The taxonomic structure but not the functioning of riparian herbaceous communities varies with hydrological conditions on a large, highly regulated river: Evidence from a 2‐year replicated study

International audienceRiparian vegetation encompasses dynamic communities structured by strong environmental gradients. Due to anthropogenic changes, a greater uniformity of environmental conditions is observed along local gradients. To maintain diverse and functional communities in regulated rivers, there is an urgent need to finely characterize the factors controlling the spatial and temporal dynamics of riparian vegetation. In the summer of 2017 and 2019, we repeatedly sampled eight gravel bars along the regulated Rhone River. Using a taxonomic and functional approach, we assessed whether the diversity and composition of species and traits varied between the 2 years at the corridor and site scales. Taxonomic diversity decreased and composition changed between years, while functional diversity and composition did not. Specifically, we showed that variation in taxonomic diversity was largely limited to species sharing the same trait values, suggesting some functional resilience of riparian communities to environmental changes. In addition, we found that changes between the 2 years concerned primarily the communities established on low-lying bars made up of coarse-grained sediments, more frequently and intensely inundated. Beyond being the most responsive to environmental changes, these riparian communities were also the most diverse in species and trait values. However, by inducing a strong decrease in diversity, fine sedimentation seems to be a major threat to the functional and ecological integrity of gravel bars. Overall, these results highlight the conservation priority of low-lying gravel surfaces and the need to promote more morphologically complex riparian habitats by renaturalizing flow variability and sediment regimes