Potential impacts of climate change on species richness in mountain forests - An ecological risk assessment

In this ecological risk assessment we evaluated potential climate-induced vegetation changes in mountain forests of Central Europe and possible impacts on species richness. The analysis was performed on all 1 km grid points of the Swiss forest inventory (c. 11,500 points) as well as on two subsets representing the forested points within the geographical limits of two inventories of legally protected reserve areas. The core of the approach is (a) a conceptual model of the movement of climatic ranges along altitudinal gradients as a result of global warming and (b) a spatially explicit forest community simulator that predicts a potential natural vegetation unit for any given 1 km grid point of the Swiss forest inventory for both current climate and for altered climate regimes, The model is derived from empirical data featuring the relationships between quasi-natural vegetation types and measured site variables. Species richness for any modelled forest community is estimated on the basis of phytosociological releves. The conceptual model showed that out of 109 legally protected landscapes, sites and natural monuments of national importance (minimum area > 1 km(2)) about 40-50% have an altitudinal or degree-day range that is able to host migrating species within the reserve limits if climate is warming in a moderate way (increase of mean annual temperature between +1.0 and + 1.4 degrees C). In the case of strong warming (increase of mean annual temperature between + 2.0 and + 2.8 degrees C) only 20-30% fulfil this criterion. The spatially explicit forest simulator showed that under a temperature increase without simultaneous increase in precipitation (warmer and more xeric due to increased evapotranspiration, Fagus-dominated communities in the colline-submontane belt might eventually be replaced by oak-hornbeam (Carpinion) communities. In the montane belt, the dominance of conifers will be seriously threatened by an invasion of deciduous species from the low montane and submontane belt. Under warmer and wetter conditions the vegetation shifts might not be as drastic as under warmer and more xeric conditions and the shift towards oak and oak-hornbeam communities on the Plateau is not supported. Concerning species richness, the models showed that in the case of warmer temperatures and constant precipitation (warmer and more xeric due to increased evapotranspiration) overall species richness is increasing on all I km points of the Swiss Forest Inventory, as well as on all selected subsets representing the forested points within legally protected reserve areas, In the case of warmer and wetter conditions the risk assessment does not show any drastic changes in the long-term species richness. (C) 1998 Published by Elsevier Science Ltd.