Seed deposition in drift lines during an extreme flooding event — Evidence for hydrochorous dispersal?

Summary Extreme flooding events can be of major importance for dispersal processes in today's fragmented landscape. We studied long- and short-distance seed dispersal during an extraordinary flooding of the Elbe River in summer 2002. In addition we evaluated the effect of different species traits (seed buoyancy, dispersal type, Ellenberg moisture values) on dispersal and discussed the relevance of hydrochorous dispersal during extreme flooding events for nature conservation and restoration. Along a transect of 400 km of the Middle Elbe River (Eastern Germany) drift line material from three habitats (arable field, grassland, river bank) was collected at five locations. Its seed content was analysed and compared with vegetation communities of the same region with respect to species composition. One part of the drift line samples was scanned for seeds and seed fragments visually, while the other part was exposed for germination. A total of approximately 1500 seedlings of about 70 species emerged from the samples. The most frequent species were Deschampsia cespitosa , Urtica dioica, Ranunculus repens and Lycopus europaeus . Hand-sorting of drift-line material revealed that only 1.5% of the seeds occurring in the samples recruited in the germination trials. Although most seeds and seedlings from the drift line samples belonged to common species, some seeds of rare and endangered species were found. Species composition of the drift line samples differed only marginally between the studied habitats and locations, which is interpreted as evidence for short- and long-distance seed dispersal. In contrast, the vegetation of the analysed habitats differed significantly. The studied species traits did not differ between drift line material from different habitats, but rather between the differentiated vegetation communities. We conclude that seeds from different habitats and locations are transported together during heavy flooding and that these floodings might thus expand the usual dispersal ranges of plant species.