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Peringer, A., S. Siehoff, J. Chételat, T. Spiegelberger, A. Buttler, and F. Gillet. 2013. Past and future landscape dynamics in pasture-woodlands of the Swiss Jura Mountains under climate change. Ecology and Society 18(3): 11.
, part of a special feature on Sustainable Land-Use Practices in Mountain Regions: Integrative Analysis of Ecosystem Dynamics Under Global Change, Social-Economic Impacts, and Policy Implications
Past and future landscape dynamics in pasture-woodlands of the Swiss Jura Mountains under climate change
1Ecole Polytechnique Fédérale de Lausanne EPFL, School of Architecture, Civil and Environmental Engineering ENAC, Laboratory of Ecological Systems ECOS, Switzerland, 2University of Stuttgart, Institute for Landscape Planning ILPO, Germany, 3Aachen University RWTH, Research Institute for Ecosystem Analysis and Assessment (gaiac), Germany, 4Microgis Foundation for Spatial Analysis MFSA, St-Sulpice, Switzerland, 5IRSTEA, Research Unit Mountain Ecosystems, Saint-Martin dHères, France, 6Swiss Federal Institute for Forest, Snow and Landscape Research WSL, Switzerland, 7Université de Franche-Comté CNRS, Chrono-environnement, France
Silvopastoral systems are traditional components of the landscape in the Swiss Jura Mountains, and are promising approaches for the sustainable management of mountain areas worldwide. Due to complex vegetation dynamics, pasture-woodlands are very vulnerable to the currently occurring land use and climate changes. Therefore, management requires integrative long-term predictions of successional trends.
We present a refined version of the spatially explicit, dynamic simulation model WoodPaM with improved climate sensitivity of simulated vegetation. We investigate pasture-woodland dynamics by applying an innovative combination of retrospective simulations starting in the Middle Ages with prospective simulations following two climate change scenarios. The retrospective simulations demonstrate the strong dependency of the landscape mosaic on both climate and management. In high elevation mountain pastures, climate cooling during the Little Ice Age hindered simulated tree regeneration and reduced forage production of grasslands. Both led to an increase in open grassland and to a structural simplification of the landscape. In turn, climate warming afterwards showed the opposite effect. At lower elevations, high cattle stocking rates generally dominate simulated succession, leading to a slow development of quite homogenous landscapes whose structures are hardly affected by historical climate variability. Aerial photographs suggest that logging and windstorms critically shaped the current landscape, both homogenizing mosaic structures that emerge from selective grazing. Simulations of climate change scenarios suggest delayed but inevitable structural changes in the landscape mosaic and a temporary breakdown of the ecosystem service wood production. The population of currently dominating Norway spruce collapses due to simulated drought. Spruce is only slowly replaced either by beech under moderate warming or by Scots pine under extreme warming. In general, the shift in tree species dominance results in landscapes of less structural richness than today. In order to maintain the mosaic structure of pasture-woodlands, we recommend a future increase in cattle stocking on mountain pastures. The (re-) introduction of mixed herds (cattle with horses, sheep, and goats) could mitigate the simulated trend towards structural homogenization of the forest-grassland mosaic because diverse browsing effects selectively control tree regeneration and would counteract simulated forest encroachment. This could prevent the loss of species-rich open grasslands and forest-grassland ecotones. Forest management should respect forest-grassland mosaics and ecotones by following the traditional selective felling of single trees instead of large clear-cutting. Additionally, beech regeneration should be promoted from now on in order to smoothen tree species replacement with warming and to ensure the continuous provision of forest ecosystem services.
Key words: climate warming; landscape structure; management; pasture-woodland; silvopastoral system; species shift; retrospective simulation; vegetation dynamics
In most developed countries, the intensification of agriculture has generally resulted in landscape changes from small-grained heterogeneous patterns towards more monotonous and mono-functional landscapes (Brandt 2003). Particularly, the multifunctional pasture-woodlands of Western Europe are threatened by a strong tendency towards segregation of land use (van Lier 1998), which leads to the spatial and functional isolation of large patches of intensively used grasslands from forests. Pasture-woodlands are ecosystems of high conservation value (Olff and Ritchie 1998, Svenning 2002, Poschlod et al. 2005, Kleyer et al. 2007) and serve as refuge habitats for threatened species because of their richness in vegetation types embedded into a dynamic forest-grassland mosaic. Furthermore, silvopastoral systems are among the most promising approaches for sustainable management of mountain areas worldwide (Etienne 1996, Mosquera-Losada et al. 2005).
