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

The vegetation dynamics in two former braided channels of the Rhone Ri ver was studied at two time scales in order to test the following hypo thesis: fluctuations would occur within seasons (flood disturbances, h ydrological fluctuations, phenology) while successions would occur bet ween years. The vegetation was surveyed in 1983, 1988 and 1989 during summer for the interannual investigation, and in spring 1989, summer 1 989, winter 1989 and spring 1990 for the seasonal investigation. Terre strialization, which was observed within the same period in other brai ded former channels of that river, did not happen here despite the 198 9 drought. However, a vegetation zone situated in the upstream part on e channel seems to represent some successional trend, resulting in the establishment of Nasturtium officinale and the increasing abundance o f Chara vulgaris. In disagreement with the tested hypothesis, only flu ctuations are observed at the two temporal scales in the other vegetat ion zones. The amplitude of cyclic trajectories observed in the season al study depends of the degree of hydraulic disturbances (floods, drou ght) 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 disturbance s wash away fine deposits and rejuvenate the vegetation mosaic. In the other former channel that is less disturbed by floods and is characte rized by a thick layer of fine sediments, the groundwater inputs from numerous limnocrene springs carry away organic matter and slow down ec ological successions.