Effects of different seasonal cutting regimes on the regrowth of black alder and on plant species diversity

Abstract

The commonly managed and extensively grazed pastures in Upper Bavaria have become a valuable remainder of a heterogeneous and diverse landscape, which exhibits an exceptionally high diversity of habitats for plant and animal species. Characteristic for this landscape is the spatial and temporal coexistence of different successional stages. However, in the wetland areas of these pastures open fen grasslands are at danger of disappearing, because they are rapidly colonised by black alder. This would result in a loss of typical fen plant communities (e.g. Caricion davallianae) adapted to this open habitat. As the current grazing regime is not suitable to keep the fens open, which have already been colonised by young alder trees, an alternative method has to be found. For this reason it was investigated in this thesis whether different seasonal cutting regimes can be an efficient measure to eliminate alder trees that have colonised mesotrophic calcareous fens. Additionally, it was studied what effect these cutting regimes have on the plant species composition after two and a half years of tree cut. To compare the efficiency of a cutting regime in winter with a cutting regime in summer the number of shoots, the height and the crown area of alder trees was measured after repeated cutting. This could indicate whether the cutting resulted in a reduced regenerative ability of the trees. The impact on the vegetation was analysed by comparing vegetation relevés in the surrounding of alder trees that were repeatedly cut back and trees that were not cut back. These investigations could demonstrate that repeatedly cutting back alder trees in summer leads to a much stronger weakening of alder trees than cutting them back in winter. After being cut back twice in summer about 40 % of the trees had died completely, while only 15 % had died when cut back in winter. Additionally, in the surrounding of the cut trees more plant species could be found, indicating that the cutting regime already had a positive effect on the fen vegetation. However, only a slight increase of light-demanding species could be observed, where alder trees had been cut. Finally, options and possible challenges of managing these open fen grasslands in a sustainable way were discussed.