Riparian 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 Rhône 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.

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Maintenance operations in regulated rivers are often implemented as preventive measure to maintain wide, smooth channels and to limit the risk of flooding. By clearing vegetation, removing roots and lowering sediment bar elevation, these works can recreate pioneer habitats and related plant communities. Using a before-after-control-impact design with monitoring of riparian vegetation over 10 years, we evaluated the effects of fluvial maintenance works on plant richness and composition in an island complex located in a Nature Reserve along the Middle Loire River. Our results showed that artificial rejuvenation operations had a significant shortterm negative effect on richness. However, in the few years following the work, the riparian communities established on the rejuvenated island became more diverse, with a large proportion of annual plants, contributing to the increased species pool of the Nature Reserve. In addition, we found that temporal changes in species richness were strongly influenced by hydrological conditions, with a peak in biodiversity observed during prolonged summer low water. This highlights the importance of considering flow variations to more accurately assess the ecological benefit of restoration operations on riparian vegetation. Overall, as successional stages were more advanced at the island complex scale, fluvial maintenance works have allowed the recreation of sandy pioneer habitats exposed to flooding. These habitats have promoted the establishment of species-rich communities without favoring the installation of invasive alien species. From this view, the maintenance works have increased the mosaic of habitats within the Nature Reserve and have restored environmental conditions favorable to early-successional species.

Managing invasive exotic plant species is a complex challenge, especially for Asian knotweeds (Reynoutria spp.). Tarping is a regularly cited but poorly documented control method, which consists of covering the ground with a tarp (agricultural tarp, geotextile, geomembrane, etc.) to create a physical barrier to hinder plant growth and deprive the plants of light in order to deplete their rhizomatous reserves. To improve our knowledge of tarping in order to identify the key factors of its success or failure, we reviewed the relevant grey and scientific literature and conducted an international survey among managers to collect feedback on tarping experiments. In the literature, as well as in the field, practices are quite heterogeneous, and the method’s effectiveness is highly contrasted. A better consideration of knotweed biology may improve the efficacy of the method. Based on the bibliography and survey work, we propose practical recommendations including covering the entire stand, extending the tarping up to 2.5 m beyond its edges for a period of at least six years, and ensuring regular monitoring. Even though tarping does not seem to be a one-size-fits-all solution to eradicate knotweed, it could still be a useful control method once knotweed has become a critical management issue.