Pasture-woodlands are dynamic systems with progressive and regressive successions being driven by the grazing impacts of cattle. The landscape structure is a dynamic patch-mosaic of different successional stages between grassland and woodland (Olff et al. 1999, Mouissie et al. 2008). In mountain pastures, vegetation dynamics are very slow because of the longevity of trees and the harsh climate. As a consequence, the impact of management actions and climate change on landscape structure may be delayed for decades. Nevertheless, due to the complex interactions that drive the dynamics of the grassland-forest mosaic, pasture-woodlands are supposed to be very vulnerable to the currently occurring coincidence of land use and climate changes (Buttler et al. 2009). Overall, livestock stocking rate and climate both directly influence the establishment of trees, and their regeneration is a critical stage in grazed systems (Vandenberghe 2006, Vandenberghe et al. 2007, 2008) that determines landscape structure in the long run. Moreover, climate also determines forage production of the herb layer, which influences the spatial pattern of cattle activities such as grazing, dunging, trampling, and browsing (Kohler et al. 2006). Considering such feedback loops and the unknown balance of counteracting processes, integrative long-term predictions of successional trends are required to support management decisions (Swetnam et al. 1999).
In the Swiss Jura Mountains, pasture-woodlands represent a traditional form of seminatural landscape that is rich in biodiversity (Gillet et al. 1999, Dufour et al. 2006). They depend on multiple and extensive land use, including cattle stocking (cows, heifers, and calves) and logging, that is still practiced or has been practiced until recent times. During the last century, socioeconomic constraints on agriculture in mountain areas have changed considerably, which has affected both qualitative and quantitative aspects of silvopastoral management (Huber et al. 2013) and landscape structure (Chételat et al. 2013). However, time series of aerial photographs can show only the combined consequences of recent land use extensification and climate warming, i.e., forest encroachment (Béguin 2007) and homogenization of the grassland-forest mosaic during the last century (Chételat et al. 2013). The impacts of these two factors have not been disentangled yet.
Thanks to many previous research projects on pasture-woodlands in the Jura Mountains, the interactions between vegetation patterns, silvopastoral management, cattle activity, grassland dynamics, and tree regeneration are well known (Gillet and Gallandat 1996, Perrenoud et al. 2003, Buttler et al. 2009). The spatially explicit dynamic model WoodPaM (Gillet 2008) incorporates most of the processes necessary to predict the long-term impact of silvopastoral management and temperature increase on plant communities and cattle habitat use in heterogeneous pasture-woodlands. However, combined impacts of climate change (temperature rise, precipitation shift from summer to winter, and drought) were not implemented into this model, and simulation studies based on explicit time series of historical and projected climate have not been carried out yet.
We report recent refinements of WoodPaM that have been made to investigate climate change impacts on pasture-woodlands more explicitly. Based on the forest landscape model LandClim (Schumacher et al. 2004, 2006, Schumacher and Bugmann 2006), the climate sensitivity of simulated tree species has been improved. Based on newly established regressions, the delayed response of herb layer succession and productivity to climate warming has been implemented. With a retrospective analysis of the development of the grassland-forest mosaic in four pastures of the Jura Mountains since the Middle Ages (1100 AD) (Sjogren 2005, 2006), we assess the causal relationships between dynamics of land use, climate, and vegetation. Through simulations of two future scenarios of climate change (A1FI and B2 according to IPCC SRES 2000) we pinpoint future successional trends and the sensitivity of pasture-woodlands to climate change impacts. From this innovative combination of retrospective and prospective simulation experiments (Leeuw et al. 2011), we derive guidelines for future adaptive management of silvopastoral systems that aim at the maintenance of the landscape mosaic.
WoodPaM (Gillet et al. 2002, Gillet 2008) is a spatially explicit model of pasture-woodland dynamics that is able to simulate the emergence of a semi-open landscape structure from the interactions between vegetation and large herbivores (cattle). During simulations, selective foraging of cattle causes local impacts on vegetation (grazing, browsing, trampling, and dunging), which in turn and together with the natural successional trend of forest development drive the dynamics of the landscape structure (Fig. 1).
In WoodPaM, a pasture is represented by a mosaic of square grid cells, each 25 m wide. Each cell features three landscape compartments, which are represented as submodels: the herb layer (consisting of four ecological community types: eutrophic pastureland M, oligotrophic pastureland L, fallow F, and understory U), shrubs (gathering all kinds of mostly thorny shrubs, which are frequent on pastures and provide safe sites for tree recruitment through protection from browsing), and tree populations (13 species, divided into four life stages: seedlings, saplings, small trees, and big mature trees). At this cell level, another submodel represents local herbivore impacts (grazing, browsing, dunging, trampling). The intensity of herbivore impacts depends on the attractiveness of the grid cell for cattle. Local succession in the herb layer is driven by local intensity of grazing, trampling, dunging, and shading. Local woody plant succession is driven by seeding input, safe-site availability in the herb layer, and browsing intensity. Local successions within cells are influenced by neighboring cells through seed dispersal of trees (considering only the four adjacent cells orthogonally to the central cell) and are connected at paddock level by cattle behavior. Selective habitat use by cattle among cells within each paddock considers the attractiveness of each cell, which depends on local forage production, distance to watering points, tree cover, and geomorphology (slope, rock outcrops). We compute a local livestock density, which is high in attractive cells and leads to strong grazing impacts there, from which in turn treeless eutrophic pastures emerge. In unattractive cells, for example, on steep and rocky slopes, trees and shrubs can regenerate.
