The impact of flooding regime on the soil seed bank of flood-meadows

We assessed the significance of flooding for the floristic composition of s eed banks in flood-meadows of the northern valley of the Upper Rhine. We co mpared three hydrological compartments of the alluvial plain, consisting of the regularly flooded land between the river and low summer dykes (functio nal flood-plain), the occasionally flooded land between summer dykes and hi gh winter dykes (hybrid flood-plain) and the land behind the winter dykes, which is now only submerged by ascending groundwater (fossil flood-plain). Due to their different flooding regime, the three compartments should diffe r with respect to the prevailing conditions of diaspore input. The seed density of soil samples increased with the duration of flooding in the three compartments, while species richness and the proportion of speci es not occurring in the vegetation was constant. The increase in seed densi ty can be largely attributed to an increase of disturbance indicators, whic h are present in the above-ground vegetation and capable of forming a long- term persistent seed bank. No effects of flooding on the composition of seed banks in the three flood- plain compartments were found. The differences in seed bank composition can be largely explained by corresponding differences in above-ground vegetati on and former and present-day meadow management. Seeds of species absent fr om above-ground vegetation can be attributed to the local species pool pres ent in the immediate vicinity of the study plots. We discuss consequences o f the results for the restoration of species-rich flood-plain meadows.