Assessing the context and ecological effects of river restoration – A meta-analysis

A large number of qualitative and semi-quantitative analyses have been performed to summarize the effects of river restoration, while systematic and quantitative assessments of the literature on river restoration are still rare. To fill this gap, we conducted a meta-analysis of 55 peer-reviewed publications describing assessments of river restoration with indicators of biodiversity or ecosystem services (ES). The core of the study was to identify: (i) the overall ecological effects of river restoration; (ii) the restoration effects on different organism groups and types of ES, including the relationship between biodiversity and ES; and (iii) the influence of the restoration context, that is degradation types, restoration techniques and restoration age. The meta-analysis revealed that river restoration significantly enhanced ecological conditions. However, restoration effects varied among organism groups and ES, with plant abundance and regulating services showing higher responses to restoration. Moreover, a trend of a positive correlation was found between biodiversity and ES. Restoration activities showed most significant effects in the context of hydromorphological degradation or land-use changes, and channel reconfiguration and riparian buffer establishment had a particularly favourable outcomes. Only regulating services showed a positive correlation with restoration age; thus, long-time monitoring is necessary to fully understand restoration effects. Overall, our meta-analysis provides evidence-based advice for future projects and assessment of river restoration.