Influence of local environmental gradients on ecological strategies of herbaceous communities in riverine side channels

Questions In riverine side channels (secondary and former arms), the alternation of lotic, lentic and dry phases produces disturbance and resource gradients that filter species and community strategies. To better understand the impact of this alternation of functioning on plant communities at a fine spatial scale, we tested for the effects of elevation (linked to water level), slope, longitudinal position, light availability and sediment grain size on (i) taxonomic composition and (ii) competition, stress tolerance, and ruderal strategies (C, S and R strategies) at the community level. We hypothesized that fewer resources would increase the stress‐tolerant prevalence (i.e., community‐weighted mean value), while topographic gradients would affect ruderal prevalence. In addition, we hypothesized that strong and directional environmental filtering should reduce the co‐occurrence of species with different CSR strategy values (i.e., low C, S and R functional dispersion). Location Fourteen side channels on the lower reach of the Loire river (France). Methods Taxonomic composition of the herb layer and environmental variables were assessed on 474 plots. The relations between species composition and environmental gradients were investigated by a constrained analysis of principal coordinates. The effects of environmental gradients on community CSR strategies were assessed using mixed‐effects models and a model‐averaging procedure. Results Taxonomic community composition was shaped mainly by light availability, topographic elevation and sediment grain size. Slope and the interaction between light and elevation influenced the community‐weighted mean and functional dispersion values of S and R strategies. Among other interactions, those involving sediment grain size shaped the dispersion values for C and R strategies. Conclusion The study identified shading and fine sediment grain size as key drivers of biotic homogenization in the riverscape. Increasing the disturbance frequency and magnitude would limit the progression of the ecological succession and favor the co‐occurrence of taxonomically and functionally diverse communities within the side channels. Former and secondary arms concentrate strong environmental gradients and plant diversity at fine spatial scale. The role of disturbance and stress‐related parameters for community structure and ecological strategies was assessed. Results highlighted five structuring parameters for taxonomic composition. Interactions