Twenty-Years’ Changes of Wetland Vegetation: Effects of Floodplain-Level Threats

Wetlands in large river floodplains are endangered biodiversity hotspots in need of active conservation measures. Most rehabilitation interventions target water replenishment and disregard other possible threats, especially those which operate at the landscape level and cause changes detectable over a period of decades. Twenty years’ changes of wetland vegetation were evaluated on the Drava floodplain, Hungary, where large-scale hydrological interventions have not been implemented during this time. Archive data from the 1990s were compared to data from 32 oxbows resurveyed in 2015. After scrutinizing scientific values and limits of archive datasets, we judged them appropriate for drawing robust landscape-level conclusions. Frequency of habitats and of selected species, diversity indices and diversity profiles were compared. Increased frequency of shrub and forest habitats, decreased frequency of dystrophic and protected species, and decreasing diversity of habitats and communities were detected. In the Drava floodplain hydroseral succession is the key natural process. Drought, riverbed incision, nutrient pollution, spread of invasive species, succession, and game and fish overpopulation were identified as further landscape-level threats. Water replenishment schemes can improve the conditions through slowing down or reverse succession. However, habitat and vegetation changes indicated that other threats to biodiversity are still present.