In order to simulate the dynamics of pasture-woodlands for alternative scenarios of climate change, we developed a climate submodel and refined the tree submodel, both based on the forest landscape model LandClim (Schumacher et al. 2004, 2006, Schumacher and Bugmann 2006). Climate impacts on tree establishment and growth in semi-open landscapes have been calibrated to the suboceanic climate prevailing in the Jura Mountains (Appendix). We introduced a delayed response of herb layer succession and forage production, taking into account that grassland plant communities slowly adapt to the general trend of temperature rise rather than spontaneously adapt to temperature fluctuations in single (warm) years.
For a detailed description of the first version of the model, we refer to Gillet (2008). All model refinements are presented in the Appendix.
Design of simulations
We applied the model to four pastures in the Jura Mountains. The study sites are located at different altitudes (1200 to 1350 meters above sea level [m a.s.l.]) and differ in land use history and current land use intensity (livestock management, logging) due to natural (topography) and socioeconomic constraints (distance to the closest village, proprietary). We performed a retrospective analysis of vegetation dynamics in the pastures since the Middle Ages (far past) using the results of a palaeoecological study in the same region (Sjogren 2005, 2006), as well as historical time series for stocking and climate (Moberg et al. 2005). We verified the model by comparing the outcome of our simulations with aerial photographs taken between 1934 and 2000 (for image processing, see Chételat et al. 2013). In one pasture, where historical data on logging and windstorms were available in the near past, we assessed the importance of logging and catastrophic events (storms) in explaining landscape structure.
Based on this thorough evaluation of our modeling approach, we simulated two future scenarios of climate change (IPCC-SRES scenarios A1FI and B2), assuming the maintenance of current constraints on cattle stocking, in order to assess the sensitivity of the silvopastoral systems to climate change impacts.
For the study sites, geomorphology (altitude, slope, curvature of the landscape surface, and rock outcrops) and farming constraints (stone walls and fences of pastures and paddocks, watering points) were arranged in a geographical information system and used to initialize the model.
While in historical simulations the pastures were simulated as a single management unit, for future scenarios, the pastures were divided into paddocks and a rotational grazing system was simulated because it has been the practice used since the 1970s. Table 1 summarizes this hierarchical structure of management units and provides information on historical and recent stocking rates used in simulations. Today, pastures are subdivided into sectors for each cattle herd, and each sector is subdivided into several paddocks. Stocking densities are expressed in livestock units (LU) per hectare, stocking periods in days per year, and stocking rates in LU day ha-1
(Allen et al. 2011).
Information on logging and windstorms was available only for the pasture Rionde (Table 2).
Time series of climate variability
For retrospective simulations in the far past, we used a reconstructed climate time series (Fig. 2) starting from the Middle Ages (1100 AD) until today (2000 AD), which was compiled from two different sources. From 1100 until 1900 AD, a dendrochronological reconstruction of Northern Hemisphere temperature deviations from the 1961–1990 mean with yearly resolution was used (Moberg et al. 2005). In order to achieve monthly values required for simulation, seasonal temperature variability was generated based on regionalized monthly observed values of temperature from 1901 until 2000 (MPI-M 2006, 2009; D. Schmatz unpublished manuscript
). Precipitation from 1100 AD until 1900 was a stochastically generated time series based on the observed data for the 1901–2000 period (monthly means and corresponding standard deviation). From 1901 on (near past), we explicitly used regionalized monthly observed values of temperature and precipitation. Future climate scenarios (Fig. 3) started with regionalized monthly temperature and precipitation time series for the 2001–2100 period according to two IPCC emission scenarios (A1FI and B2 [see IPCC SRES 2000, D. Schmatz unpublished manuscript
]). Afterwards, a stochastic climate time series was generated based on the means of temperature and precipitation of the 2091–2100 period of each scenario and corresponding standard deviations. The two climate change scenarios pinpoint two extreme possible futures of our world: a fuel-intensive future with drastic warming (A1FI, approximately +8 K from 2000 to 2100 [Fig. 3]), and a moderate development with less warming (B2, approximately +4 K from 2000 to 2100 [Fig. 3]). We assume the real development to be situated in the range between the two scenarios.
Observed climate data and climate change scenarios are based on regionalized climate data (data points in a grid of 100 m width). From this data set we use climate time series of one reference point located close to the study sites, while the climate submodel corrects for fine scale differences in altitude and exposition (slope, aspect) due to local conditions in the study sites.
Landscape history in study sites
Mosaic development since the Middle Ages (far past)
We performed a retrospective analysis of the historical emergence of the grassland-forest mosaic of the pastures between 1100 AD and 2000 AD. The study design disentangles the synchronous effects of climate and land use change based on different rates of change of these two main drivers of vegetation dynamics. We simulated the development of the grassland-forest mosaic starting from an assumed clearing (random distribution of seedlings of all simulated tree species but no juvenile or adult trees). In order to separate the influence of climate variability and land use change on vegetation dynamics, we assumed a very slow and linear increase in stocking density between 1100 AD and 1900 AD (Table 1), whereas (reconstructed) climate time series is based on yearly fluctuating mean annual temperatures from 1100 AD on (Fig. 2). In the twentieth century, observed stocking densities and observed climate applied. Logging and windstorms were not considered due to the lack of data over this long period of time.
