SUCCESSION AND FLUCTUATION IN THE AQUATIC VEGETATION OF 2 FORMER RHONE RIVER CHANNELS

The vegetation dynamics in two former braided channels of the Rhone River was studied at two time scales in order to test the following hypothesis: fluctuations would occur within seasons (flood disturbances, hydrological fluctuations, phenology) while successions would occur between years. The vegetation was surveyed in 1983, 1988 and 1989 during summer for the interannual investigation, and in spring 1989, summer 1989, winter 1989 and spring 1990 for the seasonal investigation. Terrestrialization, which was observed within the same period in other braided former channels of that river, did not happen here despite the 1989 drought. However, a vegetation zone situated in the upstream part one channel seems to represent some successional trend, resulting in the establishment of Nasturtium officinale and the increasing abundance of Chara vulgaris. In disagreement with the tested hypothesis, only fluctuations are observed at the two temporal scales in the other vegetation zones. The amplitude of cyclic trajectories observed in the seasonal study depends of the degree of hydraulic disturbances (floods, drought) that affects each vegetation zone. The channel that is closer to the river is maintained at a steady state by the periodical inputs of kinetic energy during river overflows and fast floods; the disturbances wash away fine deposits and rejuvenate the vegetation mosaic. In the other former channel that is less disturbed by floods and is characterized by a thick layer of fine sediments, the groundwater inputs from numerous limnocrene springs carry away organic matter and slow down ecological successions.