Consequences of propagule dispersal and river fragmentation for riparian plant community diversity and turnover

The spatial distribution and temporal availability of propagules fundamentally constrain plant community development. This study experimentally tested several hypotheses about the relative roles of wind and water dispersal in colonization and development of riparian communities along rivers. Through controlling the source of propagules (dispersed by wind, water, or both) reaching newly created, bare river margin sites, we isolated the relative roles of dispersal and other factors in plant community development over five years. Replicated treatments were established at 12 sites spanning 400 km along two adjacent rivers in northern Sweden, one fragmented by a series of dams, the other free-flowing. Bare river margins receiving only water-dispersed propagules had significantly higher species richness compared to plots receiving only wind-dispersed propagules during the initial two years of colonization. Species richness increased annually throughout the study along tranquil and turbulent reaches of the free-flowing river but reached an asymptote at comparatively low richness after a single year on the impounded river. Propagule source strongly influenced species richness during the initial establishment along both rivers, with richness being significantly higher in plots receiving water-dispersed seeds. This strong treatment effect continued to be important through time along the regulated river but diminished in importance along the free-flowing river where other factors such as soil moisture, light availability, and exposure of sites to fluvial disturbance overshadowed the influence of dispersal pathway in mediating species richness. This suggests that hydrochory (plant dispersal by water) may be more important for maintenance of diversity in regulated systems where long-distance dispersal is absent or negligible, but that the rich local propagule source along free-flowing rivers supports high species richness. The number of unique species was higher in water-dispersal plots along both the regulated and free-flowing rivers. This result suggests that hydrochory may contribute to temporal variability of sites, may enhance richness over time, and may have an important role in meta-population and meta-community dynamics of plant communities through long-distance (and local) dispersal and chance colonization. Our findings provide experimental evidence that water dispersal of plant propagules influences colonization dynamics and is important for long-term community development in riparian zones.