By this means, our simulation results show the effects of historical climate variability (climate cooling during the Little Ice Age and warming afterwards, both within a few centuries) under contemporary but quasi static constraints: low stocking densities in the Middle Ages increase to observed densities very slowly (approximately 0.05 LU/ha in a century) based on assumed continuous improvements in agricultural practice, such as fertilization. Subsequently, when interpreting the simulation results, any shifts in the landscape mosaic within a time frame of centuries can clearly be dedicated to climatic fluctuations because simulated land use hardly changed over such a period of time. This procedure of disentangling two synchronously acting drivers of vegetation dynamics is additionally supported by the fact that historical climate change consists of alternating periods of cooling and warming and thus triggers corresponding regressive and progressive successions, while land use change is simulated as increasing unidirectionally, from which only one kind of system dynamics can emerge (either regressive or progressive—in our case, regressive). We compared the outcome of simulations to aerial photographs from the year 2000 (for details, see the Appendix).
Recent history of grazing, logging, and windstorms in the Rionde pasture (near past)
We used the synthesis of land use change and landscape structural development provided by Chételat et al. (2013) to further detail the retrospective analysis for the twentieth century (1934 AD—2000 AD) and disentangle the effect of grazing from the combined effects of logging and windstorms. We simulated the development of the grassland-forest mosaic of the pasture Rionde based on a time series of grazing and climate alone and compared the results to aerial photographs taken in 1934, 1954, 1968, 1985, and 2000. Simulation started from the observed state of the forest-grassland mosaic in 1934. A model spin-up from 1900 until 1934 was performed and simulation proceeded until 2000. During spin-up, the model approximated the full demography of forest stands, especially the abundance of juvenile trees (old trees had been initialized from the aerial photograph interpretation) and the spatial configuration of herb layer compartments. Both processes reasonably reached equilibrium states within a few decades. The difference between simulated and observed pattern was interpreted as the impact of logging and windstorms on the mosaic development.
Climate change scenarios for study sites
We investigated successional dynamics in the pasture mosaics in the light of two climate change scenarios (B2 and A1FI, Fig. 3) and under the current grazing regime alone (without considering logging or windstorms). Stocking rates and the rotational system of cattle herding were fixed to the values and structures that had been observed around the year 2000 (Table 1). Accordingly, our simulations assumed agronomic business-as-usual, and undesirable simulation outcomes would reflect the need for adaptive management to face climate change. Simulation started from the observed state of the forest-grassland mosaic in 2000. Again, a model spin-up from 1950 until 2000 was performed. Simulations proceeded until calendar year 3000 in order to investigate long-term successional trends towards hypothetical equilibrium states of the system.
Maps of simulated landscapes
Although WoodPaM tracks the number of trees per grid cell, for visualization of landscape mosaics, four phytocoenosis types are used, which represented main plant communities associated with classes of tree cover (Table 3).
Comparison of long-term simulations with observed patterns in the year 2000 (far past)
The reconstruction of the current pasture-woodland mosaic from climate and grazing history since the Middle Ages delivered plausible results in the pastures Planets-Cluds, Bullatonne, and Pré aux Veaux, however not in Rionde (Fig. 4). In Planets-Cluds, Bullatonne, and Pré aux Veaux, the simulated landscape structure fitted aerial photograph interpretation in the year 2000 generally well: we found densely wooded pasture in the less frequently grazed corners of the pastures and along ridges of rock outcrops.
When comparing proportions of phytocoenosis types (for definitions, see Table 3) between simulated and observed landscapes, some important differences appeared, however (Table 4). Simulations of the Rionde pasture created far too densely wooded areas as compared to the observed pattern. In the pastures Planets-Cluds, Bullatonne, and Pré aux Veaux, the model simulated generally too much sparsely wooded pastures at the cost of unwooded pastures.
Landscape structural dynamics in the far past
The emergence of a well expressed pasture-woodland mosaic from open grassland alone is a process that takes centuries (Fig. 5). In our simulations, it took 100–150 years for the first densely wooded pasture to appear after clearing, which left tree regeneration to start from a random distribution of seedlings. The development of a complete mosaic consisting of some unwooded pastures, 75% of sparsely wooded pastures, 20–25% of densely wooded pastures, and some grazed forests (the latter in Pré aux Veaux and Rionde only) took about 400 years in extensively grazed mountain pastures (Bullatonne, Pré aux Veaux, and Rionde) and even longer under intensive grazing (Planets-Cluds).
