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Soil seed banks have a high potential for vegetation re-establishment in restoration projects. We studied the soil seed bank in an oxbow system of a disconnected floodplain of the Danube River in Southern Germany. The aim of the study was to analyze if floodplain target species were still present in the seed bank after more than 150 years of embankment and disconnection from fluvial dynamics. In this context we investigated seed density, seed bank species richness and species composition in four broad habitat types with and without water-level fluctuations during the time of embankment (permanent water, fluctuating water, reed bed, hardwood floodplain forest). In addition, the similarity between seed bank and above-ground vegetation in these habitat types was studied in order to predict the success of future restoration measures. In total, 124 vascular plant species were determined in the seed bank samples. More than 50 % (66 species) were target species typical for floodplain habitats and 26 of these target species were lost or very rare in the above-ground vegetation. The four habitat types differed significantly in mean seed density and mean species richness. Mean species richness and the number of target species in the seed bank as well as the mean seed density were greatest in the habitats with fluctuating water level whereas mean seed density was much lower in the parts with more or less stable conditions like permanently standing water and hardwood floodplain forest. Sørensen similarity between seed bank and above-ground vegetation was very low in habitats with more or less stable water levels and desirable floodplain target species were very rare or completely absent. Our results indicate that the soil seed bank can be an important seed reservoir for the ecological restoration of floodplain plant communities especially for habitats with unstable environmental conditions during the period of disconnection. Restoration of water level dynamics is important to maintain the seed bank of populations of floodplain target species.
Reconnection of floodplains to rivers to enhance fluvial dynamics is a favored method of floodplain restoration in Europe. It is believed that the restoration of hydrological conditions of the floodplain facilitates natural dispersal of target species, and hence the reestablishment, of typical plant communities. The aim of our study was to investigate whether floodplain target species could reach restoration sites via hydrochorous dispersal. We analyzed seed inflow from the river and seed dispersal in different sectors of a new watercourse in the Danube floodplain. Seeds were captured using 27 seed traps during three sampling periods of 3 weeks each from summer 2011 to spring 2012. After germination seedlings were identified, we detected a total of almost 39,000 seeds of 176 species, including 80 target species of riparian habitats. We found significant differences between seasons (most seeds in autumn/winter) and between stream sectors. Fewer seeds came in from the Danube (2,800 seeds) than were transported within the floodplain. Several new floodplain target species were detected, which had not been found in the aboveground vegetation or soil seed bank before the start of the restoration. Seeds of nonnative species did not disperse further than approximately 1 km. Our results indicated that hydrochorous seed dispersal from upstream habitats along the new watercourse was important for the establishment of target species and hence for the success of floodplain restoration. Technical water diversion weirs must be traversable for seeds, and small donor sectors upstream might enhance the reestablishment of target vegetation along new sectors downstream.