Broad-Scale Meta-Analysis of Drivers Mediating Adverse Impacts of Flow Regulation on Riparian Vegetation

Over two-thirds of global rivers are subjected to flow regulation. Although it is widely recognized that flow regulation can adversely affect riparian vegetation-a critical component of river ecosystems-the specific roles of various drivers remain poorly understood. To address this gap, we conducted a broad-scale meta-analysis, aiming to elucidate how different factors mediate the adverse impacts of flow regulation on riparian vegetation. This meta-analysis encompassed 59 papers, spanning 278 dams constructed on 146 rivers. We extracted data on four key indices of riparian vegetation: species richness and abundance of all riparian species, and those indices exclusively for non-native species. Indices were compared between regulated and free-flowing or pre-damming rivers to quantify the impact of flow regulation. Our meta-analysis revealed a moderate but significant reduction in the richness and abundance of all riparian species under flow regulation, coupled with a strong increase in the abundance of non-native species. Riparian vegetation in arid and continental climate regions experienced stronger negative impacts than those in tropical and temperate climates. Furthermore, the adverse effects on riparian vegetation were more pronounced downstream of dams than upstream. Considering climate region, study identity, and relative position to the dam as random variables, it became evident that years since flow regulation emerged as the most important factor influencing species richness. Over time, richness gradually recovered from initially low levels. However, this recovery was slowed by increasing flow regulation intensity (percentage of annual runoff stored). Additionally, the impact was more evident in larger rivers. To support regulated river management, we recommend prioritizing the protection of riparian vegetation in arid and continental climates, with emphasis on areas downstream of dams, limiting flow regulation intensity, particularly in larger rivers, and monitoring non-native species to prevent disproportionate spread.