Ecological complexity of wetlands within a river landscape

The sustainable conservation of diversity in riverine wetlands implies knowledge of the basic geomorphological and ecological processes that inter play at the landscape scale. Eight hypothetical types of wetlands, predicted as differing in their plant communities, were proposed by combination of three factors (geomorphological pattern, river incision vs aggradation, groundwater origin), selected because they determine the effects of flood disturbances (intensity, frequency) and the water supplies. This hypothesis was tested through the comparison of the vegetation and the physico-chemistry of cut-off channels of the Ain River in France. Cutoff meanders were related to meso-eutraphent plant species of standing water, aggraded meanders being more frequently overflowed than incised ones. Braided channels were characterized by flood-tolerant species, some of the incised ones being related to oligotraphent communities, but some remaining connected, through seepage and floods, to the river (mesotraphent species). The ratio of side-hill aquifer to seepage supplying these channels depends on river vs aquifer levels but also on their elevation from the river level, determined by their age. The study demonstrates the effect of geomorphology and of water supplies on floristic assemblages, but some other features must also be considered. The high diversity of functioning implies that conservation of such ecosystems should consider all the parts of the river landscape and all the processes that interfere on several time-scales. (C) 1998 Elsevier Science Ltd. All rights reserved.