In the extensively grazed mountain pastures (Bullatonne, Rionde, and Pré aux Veaux), forest regeneration (progressive succession) stopped around 1500 AD with the beginning of the Little Ice Age. During the following centuries of cool climate, the cover of densely wooded pasture decreased (regressive succession). Sparsely wooded pastures dominated the pattern and open grasslands slightly increased. Recent climate warming from 1900 on triggered the inverse development in Rionde and Pré aux Veaux, i.e., densification of sparsely wooded pastures and forest encroachment at the cost of unwooded pastures. In Bullatonne, stocking remained constant during this period, and recent climate warming of 1 K promoted progressive succession only slightly. In Pré aux Veaux and Rionde, however, temporary extensification during the middle of the twentieth century, additional to warming (Table 1), led to a drastic increase in tree cover.
The intensively used pasture Planets-Cluds at lower altitude behaved differently: historical climate variability (warm Middle Ages vs. cool Little Ice Age) had little impact on the slow but continuous emergence of the mosaic. Landscape structural dynamics were governed by the slow increase in stocking density, which demanded a closely coupled increase in the cover of open grasslands. Interestingly, a slow but continuous segregation of the landscape into grassland and densely wooded pastures at the cost of sparsely wooded pastures was simulated.
As shown by the landscape aggregation index AIL (Fig. 5), the forest regeneration and mosaic development until 1500 AD increased landscape structural diversity, indicated by a low AIL. During climate cooling, landscape structural diversity remained constant (Pré aux Veaux, Rionde) or decreased due to regressive forest succession (temporary increase in AIL in Bullatonne and Rionde). Recent climate warming from 1900 on positively contributed to landscape diversity, as the segregation into grassland and forest did in Planets-Cluds.
Effects of logging and windstorms in the twentieth century (near past)
When comparing simulated landscape dynamics during the twentieth century in the pasture Rionde with corresponding aerial photographs (Fig. 6), it became obvious that grazing and recent climate warming alone were not able to explain the currently observed open state of the pasture. The simulations showed a slow maturing of the landscape mosaic to a diverse pattern following geomorphological conditions and driven by cattle behavior (forest developed in the corners of the pasture, along fences and on rocky ridges). To the contrary, aerial photographs showed a decrease in forest cover from 1934 until 1968 and from 1985 until 2000, with an increase in between. Data on logging and windstorms (Table 2) from Chételat et al. (2013) showed that in these two time intervals, the landscape development was disturbed, the landscape mosaic was homogenized, and the diversification trend shown by simulations was overrun. In between (1968–1985), reduced logging intensity and extensification of grazing was followed by forest encroachment (Chételat et al. 2013), which allowed the emergence of a typical mosaic pattern again. Recent logging between 1985 and 2000 in the north of the pasture did not wipe out this pattern completely, as storms and intense logging did in the first half of the twentieth century. Forest cover was reduced selectively, turning dense (old) stands of trees (grazed forest) into lighter stands (densely wooded pasture). In the south of the pasture, densely wooded pasture was allowed to mature to grazed forest.
Climate change scenarios
The simulations showed that the future landscape structure and vegetation types strongly depend on the degree of simulated climate warming (Fig. 7, Fig. 8; cattle stocking densities were held constant during simulations), while until 2150, only minor changes occurred with transformations of unwooded pastures into sparsely wooded pastures (Bullatonne and Planets-Cluds) and densification of densely wooded pastures into grazed forests (Pré aux Veaux). This initial phase of forest encroachment was initiated by both the contemporary climate warming from 1950 until 2000 during model spin-up and the climate warming during the first half of the twenty-first century, which facilitated establishment and growth of spruce. Afterwards, successional trajectories diverged according to the degree of warming. By this period, in the moderate scenario B2, sparsely wooded pastures transformed into densely wooded pastures. Grazed forests developed centuries later (2400 in Bullatonne, 2450 in Planets-Cluds), with the extensive mountain pasture Pré aux Veaux being the exception (2150). In the extreme warming scenario A1FI, however, forests as well as densely wooded pastures decreased after 2150 and sparsely wooded pastures temporarily dominated (around year 2200). From 2250 on, densely wooded pastures developed to high cover. In both scenarios, progressive forest succession took place in the long run, however towards very different forest types and landscape patterns: in the B2 scenario, a segregation into grazed forest and sparsely wooded pastures with some open grasslands took place, while in the A1FI scenario, grazed forests and open grassland did not develop even after 1000 simulation years (calendar year 3000) and the mosaic turned into a homogenous landscape consisting of densely wooded pastures only (Fig. 8).
This purely climate-driven dynamic was best expressed in the subalpine pasture Bullatonne, while in the extensively grazed subalpine pasture Pré aux Veaux, at higher elevation geomorphological conditions triggered some initial fluctuations (see the sensitivity analysis in the Appendix for details). In the pasture Planets-Cluds at lower altitude and with high grazing intensity, forest encroachment was much slower, but regressive succession in the scenario A1FI occurred at a similar speed. Here, a comparatively large proportion of unwooded and sparsely wooded pastures was maintained.
