Seed inputs in riparian zones: Implications for plant invasion

Seed inputs were studied from January 2001 to August 2002, along a tranverse successional gradient of a riparian zone within the mid-reaches of the Garonne River (SW France). Seeds were trapped monthly at ten sites distributed along the gradient. In total, 117 000 seeds belonging to 296 species were collected, among which 145 were exotic species. Seed inputs were overall dissimilar to the extant vegetation; however, similarity among seed input samples was higher than similarity among the extant vegetation samples. Exotic seed inputs resulted from distinct modes of dispersal in comparison to the native ones. Exotic guilds also comprised more short-lived species. A significant positive linear relationship was observed between exotic and native species richness for both seed inputs and extant vegetation. Flood events were responsible for species richness peaks in seed inputs. They also contributed towards peaks in exotic species and individuals. However, most exotic seeds were recruited during low water period, with a significant lag in time compared to native inputs. Species composition, densities of species and individuals as well as the percentages of exotic species distinguished the infrequently agricultural zone from the frequently disturbed riparian zone. The latter zone was the most invaded both in terms of seed input and extant vegetation. Copyright (c) 2005 John Wiley & Sons, Ltd.