2M), βj is the effect of revision j, εij is the random error for revision j of forest i, with distribution N (0, σ2) in which εij are independent and equally distributed, σ2 is the random variance within revisions of a forest and σ2M is the random variance between forests. Standing volume and allowable and actual harvest volumes The changes in allowable and actual har vest volumes (both expressed as m3/ha and year) compared to the standing volume were evaluated on the basis of a multivariate ana lysis of variance (MANOVA). This made it possible to determine not only whether each of the factors affects the dependent variable separately, but also whether interaction among them modifies that effect (Hair et al. 2006). MANOVA hypotheses compare the equality of vectors for measurements of mul tiple dependent variables among revisions; MANOVA uses various criteria with which it evaluates multivariate differences among revisions. In this case, the Lambda criterion of Wilks was used, because it is the most commonly used comparison for global MANOVA significance. Multivariate ana lysis of variance is an extension of ANOVA, in which more than one dependent variable is taken into account, which can be represen ted using the following formula (eqn. 2). in which Yij, μ, αi, βj, εij are vectors of lR3. For example, with: • y(k)ij = value observed for variable k (k=1: standing volumes; k=2: allowed harvest volume; k=3: actual harvest volume) in forest i and revision j. • μ(k) = effect of the general mean for vari able k. • α(k)i = random effect of forest i on variable k [α(k) i ~ N (0,σ2 MK)]. • β(k)j = effect of revision j on variable k. • ε(k)ij = random error for forest i and revision j on variable k. It is supposed that for each fixed k, ε(k)ij is independent and equally distributed with the distribution N (0, σ2k), in which σ2k is the random variance between revisions of a forest for variable k, while σ2MK is the ran dom variance between forests for variable k. © SISEF http://www.sisef.it/iforest/ 35 iForest (2010) 3: 33-38 Fig. 2 - Change in merchantables trees (showing interval of confid ence): (a) group of Almazán, (b) Burgo de Osma and (c) Albarracín. Tab. 3 - Univariate analysis of variance by standing volume. Group Effect Sum of squares Degrees of Freedom Mean Square p-value Almazán Forest 21766 3 7255.35 0.0001 Revision 17010 8 2126.35 Error 3693 24 153.91 Burgo de Osma Forest 4123 3 1374.39 0.0001 Revision 15758 6 2626.49 Error 1974 18 109.71 Albarracín Forest 2947 5 589.58 0.0001 Revision 9356 3 1559.49 Error 1700 18 94.45 Yij =ijij Yij =ijij Bravo F et al. - iForest 3: 33-38 This evaluation was only performed on Al mazán and Burgo de Osma groups, because continuous, reliable information was avai lable. Analysis was performed with STA TISTICA 5.5 (Statsoft 1995) and SAS/STAT 9.1 (SAS Institute 2006). Results Change in number of merchantable trees Steady implementation of forest manage ment plans (expressed by number of revi sions) has a significant impact on merchan table trees. Revision impact is significant in all forests (p < 0.05) but is less evident in Albarracín forests (p = 0.0376 - Tab. 2). The number of merchantable trees increases over time in the forests analyzed (Fig. 2). For example, Almazán and Albarracín forests currently show a density of over 150 mer chantable trees per hectare, while forest density is 100 trees/ha in Burgo de Osma. In contrast, at the time of organization (late 19th or early 20th centuries), the first two forests had 100 trees/ha, while that of Burgo de Osma had fewer than 50. Two different pro cesses can interact to increase the number of merchantable trees per ha: increasing forest density at stand level and generating a ba lanced forest age structure at forest level. When regeneration harvests were relatively scarce, the allowable yield was completed by dispersed harvest operation throughout the forests. It is also possible to observe that there are no significant differences in the changes in merchantable trees from applying different regeneration treatments, such as shelterwood and seed trees methods. This may be because operational application of these two treatments shows little difference, because the establishment and final cuttings are grouped into one single intervention in the shelterwood method, after preparatory cuttings with a double objective: increase tree vigour and seed production and allow early seedling establishment. In the three forest groups analysed, there is a trend to wards an increase in merchantable trees that verifies the importance of the organizations in the increase in forest density. Change in standing volume Standing volume, like