Between the years 2100 and 2500, a shift in tree species composition driven by climate change took place, which came with a temporary decrease in cover of trees producing high quality wood (Fig. 9). In all scenarios, Norway spruce (Picea abies
) declined from around 2100 on, and its decline was accelerated with warming. In the moderate scenario B2, spruce was replaced by beech (Fagus sylvatica
) from year 2100 on. However, it took several hundred years before beech reached significant cover at landscape scale because beech is currently hardly present in the landscape, its migration rate is slow, and its establishment is hindered by browsing. The sensitivity of the establishment of beech to stocking densities is shown by the extensively grazed mountain pasture Pré aux Veaux, where beech could reach significant cover quite early. To the contrary, the immigration of beech was slowest in the intense pasture Planets-Cluds. During the temporal gap in the transition from spruce to beech forests, rowan berry (Sorbus aucuparia
) as a pioneer tree dominated the landscape. In the extreme warming scenario A1FI, spruce was replaced by Scots pine (Pinus sylvestris
) comparatively rapidly due to its high dispersal rate, and rowan berry failed to form a transient phase of forest succession. Interestingly, the cover of pine at the landscape scale shows fluctuations in the long run, which indicates a shifting mosaic cycle. Although the landscape appeared to be dominated by densely wooded pastures in Fig. 7, the cover of pine was only 20–30% (Fig. 9). In the mountain pastures (Bullatonne and Pré aux Veaux), pine was accompanied by downy oak (Quercus pubescens
) and additionally by beech in Pré aux Veaux, where climate is a little cooler because the site is at a higher altitude above sea level.
The landscape structural heterogeneity generally decreased in climate change simulations, as indicated by a higher landscape aggregation index in Fig. 7. Ongoing tree species shift temporarily increased landscape heterogeneity (around 2100 in the moderate scenario B2 due to the spread of rowan berry and around 2300 in the extreme scenario A1FI due to the establishment of pine). In the long run, the segregation of beech forest from sparsely wooded pastures with some open grasslands increased landscape heterogeneity again to some extent in the moderate scenario B2, while in the extreme warming scenario A1FI, the development of homogenous pine forests generally simplified the landscape pattern.
Model refinements as listed in the Appendix allowed for the successful reproduction of natural vegetation and of grazing impacts on tree species composition and landscape structure (results of calibration will be shown in detail in a further paper), which demonstrates the capability of WoodPaM to adequately simulate vegetation zonation along multifactorial gradients of altitude, microclimate, and grazing intensity.
However, two major limitations of the model remain that have to be considered when interpreting the simulation results presented. First, retrospective simulations of the pasture Rionde (near past: 1900–2000) show that logging and windstorms play an important role in shaping landscape patterns and closely interact with simulated successional trends of pattern formation under grazing. Hence, the understanding of historical and future landscape dynamics would profit from the consideration of a further extended multiple disturbance regime, i.e., windstorms and logging in addition to grazing and drought, which both are currently implemented in the model (drought impacts on tree species only). By this means, we would cope with current research needs (Seidl et al. 2011). We recently investigated the role of logging and promoted tree species migration through forest management on landscape structural dynamics from a strategic perspective (Gillet and Peringer 2012) and found that both have the potential to mitigate climate change impacts, when carefully applied. In this study, however, we simulated vegetation dynamics driven by grazing alone in order to learn about the importance of logging and windstorms from the poor reproduction of open grassland in the historical analysis.
Furthermore, when interpreting simulation results, the indirect effects of logging have to be kept in mind: logging is the traditional complementary land use in pasture-woodlands, and isolated trees in sparsely wooded pastures have been preferably felled for fire wood for reasons of low effort and improvement of forage production of the pasture. Cattle then focus on such cleared area, which in turn allows the development of dense stands in subsequently neglected areas (rocky sites, slopes, corners of the paddocks). Logging therefore might explain both the general underestimation of forests and the overestimation of sparsely wooded pasture by simulations. To the contrary, windstorms show a homogenizing effect on landscape structure and can wipe out such patterns of grassland-forest segregation (results from the near past in the pasture Rionde).
The second limitation is the overestimation of forage production because we did not consider drought effects on the primary production of grasslands. Drought periods are supposed to heavily impact forage provision in the future (see Gavazov et al.  for a study in which we addressed this issue explicitly). Such an overestimation of productivity promotes forest encroachment because the overall browsing pressure is reduced (Vandenberghe et al. 2007, 2008, 2009). We tried to balance this bias with an assumed slow adaptation of the productivity of the herb layer to the increasing length of the growing season. However, assuming grassland communities to be adapted to the climate of the past 50 years is a rough estimate, which has not been verified yet. The community-based modeling of the herb layer by WoodPaM does not explicitly consider plant species or plant functional groups (as, for example, in the GraS model by Siehoff et al. 2011). Subsequently, a rebuild of new plant grassland communities that form under future climatic conditions cannot be simulated similarly to tree communities in a mechanistic way.
