Forest disturbance and salvage logging have neutral long-term effects on drinking water quality but alter biodiversity

Abstract Forests host most terrestrial biodiversity and provide important ecosystem services, including the provision of drinking water. Increasing frequency and intensity of natural disturbances and subsequent salvage logging may impact both biodiversity and drinking-water quality. However, empirical evidence and particularly that generated from long-term studies, is scarce. Using data obtained from the monitoring of streamwater between 1985 and 2018 and mid-term data on biodiversity of twelve species groups, we quantified the combined effects of natural disturbances and salvage logging. We used generalized additive models to test the effects of cumulative disturbed and salvage-logged areas on annual maximum nitrate and dissolved organic carbon (DOC) concentrations. We used generalized mixed-effects models to test the effect of management (disturbed unlogged, disturbed logged and undisturbed, intact forest) on species numbers of studied taxa. We found that forest disturbances led to a temporal increase of nitrate concentration in streamwater, yet remaining far below 50 mg L−1, the limits recommended by the World Health Organization. Salvage logging did not exert any additional impact on nitrate and DOC concentrations, and hence did not affect streamwater quality. Natural disturbances increased the biodiversity in eight out of twelve species groups. Salvage logging additionally increased the biodiversity of five species groups related to open habitats, but decreased the biodiversity of three deadwood-dependent species groups. We conclude that neither natural forest disturbances in watersheds nor associated salvage logging have a harmful effect on the quality of the streamwater, which is used for drinking water. Setting aside naturally disturbed areas would promote the conservation of deadwood-dependent species.