Fragmentation of hardwood floodplain forests — how does it affect species composition?

The present study focuses on how spatial patch characteristics, such as patch area, shape and isolation, affect the natural species composition of hardwood floodplain forests. The natural species composition is defined according to species groups obtained using phytocoenological methods. The aim of the study was to establish the relationship between fragmentation indices and the number and proportion of species in each functional species group stated in this paper. This study is based on a dataset of 118 phytocoenological relevés sampled using the standard methodology of the Zürich-Montpellier School, ordered within the suballiance Ulmenion (mixed oak-elm-ash forests along the great rivers). The study area is situated in Central Europe, in the northern part of the Pannonian biogeographic region. The digital map of hardwood floodplain forests was rasterized to 25 m cell size. The FRAGSTATS software was used to obtain fragmentation indices, and generalised linear models tested the influence of forest patch fragmentation indices on species composition. Our analyses confirm that large hardwood floodplain forests are essential for natural species composition conservation, and that large fragment areas are highly susceptible to non-native species penetration. We also determined that small, compact fragments contain very valuable remnants of well-preserved natural hardwood floodplain forests with a high proportion of specialised Ulmenion species. However, disruption to hardwood floodplain forest natural borders engenders a greater threat to its natural species composition than decline in patch area, because disruption results in increased Shape index, increased contact with the surrounding environment, greater edge effect and a higher proportion of alien species in the forest community.