merchantable trees, shows a considerable increase in the period analysed (Tab. 3). The analyses of variance among forests in the same group and within the same forest are highly significant (p > 0.05 in all cases). Fig. 3 shows how standing volume gradually increased during the management period in the three groups analysed. During the period studied, standing volume increased from an average of 20 m3/ha to 80 m3/ha in the decade of the six ties, stabilizing later at approximately 70 m3. However, the upper limit reaches 140 m3/ha in Soria, while the maximum value is 90 m3/ha for the forests in Teruel. It is important to mention that a section of the forests in Al barracín has become part of a natural re serve; the handling goal is consequently changing now to conservation in these forests, and the silvicultural treatments ap plied are therefore changing as well. Change in pine resin production in the Almazán and Burgo de Osma groups Analysis of changes in resin production is only performed on the Soria groups (Al mazán and Burgo de Osma), because ongo ing, reliable information is only available for them. The analysis focuses on the changes in resin production per tree throughout the pe riod in which this activity took place. Tab. 4 shows the variance analysis, which has high significance levels. The p-values are far be low 0.05, indicative a high level of variabi lity among the forests and within the forests for each group. Resin production is stable in the Almazán group, while it peaks during the fifties in the Burgo de Osma forest (Fig. 4). In both cases, resin production stabilizes at around 2.5 kg/tree. After the seventies, pro duction levels dropped due to decreasing tapping profitability and to the introduction of synthetic resins in the market. iForest (2010) 3: 33-38 36 © SISEF http://www.sisef.it/iforest/ Fig. 3 - Standing volume evolution: (a) group of Almazán, (b) Burgo de Osma and (c) group of Albarracín. Tab. 4 - Univariate analysis of variance for resin production. Group Effect Sum of squares Degrees of Freedom Mean Square p-value Almazán Forest 2.436 3 0.812 0.006 Revision 0.564 7 0.080 Error 0.426 21 0.020 Burgo de Osma Forest 0.089 3 0.029 0.000 Revision 8.772 6 1.462 Error 0.587 18 0.032 Long-term implications of traditional forest regulation methods in Spain Changes in allowable and actual har, vest volumes There are significant changes in both al lowable and actual harvest volumes. In ac cordance with Wilks’ Lambda criteria, as a result of the multiple variant analysis, there is a value of 0.0551 and a statistic of F < 0.0001, in which this figure is significant for three reasons: it allows us to interpret that the means among the groups are diffe rent, that there are significant variations among plan revisions, and that the changes in each of the variables are not the same (Fig. 5), with the exception of Burgo de Osma’s forests during the sixties. Results from the Burgo de Osma data set, obtained using Wilks’ Lambda criteria, are similar to those obtained for the Almazán group, in the sense that they have low Lambda values and the statistic F < 0.0001 remains significant. These results suggest that the changes in these three variables do not share a common pattern, despite their in crease over time. Standing and harvest (both actual and allowable) volumes increase gradually, but they do not follow the same trend or have the same fluctuations in each period (Fig. 5). This situation simply demon strates the difficulties that managers face when carrying out organization projects and their constant revisions. Discussion Our study, focused on a wide set of indica tors, shows the importance of taking the multidimensionality of forest management effects into account. Timber production, stand density, and non-timber products (re sin) have been analysed. Our main finding is that the long-term nature of forest manage ment leads to a steady implementation of its prescriptions, because the forest and forestry need time to adapt to a new situation when goals and procedures change dramatically. Joint monitoring, with experimentation and evaluation, is a core element in adaptive management. It should be considered as a key element in management plans because foresters can implement sound practices ori ented to previously-defined goals by gathe ring information from the past forestry acti vities. In the forest areas analysed, stakeholder goals have shifted in the last few decades to a multi-objective approach because of the disappearance of the resin market, the de crease in firewood demand, the restrictions imposed on grazing (which helps the success of regeneration), and the increase in use of forests