1 The importance of dispersal for plant community structure is poorly understood. Previous studies have hypothesized that patterns in the distribution and genetic structure of riparian plant communities were caused by hydrochory, i.e. plant dispersal by water. We separated the relative contributions of propagules from hydrochory and other dispersal vectors by comparing colonization in pairs of plots, one subject to flooding and deposition of hydrochores and the other unflooded. 2 The number of colonizing individuals and the mortality rate of individuals per year did not differ significantly with flooding, but hydrochory increased the number of colonizing species per year and plot by 40-200%. The pool of colonizing species was 36-58% larger per year for flooded than for unflooded plots, indicating that hydrochory increased the diversity by facilitating long-distance dispersal. Hydrochory resulted in more diverse plant communities after 3 years of succession at both plot and reach scales, despite the fact that flooding caused plant mortality. 3 We found no evidence that dams reduce the abundance and diversity of water-dispersed propagules by acting as barriers for plant dispersal. The role of hydrochory for plant colonization was similar between a free-flowing and a regulated river, although in fragmented rivers propagule sources are likely to be more local (within-impoundment). 4 We conclude that plant dispersal by water, as well as fluvial disturbance, is important for enhancing species richness in riparian plant communities. As flowing water may carry buoyant seeds long distances, riparian plant communities may receive a comparatively large proportion of their seeds by long-distance dispersal.
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