The hydrochorous dispersal of plant propagules in a giant river reservoir: implications for restoration of riparian vegetation.
Abstract
The riparian vegetation of many rivers around the world is impacted by flow regulation for hydropower. Water levels behind dams are being raised to generate electric energy, forming river reservoirs. River regulation has a large impact on the riparian vegetation, which influences both the adjacent aquatic and terrestrial ecosystems. Therefore, the restoration of degraded riparian vegetation in river reservoirs has been of increasing research interest. Propagules dispersed from connected tributaries via water (hydrochory) are considered a vital source for the recovery of riparian vegetation in regulated rivers. However, the hydrochorous dispersal of plant propagules in river reservoirs is unclear. We explored the dispersal distance and deposition patterns of hydrochorous propagule mimics in three tributaries that are regulated by the Three Gorges Reservoir (TGR) in China. In eight out of nine release experiments, 95% of propagule mimics were found within 3 km downstream from the release points. Cumulative wind speed was the most important factor affecting the dispersal distance of propagule mimics in the TGR. However, the dispersal distance of propagule mimics was not significantly affected by water-level variation and channel sinuosity. Variations in water level strongly affected the deposition pattern of propagule mimics, with only 6.6% of the propagule mimics stranding on the riparian zones under raised water level and 83.8% stranding under declined water levels. The majority of stranded propagule mimics were deposited at gentle slopes (0-20°). Synthesis and applications. Our results suggest that the majority of propagules that enter river reservoirs via water would be retained within the first few kilometres. Wind and water level variation are the main factors determining the dispersal distance and deposition pattern of propagules. Our findings have applications for riparian vegetation restoration in river reservoirs. The vegetation in steep riparian zones distant from free-flowing tributaries should be the priority for restoration actions because these areas receive limited hydrochorous propagules. The plant biodiversity and hydrological connectivity of connected tributaries, which is an important propagule sources, should be protected. The seasonal dominant wind pattern should be considered when evaluating the importance of tributaries as the source of hydrochorous propagules for river reservoirs.