Modelling vegetation dynamics in heterogeneous pasture-woodland landscapes

Recent knowledge about tree regeneration, shifting mosaic in the herb layer or complex interactions between cattle activities, vegetation and landscape structure has allowed the development of a novel, spatially explicit, mosaic compartment model of the dynamics of silvopastoral ecosystems (WOODPAM). This deterministic model considers three hierarchical levels: the focal level is the phytocoenosis, represented by a cell or a patch in the landscape with a variable local stock density; herb and shrub communities as well as size-structured tree populations are the components of each patch at the lower level, spatially implicit; patches are aggregated in a pastoral management unit building the higher level, with an externally controlled global stock density. Simulation experiments were applied to a pasture-woodland landscape in order to assess the consequences of hierarchically organized ecological interactions on spatio-temporal patterns of vegetation and cattle habitat use. In the first scenario, the global environmental and management conditions were fixed to their observed initial values. The patterns of vegetation and cattle habitat use evolved at very long term toward a permanent state dominated by wood-pastures, strongly dependent on the spatial configuration of the environment. Landscape dynamics being very slow, the second scenario took into account the effect of climate change, by considering a linear increase of the annual temperature, which influenced forage production and the speed of vegetation dynamics. in this simulation, closed forests and densely wooded pastures tended to dominate, vegetation diversity decreased, the patterns became unstable and long-period cyclic successions appeared at patch level. Model simulations emphasized the role of livestock selectivity to preserve vegetation variability and heterogeneity at long term. A high selectivity occurs at low overall stocking rate, inducing a slow fragmentation of the landscape into open pastures and closed forests and a decline of wood-pastures. A low selectivity occurs at high utilization rates, decreasing the heterogeneity of the landscape. These results are discussed in the light of the recent theories binding grazing ecology and woodland dynamics. (C) 2008 Elsevier B.V. All rights reserved.