Effect of planting alien Robinia pseudoacacia trees on homogenization of Central European forest vegetation

Biological homogenization is a process of biodiversity loss driven by the introduction and invasion of widespread species and the extinction of specialized, endemic species. This process has accelerated in recent years due to intensive human activities. We focused our study on large areas of forest vegetation that have not yet been intensively studied. Forest management, especially the planting of alien trees, could play a key role in the homogenization process because alien trees can act as habitat 'transformers' influencing vegetation through creating different environmental conditions. Several types of native forests (hardwood floodplain forests, oak forests, and oak-hornbeam forests) have in many regions been replaced by Robinia pseudoacacia plantations. The huge diversity of native broadleaved deciduous forests in the Pannonian and Carpathian regions, with many local differences and considerable geographical variability, could be exposed to the homogenization process due to the planting of Robinia pseudoacacia. We used 282 paired plots of Robinia pseudoacacia-dominated forests and native forests with a distance of 50-250 m among them under the same environmental conditions to avoid the influence of the variability of local environmental conditions on the forest undergrowth. We found out that the replacement of native forests by plantations of Robinia pseudoacacia plays a crucial role in the homogenization process in forest vegetation by unifying microenvironmental conditions of stands and removing the geographically specified variability of plant communities from previous four classes to single one. The replacement reduced total species pool from 422 to 372 species and supported the occurrence of widespread, generalist plant species in the undergrowth.