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climate change induced vegetation shifts

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Helge Walentowski
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Anthropogenic impacts such as global warming, land-use change and nitrogen deposition are affecting plant communities worldwide, and particularly alpine communities have been undergoing significant changes in recent years. We therefore analysed changes in the alpine calcareous grassland vegetation (Carex sempervirens and Carex firma communities) in the Berchtesgaden National Park over the last 30 years. We used data from permanent plots to produce a "floristic time series" from the 1980s to the present. We analysed the changes in the composition and species richness and tried to link these to autogenic or allogenic processes, with special focus on anthropogenic nitrogen deposition as a potential major cause. We found clear floristic changes over the study period, with an increase in average species richness of more than 10 species per plot in both communities. The Carex firma communities also showed a significant decrease in the average indicator values for continentality, suggesting that the floristic changes might be related to global warming and the resulting climatic effects. Global warming may have also accelerated the natural successional processes affecting the vegetation, but the time frame of 30 years is too short to consider this autogenic succession as one of the main factors. In contrast, nitrogen deposition or changes in land use appear to have played a smaller role in the vegetation changes.
European beech (Fagus sylvatica L.) is the prevailing tree species of mesic forests in Central Europe. Increasing summer temperatures and decreasing precipitation, as climate change scenarios predict, may, however, negatively influence beech growth and induce a shift to more thermophilous forest communities. Temperatures as expected in the future for western Central Europe are currently found in parts of western Romania. In light of this climate analogy we investigated forest vegetation as an indicator for future vegetation changes in five regions of western Romania representing a climatic gradient. We related species composition to climate variables and examined if tree and understorey species composition respond similarly to the climatic gradient. We further analysed if tree species occurrences correspond with their modelled distance to the rear niche edge. We found evidence for climatic effects on vegetation composition among regions as well as within deciduous and pine forests, respectively. This underlines that vegetation composition is a useful indicator for environmental change. Tree and understorey species compositions were closely linked showing that community-based characterization of forest stands can provide additional information on tree species suitability along environmental gradients. Both, vegetation composition and a climatic marginality index demonstrate the rear niche edge occurrence of beech in the studied sites of Romania and can predict the site suitability for different tree species. While vegetation surveys indicate Quercus petraea to be associated to moderately mesic forests, the marginality index suggested an inner niche position of sessile oak along the climatic gradient. Phytosociological relevés that differentiate between subspecies (or microspecies) of sessile oak with differing habitat requirements should be considered to complement national forest inventories and species distribution maps when modelling rear distribution edges. We conclude that climate driven forest vegetation composition in western Romania is a suitable analogon and may indicate future forest development in western Central Europe.