How maintenance and restoration measures mediate the response of riparian plant functional composition to environmental gradients on channel margins: Insights from a highly degraded large river.

Riparian habitats are transitional zones where strong environmental gradients shape community. To prevent flood risks and channel migration on managed rivers, civil engineering techniques have been widely used. Recently, ecological restoration of rivers has become a major issue. However, given the alteration of natural disturbance regimes induced by human infrastructures, the real added-value of these restoration actions is questionable. Thus, a major challenge is to better understand whether changes in abiotic conditions induced by human activity influence the response of plant communities to environmental gradients. Studying a highly degraded large river, we evaluated the effect of the elevation and soil texture gradients on plant functional composition and assessed whether human-mediated environment gradients, achieved through maintenance and restoration measures, shape community structure. In the summer of 2017, we sampled 17 geomorphic surfaces, mostly gravel bars, along the Rhône River and its tributaries that were either repeatedly cleared (brush clearing vs plowing), newly reprofiled or naturally rejuvenated by high flows. The results show shifts in trait values with elevation and convergence in plant traits with increasing proportion of fine sediments. The co-occurrence of species with contrasting traits was higher in highly disturbed environments, revealing the importance of rejuvenation processes. However, the influence of both environmental gradients was mediated by human activity. For maintenance measures, plowing was better able to promote species diversity and limited biotic homogenization along environmental gradients. Among the three geomorphic surfaces, naturally rejuvenated bars were the most stressful environments, hosting distinct functional assemblages, while communities on newly reprofiled banks were in the same ecological trajectories as repeatedly cleared bars. To promote an effective ecological restoration of riparian zones, (i) a greater variability of the minimum flow is needed, (ii) bedload transport restoration should be a priority and (iii) reprofiled banks should better mimic the landforms of natural river margins.