Long-term effects of mechanical site preparation on understorey plant communities in lowland floodplain forests

The influence of site preparation on understorey plant diversity was assessed in the commercial floodplain forest in the Thaya River alluvium near Br?eclav, Czech Republic. Phytosociological surveys were carried out during the summer of 2013 in 10-50 years old forest stands with and without mechanical site preparation (MSP). One hundred sixty releves (eighty from Quercus robur stands and eighty from Fraxinus angustifolia subsp. danubialis stands) were selected for analyses. Phytocoenological releves were analyzed using multidimensional statistics and plant community characteristics (fidelity, species richness, the percentage of non-native or endangered plant species). The results showed that mechanical site preparation significantly affects herbaceous species composition and richness, in the particular richness of non-native species and neophytes. We found a total richness of 231 herb and woody species, of which 49 (21.2 %) were non-native. Our results demonstrate that reforestation with the MSP decreased species richness of the understorey and increased the number of non-native species. The sites without MSP treatment contained significantly fewer non-native species. The invasive plants Aster lanceolatus and Acer negundo were more abundant in sites impacted by mechanical soil disturbance. Our results also show that a higher percentage of non-native plants survive longer in forest stands subjected to MSP. The abundance and cover of neophytes within MSP treatments are significantly higher in stands dominated by Fraxinus than Quercus stands.