Dynamics of Forage Production in Pasture-woodlands of the Swiss Jura Mountains under Projected Climate Change Scenarios

Silvopastoral systems of the Swiss Jura Mountains serve as a traditional source of forage and timber in the subalpine vegetation belt, but their vulnerability to land use and climate change puts their future sustainability at stake. We coupled experimental and modeling approaches to assess the impact of climate change on the pasture-woodland landscape. We drew conclusions on the resistance potential of wooded pastures with different management intensities by sampling along a canopy cover gradient. This gradient spanned from unwooded pastures associated with intensive farming to densely wooded pastures associated with extensive farming. Transplanted mesocosms of these ecosystems placed at warmer and drier conditions provided experimental evidence that climate change reduced herbaceous biomass production in unwooded pastures but had no effect in sparsely wooded pastures, and even stimulated productivity in densely wooded pastures. Through modeling these results with a spatially explicit model of wooded pastures (WoodPaM) modified for the current application, results were extrapolated to the local landscape under two regionalized Intergovernmental Panel on Climate Change scenarios for climate change. This led to the suggestion that within the Jura pasture-woodlands, forage production in the near future (2000-2050 AD) would be affected disproportionately throughout the landscape. A stable forage supply in hot, dry years would be provided only by extensive and moderate farming, which allows the development of an insulating tree cover within grazed pastures. We conclude that such structural landscape diversity would grant wood-pastures with a buffering potential in the face of climate change in the forthcoming decades.

Subalpine Pasture-Woodlands in a Changing Climate

Konstantin Svetlozarov Gavazov

Climate change in temperate mountain systems and associated increase in temperature and decrease in precipitation are expected to have strong implications for vegetation productivity, species diversity and carbon turnover in subalpine grasslands. Little is known, however, about the interaction between the effects of climate change and those of local land use management and possible changes in landscape structure. Pasture woodlands in the Swiss Jura Mountains are a traditional landscape, resulting from a long-lived sustainable use of grasslands and woodlands, and as such provide a suite of important ecosystem services to human society. These range from carbon sequestration and biodiversity preservation, to provision of timber and forage for livestock, and last but not least an aesthetic value, much appreciated by tourism. In this thesis various aspects of ecosystem functioning have been studied, investigating the combined effects of experimental climate change and land use on structurally different wooded pastures. An altitudinal gradient method has been used to simulate future climate change conditions, by imposing warmer and drier climate on subalpine turfs transplanted at lower elevation. The resulting gradient in mean annual temperature and precipitation – ranging from cold and wet in the subalpine zone, to warm and dry in the colline zone – has allowed for the detection of tipping points and altered states of ecosystem functioning in response to the treatments. The method employed provided also the possibility for a direct comparison of three land use types: unwooded pastures, sparsely wooded pastures, and densely wooded pastures (the result of pasture management intensity), in their response to climate perturbation. During the four years of experimental work, a series of observations have been made at the plot scale (square metre) in terms of plant performance and biogeochemical cycles, as well as at the landscape scale (hectare) in terms of forage production. A general threshold level for ecosystem resistance to experimental climate change was detected between the moderate IPCC scenario (+2 K mean annual temperature; -20 % annual precipitation) and the intensive IPCC scenario (+4 K mean annual temperature; -40 % annual precipitation). A concomitant gradient in ecosystem response to climate change was observed across the three land use types. The intensively managed unwooded pasture type was consistently more affected by the experimental treatment and rarely exhibited signs of resistance, especially under the intense climate change scenario. A drastic loss of plant species diversity, reduction of herbaceous biomass, impaired litter decomposition and soil microbial metabolic activity have all contributed to the altered state of ecosystem functioning. In contrast, the two extensively managed wooded pasture types showed considerable resistance to climate perturbation in terms of both above and belowground ecosystem processes. The reported inter-annual variation in herbaceous diversity and biomass production within these land use types demonstrated their resilience (recovery) potential too. Using a modelling approach for upscaling these results to the heterogeneous landscape of pasture woodlands in the Swiss Jura Mountains, has proven that extensively used wooded pastures could grant sustainable ecosystem services in terms of forage provision for cattle under climate change. Considering that the two experimental climate change intensities implemented this study are the projected ‘best’ and ‘worst’ case scenarios for the coming decades, the reported resistance of wooded pastures to climate change has to be embraced, and sustainable land use set as a goal in high altitude mountain pastures.

EPFL2013

Past and future landscape dynamics in pasture-woodlands of the Swiss Jura Mountains under climate change

Alexandre Buttler, Joël Chételat, François Gillet, Alexander Ludwig Johannes Peringer, Thomas Spiegelberger

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.

Resilience Alliance2013

Transplantation of subalpine wood-pasture turfs along a natural climatic gradient reveals lower resistance of unwooded pastures to climate change compared to wooded ones

Alexandre Buttler, Konstantin Svetlozarov Gavazov, Thomas Spiegelberger

Climate change could impact strongly on cold-adapted mountain ecosystems, but little is known about its interaction with traditional land-use practices. We used an altitudinal gradient to simulate a year-round warmer and drier climate for semi-natural subalpine grasslands across a landscape of contrasting land-use management. Turf mesocosms from three pasture-woodland land-use types-unwooded pasture, sparsely wooded pasture, and densely wooded pasture-spanning a gradient from high to low management intensity were transplanted downslope to test their resistance to two intensities of climate change. We found strong overall effects of intensive (+4 K) experimental climate change (i.e., warming and reduced precipitation) on plant community structure and function, while moderate (+2 K) climate change did not substantially affect the studied land-use types, thus indicating an ecosystem response threshold to moderate climate perturbation. The individual land-use types were affected differently under the +4 K scenario, with a 60 % decrease in aboveground biomass (AGB) in unwooded pasture turfs, a 40 % decrease in sparsely wooded pasture turfs, and none in densely wooded ones. Similarly, unwooded pasture turfs experienced a 30 % loss of species, advanced (by 30 days) phenological development, and a mid-season senescence due to drought stress, while no such effects were recorded for the other land-use types. The observed contrasting effects of climate change across the pasture-woodland landscape have important implications for future decades. The reduced impact of climate change on wooded pastures as compared to unwooded ones should promote the sustainable land use of wooded pastures by maintaining low management intensity and a sparse forest canopy, which buffer the immediate impacts of climate change on herbaceous vegetation.

Springer2014