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

We assessed the significance of flooding for the floristic composition of seed banks in flood-meadows of the northern valley of the Upper Rhine. We compared three hydrological compartments of the alluvial plain, consisting of the regularly flooded land between the river and low summer dykes (functional flood-plain), the occasionally flooded land between summer dykes and high 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 differ 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 species not occurring in the vegetation was constant. The increase in seed density can be largely attributed to an increase of disturbance indicators, which 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 vegetation and former and present-day meadow management. Seeds of species absent from above-ground vegetation can be attributed to the local species pool present in the immediate vicinity of the study plots. We discuss consequences of the results for the restoration of species-rich flood-plain meadows.