Long-term ecological studies on the oxbow ecosystems development and fire history in the Drava river valley (Central Europe): Implications for ecological restoration

Abstract

Oxbow lakes are important habitats for many rare and protected wetland organisms and play a role as a storage of pollutants in floodplain. Unfortunately, many oxbows were destroyed due to drainage works and the adaptation of riverbeds to inland transport. Multi-proxy palaeoecological analyses makes it possible to recognise the succession of flora and fauna in the oxbow lakes over centuries and millenia. Insight into the history of the destroyed wetlands ecosystems is especially important for their restoration. We used palaeoecological methods (plant macrofossils, pollen, mollusc, Cladocera, diatoms, macrocharcoal) supported by radiocarbon dating to: (i) reconstruct the succession of the local flora (macrophytes, diatoms) and fauna (molluscs, Cladocera) during the last ca. 3000 years; (ii) provide a background of local to regional vegetation composition and disturbance by fire and human impact in the Drava valleys. (iii) determine natural reference conditions as a basis for the restoration of degraded oxbow ecosystems in the region; Our study revealed that: (i) in the shallow eutrophic oxbow lakes (ca. 2 m deep) occurred numerous submerged (Najas marina, N. minor, Ceratophyllum demersum, Zannichelia palustris) and floating macrophytes (Trapa natans, Nymphaea alba, Nuphar lutea), Cladocera and molluscs species (e.g. Borysthenia naticinata, Bithynia tentaculata, Valvata cristata) and we recommend that these species should be included in the restoration of oxbows in this region; (ii) warm climate phases (Roman Period, Medieval Period) likely enhanced the terrestrialisation process and transformation of water reservoirs into peatlands between 1200 and 800 cal. BP; (iii) human activity has led to changes in overall forest composition, including riparian tree taxa, since the beginning of the record and has intensified since the Medieval Period; (iv) the difference in the time and intensity of fires between the northern and southern part of Drava river valley is most likely due to the different history of human colonisation.