Spatial patterns of water-deposited seeds control plant species richness and composition in riparian forest landscapes

Determining regional and local processes that govern the maintenance of biodiversity and assessing their relative importance remain major research challenges in landscape ecology. In riparian systems, propagule dispersal and disturbance are constrained by longitudinal and lateral water flows, but riparian forests in temperate human-dominated landscapes have received little attention so far. We investigated how the longitudinal position along the river course and the lateral position to the riverbed structure forest plant communities subjected to flooding. We studied vegetation, soil seed bank, hydrochorous seed rain and environmental conditions along 23 transects perpendicular to the river in two forest floodplains in the North of France. We used generalized linear mixed models to assess the relative importance of longitudinal distance, relative elevation, and their interaction on both species richness in a given compartment and species turnover between compartments. Relative elevation to the riverbed was the main factor explaining species richness of vegetation and water seed deposit, but not of the seed bank. Vegetation was the most species-rich at low elevations, where the number of water-dispersed seeds and related species richness were maximal. The longitudinal position within the riparian forest had no effect on vegetation and seed bank richness. Dissimilarity between the seed bank and the seed rain increased with relative elevation. Lateral much more than longitudinal movements of water create a strong gradient in seed rain intensity that structures plant communities. Flooding is important to species dispersal, hence to the conservation of species-rich plant communities within human-pressured landscapes.