Environmental factors shape the relationship between seed bank and vegetation on periodically emerged alluvial gravel bars of the Elbe river

Riparian ecosystems are among the most valuable natural ecosystems in terms of their diversity and ecosystem functions, but their intensive use by humans has led to their degradation and reduction in extent. One of the last free-flowing rivers in central Europe is the Elbe, with more than 600 km of water course without weirs. Thanks to the relatively natural fluvial regime, exposed gravel bars hosting specific vegetation including several endangered species have been preserved. In this study, we examined how factors that are related to fluvial dynamics influence plant communities, above and belowground, on gravel bars that are periodically exposed along the Elbe in the Czech Republic. This study was carried out along a 40 km long stretch of the river between Ústí nad Labem and the Czech-German border. There, 10 localities were selected where 60 plots 1 × 1 m in size were established that were arranged in transects perpendicular to the river. Plant communities were recorded in terms of their composition and richness of both the standing vegetation and the soil seed bank, which provides information on the regeneration potential of these communities. All vascular plant species were identified at the peak of vegetation development and the soil seed bank cultivated from sediment sampled next to the plots. Of the environmental factors, the texture and chemical properties of the sediment were analysed, and hydrological modelling was used to determine the duration of plot exposure. The composition of the vegetation was most influenced by the duration of plot exposure, which separated the species of long- and short-flooded sites. In contrast, the composition of the seed bank was not significantly influenced by the environmental factors studied, but by functional species traits, with stress-tolerant species capable of clonal spread clearly different from light- and moisture-demanding species with a ruderal life-strategy. It is concluded that fluctuations in water level are essential for maintaining species richness on gravel bars because they create a strong gradient, which promotes the coexistence of species with different requirements in small areas.