Comparative impacts of dam water level regimes on herbaceous plant growth strategies in cascade reservoirs and downstream reaches of a major river

Abstract

Flow regulation is a prolific and growing influence on rivers world‐wide. Nine cascade hydropower dams were constructed along the 1,150‐km Wujiang River in China over the past 30 years, disrupting longitudinal continuity. Water level fluctuations in the associated reservoirs range between daily, weekly, seasonal, and annual, depending on the type of regulation, but the comparative impacts of these regimes on plant growth strategies, or the extent of their downstream influence, is unknown. Competitor, stress‐tolerator, and ruderal (CSR) plant strategies were used to assess the impact of reservoir regulation type on the riparian herbaceous plant community based on sampling the inundation zone of nine reservoirs and their downstream river reaches during 2017 and 2018. Our results revealed profound differences in CSR plant strategies of the dominant vegetation with respect to water level regime. While ruderal plants dominated (45%–60% of species), irrespective of regulation type, vegetation in reservoirs exhibited a strong shift from stress‐tolerators (e.g., Cynodon dactylon, C‐11.9:S‐41.5:R‐46.5%) to competitors (e.g., Reynoutria japonica, C‐77.9:S‐0:R‐22.0%) with increasing intensity of water level fluctuation, reflecting the shift from annual to daily regulation. The width of the inundation zone was the best overall variable in explaining the CSR strategies of riparian vegetation, both in the reservoir inundation zone (r2‐adj = 15.4%) and the downstream river (r2‐adj = 7.3%). Retention time significantly explained variation in CSR plant strategies in the reservoir inundation zone (r2‐adj = 3.7%, p = 0.002) but not downstream (p > 0.01). There was also a clear scale dependency of CSR plant strategies, with an increase in stress tolerators (average slope = 0.7%/km) and decline of competitor (average slope = −0.3%/km) and ruderal plants (average slope = −0.9%/km) with increasing distance downstream from dams. The growth strategies of the dominant riparian vegetation changed with the magnitude and frequency of water level fluctuations caused by differences in regulation type, and local environmental conditions. Clear scale dependency in the CSR plant strategies was observed with distance from the dam, with ruderals dominating closest to the reservoirs and declining gradually downstream as stress tolerators increased. Our study helps to evaluate the impact of river damming on the functional traits of riparian vegetation and to predict the resilience and restoration potential of riparian vegetation under different forms of human disturbance.