Responses of Riparian Vegetation in Cascade Reservoirs to Contrasting Flow Regulation Types

ABSTRACT: (I) Large dams alter flow regimes and water-level fluctuations, reshaping riparian disturbance regimes and shoreline habitat conditions, with important implications for vegetation recovery, biodiversity maintenance, non-native species establishment, and reservoir shoreline management. However, how these effects vary among flow regulation types and water-level fluctuation (WLF) zonation remains insufficiently understood in cascade reservoir systems. We asked: (1) how species richness, alpha diversity, beta diversity, and the proportion of non-native species vary among flow regulation types and WLF zonation; (2) which plant species indicate different regulation types and WLF zonation; and (3) which biotic or abiotic factors best explain variation in riparian species composition. (II) WLF zonation of nine cascade reservoirs along the Wujiang River, central China. (III) We surveyed 573 quadrats across 72 sites in inundation, transitional, and unflooded zones of reservoirs with daily, weekly, seasonal, or yearly regulation. Vegetation composition and local environmental variables were recorded. ANCOVA was used to test differences in vegetation diversity and non-native species proportion among regulation types and WLF zonation. Redundancy analysis and variation partitioning were used to assess the relative effects of dam properties, hydrological alteration, and local environmental factors on species composition. (IV) Riparian vegetation differed clearly among regulation types and WLF zonation. Seasonally regulated reservoirs supported the highest herbaceous species richness in the inundation zone, followed by daily and weekly reservoirs, and then the yearly regulated reservoir. Daily reservoirs had the highest beta diversity, whereas the yearly reservoir had the lowest. Annual species were mainly associated with yearly regulated shorelines, while perennial species dominated daily and weekly reservoirs. Non-native species also varied among regulation types and WLF zonation, with Alternanthera philoxeroides and Erigeron canadensis associated with weekly and seasonally reservoirs, respectively. Indicator species differed among regulation types and WLF zonation, suggesting distinct vegetation assemblages under contrasting regulation regimes. Dam properties were the strongest predictors of riparian species composition, while hydrological alteration and local environmental conditions also contributed. (V) Riparian vegetation composition in cascade reservoirs reflects the cumulative effects of contrasting long-term flow regulation regimes, WLF zonation, and associated environmental filtering. Regulation type was linked to differences in plant diversity, annual-perennial dominance, non-native species occurrence, and community composition. These findings highlight the need to consider long-term hydrological regulation and invasion risk when managing and restoring reservoir WLF zonation.