| Summary: | International audience The removal of the two hydroelectric Sélune River dams (Normandy, West of France) –36m and 16m high, is expected by 2019 and 2021 respectively. It is a unique European renaturation project freeing an entire coastal river. This study, part of a global scientific program, focuses on 19kms of river stretch (the first disappearing lake’s length). Since 2015, the usual 1m summer tidal range of the Vezins impoundment is not established anymore in order to make sediment management easier during the emptying. Between early 2017 and late summer 2018, the water table has been slowly reduced to conduct these 700 000 m3 sediment flow prevention engineering works. The floodplain progressively goes back to its normal course. Hectares of dewatered muds were rapidly colonized by spontaneous vegetation over the following months. The former lake is slowly being replaced by a river meandering through resurgent fields and wetlands. The 4-year emptying has made a close vegetation change monitoring possible, highlighting how to ecologically restore the valley. This 2015 to 2018 survey is based on spatial (lateral and longitudinal variations) and temporal (intra- and inter-annual shifts) observations of riparian vegetation communities. The aim is to predict the vegetation’s ability to passively and ecologically restore the riparian area of the new Sélune River. It focuses on three axis: i) the temporal dynamics of the colonizing species, ii) the influence of environmental variables in spatially structuring communities, and iii) the functional successional patterns of the process. Species traits related to colonization and sediment stabilization potentials within the habitat mosaic are treated. We discuss how management decisions related to the ecological restoration of the valley could be supported by the community ecology and trait data obtained. Finally, we suggest prioritizing passive restoration issues according to the functions assigned in this new valley.
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