Are the banks a source of recolonization after disturbance: an experiment on aquatic vegetation in a former channel of the Rh�ne River

Abstract

Recolonization of vegetation on 1 m2 quadrats was surveyed after a major disturbance induced by a restoration experiment, carried out by dredging in a former river channel. Various spatial and temporal patterns of recolonization were determined by the analysis of vegetation (re-)establishment on adjacent quadrats located along transects from one bank to the other in several zones. Most species, except emergent species that remained on bank quadrats (e.g. Phalaris arundinacea), initially (re-)established on the banks and later expanded towards the center of the channel (e.g. Callitriche platycarpa). Several species (re-)established simultaneously on both bank and center quadrats, and this in all three zones (e.g. Nasturtium officinale, Potamogeton pusillus). This suggests that the fine sediment contains a propagule bank. New species (not observed prior to restoration) generally established on the center quadrats first (e.g. Groenlandia densa), suggesting immigration via propagule drift. At the channel scale, several species (re-)established first upstream and then downstream (e.g. Nasturtium officinale), confirming the role of propagule drift from upstream habitats. Few species (re-)established downstream first (e.g. Callitriche platycarpa). Recolonization patterns of dominant species (location and timing of (re-)establishment) are related to biological traits such as the efficiency and means of reproduction.