Functional significance of species composition in riparian plant communities

This paper analyzes the spatial patterns of plant species richness observed in 1989 and in 1999 alongside the Adour River (Southwest France), focussing on: (1) upstream-to-downstream changes, (2) mid-term changes, and (3) differences in observed patterns among various plant functional groups. The purpose of the research was to analyze how the proportions of invading species and/or ruderal species within riparian corridors can be related to regional environmental settings. About half of native and exotic species were ruderals. However, the proportion of competitors was higher within the native pool. No correlation was found between exotic and native functional groups. The longitudinal distribution of native and exotic ruderal species were correlated together, but they were not correlated with the total species richness. Converse to observations at the local scale in other studies, these patterns seem to be driven more by factors co-varying with the size of the river and by climatic factors than by hydrological disturbance-related factors. The role of the various plant groups studied and potential consequences on ecosystem function are discussed in the context of an increasing susceptibility of our riparian corridors to invasions by nonriparian species. Management perspectives are discussed for the purpose of biodiversity conservation.