as recreational areas. The last three are directly connected with migration from rural areas to urban and industrialized areas (which occurred mostly during the sixties and seventies in Spain). In many cases, we can observe a gradual increase in different indicator figures, such as standing volume, the number of merchantable trees, and allow able and actual harvest volumes. However, in some cases the information recorded in the various organization projects has been insufficient to evaluate these forests, even with the most characteristic organization variables. We have only a rough idea of key management activities implemented in the past. In many cases, plan guidelines have been abandoned, either because they did not achieve what was proposed or the main ob jective simply changed, making it necessary to set up a new organization project. In addi tion to the aforementioned difficulties, his torical evaluations of organizations do not possess reliable economic information. This information is sometimes limited to obtai ning economic indicators reflecting the ex pected profitability of these forests in terms of wood and resin production. However, re levant knowledge can be acquired from dif ferent sources, including less-formalized ap © SISEF http://www.sisef.it/iforest/ 37 iForest (2010) 3: 33-38 Fig. 5 - Changes in standing volume, allowable and actual harvest volume in the group and by forests of Almazan (a) and Burgo de Osma (b). Fig. 4 - Change in resin production: (a) group of Almazán and (b) group of Burgo de Osma. Bravo F et al. - iForest 3: 33-38 proaches (Davis et al. 2001, Lindenmayer & Franklin 2002) that complement information from well-designed experiments and moni toring programs. Studies like the one we present here are based on historical data recorded over a long period. It is important to stress that data management is an obscure task that normally receives little attention because managers and scientists do not obtain career rewards from doing this. Likewise, little effort has been devoted to adequate analysis of histo rical management data. As Lindenmayer & Franklin (2002) state, this situation leads to a loss of long-term environmental and mana gerial information, weakening both forestry research and practice. Analysis of variance using repeated mea surements makes it possible to group the forests and compare their measurements for each of the variables to evaluate. For the three groups of forests studied, they display a distribution between similar measurements and change curves both in merchantable feet and in standing volume that all resemble one another. All the groups begin at very low levels and reach a maximum level in the same period (the decade of the fifties) until stabilizing. In accordance with the results for the three forest groups, they display a high significance compared to the variability among and within the forests. We can also observe that there is greater significance after every second revision, showing that the effects of forest plan prescriptions are not found until two regeneration periods have taken place (40 years in this case). There are a limited number of long-term studies focused on management plan impact over more than 50 years. The limited span of these studies is due to the difficulty in ob taining long-term, reliable, and comparable data sets. Charnley et al. (2008), examining the effects of the Northwest forest plan on the well-being of local communities, used a 30-year span (from 1979 to 2003). Conclusions It is important to stress the fact that tradi tional organization methods are valid tools for managing forests, as reflected by their benefits in making ongoing usage and im provement of forest stands compatible. It is important to implement organizations and their revisions in a sustained manner over time, for at least the medium term (40 years or more), because time is needed to obtain full responses to management prescriptions. Many concerns regarding susceptibility and vulnerability to global change of different forest values (timber, recreational, bio diversity, and so on) have been related to management practices. Further research is needed to understand the complex, long-term impact of forest management fully. Acknowledgements This paper has been made possible through different research projects financed by the Spanish Research Ministry (project codes AGL-2001-1780, AGL2004-07094-C02-02, and AGL2007-65795-C02-01). L.F. Osorio acknowledges support from the AECID (Agencia Española de Cooperación Inter, nacional y Desarrollo). References Bravo F, Bravo-Oviedo A, Díaz-Balteiro L (2008). 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