Sensitivity of pasture-woodlands to land use intensity and climate change
Under stable climatic conditions, grazing pressure and tree cover are negatively correlated: pasture extensification reduces browsing pressure and leads to densification of forested areas and the loss of grassland (Pré aux Veaux and Rionde in the second half of the twentieth century in Fig. 5 [near past], Vandenberghe 2006, Peringer and Rosenthal 2011, Siehoff et al. 2011), whereas intensification suppresses tree regeneration and grasslands emerge. Once established, grasslands persist because they represent the main forage resource for cattle (Planets-Cluds in retrospective simulations of the far past). These processes take a long time under mountainous conditions: diversely structured mosaic pastures emerge very slowly due to self-organization processes under extensive grazing acting over centuries (far past [Olff et al. 1999, Mouissie et al. 2008]) and because regressive succession driven by grazing alone is slow due to the high maximum age and the enduring capability of adult trees (Olff et al. 1999, Van Uytvanck et al. 2008). To the contrary, clear-cuts and windstorms homogenize mosaic patterns very rapidly (near past, pasture Rionde). As long as small patches of woodland remain after such disturbance, however, forest encroachment can be comparatively fast in a window of opportunity of temporary extensification (Pré aux Veaux and Rionde in the second half of the twentieth century in Fig. 5 [near past], Rionde in the period after intense logging during the World War II in Fig. 6 [near past] [Keddy 1992, Allen et al. 2011]).
Slight climate fluctuations, as they happened in the far past (Little Ice Age), trigger forest progression and regression, but to a limited extent: climate cooling of approximately -1 K between the Middle Ages and the Little Ice Age (far past) led to a decrease in simulated tree cover and an increase in open grassland over centuries. Generally, reduced forage production increased overall grazing and browsing pressure, weakened tree regeneration, and—over decades and centuries—led to regressive succession. Climate warming of +1 K during the twentieth century initiated the opposite effect (Bullatonne, near past). Such tree cover fluctuations align with pollen data from a mire in the Amburnex Valley, where the mountain pastures Pré aux Veaux and Rionde are situated (Sjogren 2005, 2006).
From these interrelationships between grazing and browsing pressure, climate, and tree cover dynamics, one might conclude that enhanced grazing and browsing pressure could be an opportunity to mitigate future climate change impacts, i.e., the simulated forest progression following future climate warming. Increased cattle stocking densities might counteract the projected loss of species-rich mountain grasslands. However, all patterns on which the comparative historical analysis is based share the same general structure and tree species composition of pasture-woodlands, i.e., a mosaic of unwooded pastures and spruce forests connected by ecotones of sparsely and densely wooded pastures. Simulations of future climate change vividly show qualitative and inevitable changes in landscape structure resulting from a shift in tree species triggered by future climate warming, which is far beyond historical temperature fluctuations in both climate change scenarios (~4 K in scenario B2 and ~8 K in scenario A1FI in contrast to ~1 K in the near and far past). Moderate warming (B2) forces segregation of the pasture-mosaic into grazed forest and sparsely wooded pasture or grassland. Densely wooded pastures widely disappear. Landscape diversity and related biodiversity subsequently decrease. Extreme warming homogenizes the landscape pattern to an almost uniform distribution of densely wooded pastures. Even though densely wooded pastures imply a high structural diversity in themselves because they represent the structure of semi-open landscapes in a fine-grained way, biodiversity will lack species that require large patches of open grassland or closed forest (Lederbogen et al. 2004).
The processes behind the formation of these patterns are (1) the drought-induced collapse of currently dominating stands of spruce and (2) its slow replacement by beech following moderate warming or by Scots pine with some downy oak following extreme warming. After moderate warming of ~4 K, temperature levels in the Jura Mountains correspond to current lowland conditions (500 m a.s.l., assuming an elevation lapse rate of 0.5 K per 100 m in altitude), where beech is frequently dominant today. After extreme warming (~8 K), temperatures correspond to current sea level (e.g., La Rochelle in France at similar latitude), where pine and downy oak are currently found.
Climatic determinants of landscape structure differ between the past and the future: for the past, we found temperature was most influential, while in the future, we expect drought stress on the karstic Jura Mountains to select dominant tree species from the species pool. Although the yearly sum of precipitation is not expected to change significantly in the future (Fig. 3), seasonality and return frequencies of summer droughts are (Calanca 2007). Especially, summer droughts are causal for the decline of spruce and for the dominance of pine after extreme warming. Landscape structure changes according to the ecology of new tree species: landscape segregation in the scenario B2 is driven by the ability of beech to regenerate well in the shade of old stands, which are avoided by cattle (Smit et al. 2005) and where browsing intensity and drought stress is lower in comparison to sparsely wooded pasture or grassland. Landscape homogenization in the scenario A1FI is driven by the establishment of pine on grassland, where the high light requirements of pine seedlings for establishment are satisfied exclusively. The lack of pine regeneration in old stands is also the reason for shifting mosaic cycles that might appear in pine-dominated pasture-woodlands, as suggested by our simulations. In the very long run, such light pine forests might transform into forests of downy oak, as indicated in our simulations (Fig. 9) and recent studies in the Valais (Huber et al. 2013).
