INFLUENCE OF FLOOD TIMING ON THE RECOVERY OF MACROPHYTES IN A FORMER RIVER CHANNEL

Over two vegetation cycles we compared the recovery of macrophytes fro m flood disturbances that occurred at different seasons (July vs Decem ber) on patches of a former channel of the Rhone River, France. Some p atches were disturbed twice; others were disturbed either in summer or in winter; others were never disturbed and were used as controls. The recovery rate of the vegetation was estimated from the duration of re colonization of the disturbed areas and of growth of the recolonizing species. The influence of the summer disturbance appeared to be strong because the disturbance occurred when the development of the vegetati on was maximum. The influence of the winter disturbance was apparently much lower since most species had already declined at this time becau se of their phenology. The repetition of the two disturbances on the s ame patch had little influence on the vegetation community. In all cas es, the recovery of the vegetation occurred rapidly, both for total ve getation cover and species richness. By the following spring, no signi ficant differences appeared between disturbed and reference patches. T he effect of the disturbances varied according to the phenology of the plants, and the macrophyte community studied was more sensitive in su mmer than in winter.