Woody riparian buffers have indirect effects on macroinvertebrate assemblages of French rivers, but land use effects are much stronger.
Woody riparian buffers (hereafter, 'woody buffers') are frequently considered as important to mitigate the effects of stressors on streams and rivers. While several individual studies addressing nutrients, pesticides, water temperature and different biotic components support this conjecture, no study has addressed the effects of woody buffers on riverine biota at country-wide scales. We used a comprehensive dataset from sampling sites on 1082 catchments in France, comprising samples of benthic invertebrates, along with data on river size, physico-chemistry, hydromorphology, riparian and catchment land use and woody buffers at sampling sites and upstream. Using partial least square modelling, we delineated the effects of the different environmental variables on two benthic invertebrate metrics, separately for siliceous and calcareous rivers. Overall, models explained 49% (calcareous) and 39% (siliceous) of the variation in benthic invertebrate metrics. Direct effects of woody buffers on benthic invertebrate metrics were marginal, while physico-chemical conditions and catchment land use explained most of the deviance. Direct and indirect effects of woody buffer together covered up to 6% (upstream scale) plus 2% (local scale) of the explained variability. Synthesis and applications. In this national-scale study, on 1082 catchments, we investigated the potential of woody buffers to mitigate the effects of catchment-scale and local-scale stressors on macroinvertebrate biodiversity. Our results underline that the establishment of woody buffers is not necessarily a sufficient measure to solve the problem of deteriorating riverine macroinvertebrate communities, especially in catchments prone to intense land use. Nevertheless, two main outcomes included that local woody patches are not sufficient and that woody buffers should be established along longer river stretches. Also, accompanying catchment-scale measures should be promoted to reduce the effects of intense land use and pollution to a level that enables woody buffers to be effective as well.