Implications for ecosystem services and management
Pasture-woodlands depend on a balance between the (natural) successional trend towards forest development and the oppression of tree establishment by browsing. Our simulations underpin the observed sensitivity of pasture-woodlands to land use change (Chételat et al. 2013) and provide a mechanistic explanation for climate change impacts. Most important, climate change scenarios demonstrate that future climate will cause qualitative shifts in the landscape mosaic regardless of grazing pressure (Huber et al. 2013). Although simulations of future climate do not predict immediate landscape structural changes of similar speed and magnitude as observed after recent extensification during the last century, this does not mean that pasture-woodlands are resistant to climate change. To the contrary, shifts in landscape structure, tree species composition, and thus ecosystem service provision are predicted to occur even more rapidly and drastically than known from the past after a delay of approximately 50 years.
Especially, the decline of spruce is probably underestimated in the model because we considered neither windstorms (which are supposed to occur more frequently in the future [Matulla et al. 2008, Swiss Climate Change Scenarios 2011]), nor bark beetle outbreaks. Outbreaks are known to be triggered by windstorms and drought (Schlyter et al. 2006), and trees are more susceptible to bark beetle attack under drought stress (Wermelinger 2004). Considering the current scarcity of beech and pine in the neighborhood of the pastures, the establishment rate of both species is probably overestimated. Both model uncertainties increase the probability of simulated forest breakdown in climate change scenarios. The temporal “gap” in the provision of the ecosystem service “quality wood production” might happen to an even larger extent than simulated. The promotion of beech establishment by forest management, however, has the potential to mitigate this effect (Gillet and Peringer 2012).
Regarding the ecosystem service “forage production,” seasonal scarcity of water, as indicated by the dominance of pine following extreme warming, will reduce grassland productivity in the future and demand a reduction in cattle stocking or an innovative management scheme (cisterns capturing snow melt water, adapted breeds). While stocking rate will need to be reduced in open and nowadays intensively used pastures exposed to summer drought and subsequently collapse of herb layer productivity (Gavazov et al. 2013), it will become crucial to enhance stocking rates in more encroached areas in order to maintain a diverse pasture-woodland mosaic in the light of climate change. The biodiversity of pasture-woodlands strongly depends on landscape structures (mosaics and interconnecting ecotones), which emerge exclusively from extensive grazing (Lederbogen et al. 2004, Peringer and Rosenthal 2009, Rosenthal et al. 2012). Direct human intervention (logging, shrub cutting) can maintain open grassland. Species-rich forest-grassland ecotones, however, are usually destroyed by pasture clearing. The segregation between grassland and forest, which is predicted for moderate climate warming, comes with a drastic loss of open pastures and their ecotones to forest. Both can be mitigated with enhanced browsing pressure. Because enhanced cattle stocking can also lead to soil compaction, and root and stem damage in forest, various grazers and browsers might provide an opportunity to promote regressive succession and avoid such negative effects (Jorritsma et al. 1999, Vera, 2000). While the historical landscape dynamics in Planets-Cluds (far past) show that segregation is not reversed but promoted by intense cattle grazing, browsing animals are able to thin forests to densely wooded pastures and clear sparsely wooded pasture to open grassland. Thus, we argue for (1) extended stocking periods adapted to enlarged future vegetation periods, and (2) the (re-) introduction of horses, sheep, and goats. The differences in forage selectivity between these herbivores and cattle would counteract fallow succession of grassland (especially, horses graze where cattle do not) and tree encroachment (browsing by sheep and goats) (Rook et al. 2004, Rosenthal et al. 2012). Both constitute an opportunity to maintain the desired heterogeneous landscapes with their biodiversity, which indeed calls for reforms in policies (Huber et al. 2013). However, considering the known sensitivity of Mediterranean pine forests to devastation by overgrazing (González-Martínez and Bravo 2001) and keeping in mind that real temperature increase most probably will be in between the two simulated scenarios, intensification of stocking should be done carefully in order to not hinder the immigration of new tree species, especially browsing-sensitive beech.
If stocking turns out to be generally limited due to future seasonal scarcity of water or socioeconomic constraints, forest management becomes more important for shaping landscape structure. Though, logging should support mosaic and ecotone structures, which again would need reforms in policies (Huber et al. 2013). Selective felling of single trees instead of large clear-cuts, however, can have contrasting effects depending on the forest type: while in pine forests, which emerged following extreme warming, landscape structure can be diversified, the mosaic of beech forest and grassland, which developed following moderate warming, can be simplified into a more homogenous landscape (Gillet and Peringer 2012).
In summary, our results clearly show that inevitable shifts in landscape structure have to be expected due to climate change. Adapted land use, however, offers opportunities to mitigate climate change impacts on pasture-woodlands. Policy development should focus on the provision of favorable socioeconomic conditions for the future practice of diverse livestock breeding in mountain pastures in combination with careful logging with respect to forest-grassland ecotones.
This work was supported by the CCES (Competence Center Environment and Sustainability of the ETH Domain, Switzerland) as part of the MOUNTLAND project and by the State Secretariat for Education and Research (SER C07.0112) in the framework of the COST Action FP0603. We also thank Konstantin Gavazov, Andreas Rigling, Rober Huber, Kathryn Lannas, Jean-Bruno Wettstein, and Michael Kalbermatten.
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