Plant dispersal along rivers fragmented by dams

Abstract

To understand the influence of dams on connectivity of riparian plant communities along rivers, we examined plant dispersal by water (hydrochory) and riparian plant community attributes upstream and downstream from dams on two rivers in the southern Rocky Mountains, Colorado, USA. Drifting plant propagules were collected from the water column along reaches upstream and downstream from dams to examine the longitudinal and temporal variation in seed‐pool species composition and concentration of water‐transported seeds. Similarities between species composition of the hydrochoric seed pool and local standing riparian vegetation were used to evaluate the degree of longitudinal connectivity along river corridors and to isolate the relative contributions of local versus regional species pools to hydrochoric species composition. Furthermore, several synthetic attributes (longevity, origin, life‐form and dispersal mode) and species composition of riparian plant communities were examined to explore the effects of interrupted propagule dispersal on standing vegetation. We estimated that as many as 120 million seeds were transported via hydrochory along free‐flowing reaches of the Rocky Mountain streams in a single growing season. Seed concentration (seeds/m 3) in the water column was reduced by 70–94% along reaches downstream from dams compared to free‐flowing reaches. The similarity in species composition of hydrochoric seeds and local standing vegetation was nearly two times greater downstream from reservoirs compared to upstream. This suggests that hydrochory complements local species pools by importing seeds from throughout the upstream catchment area along free‐flowing river reaches, but that hydrochoric seeds are derived primarily from local sources along regulated river reaches. Species richness recovers as a function of downstream distance from contributions of standing vegetation and seeds from tributary streams. Hydrochory may extend the period over which viable seeds of a parent population are dispersed. Even after dispersal of parent populations has terminated, seeds may continue to be available due to residence time in water transport. This extension of the ‘effective dispersal window’ of some species may exceed two weeks or more and may influence the likelihood of successful establishment. In this study, synthetic attributes of riparian vegetation did not differ significantly between free‐flowing and regulated reaches, whereas formal statistical comparisons of community composition upstream and downstream from reservoirs indicate that there are differences in community composition upstream and downstream from dams. These findings suggest that the consequences of 50 to 100 years of fragmentation result in community‐wide effects along Rocky Mountain streams and that these effects may be partially explained by dam‐caused disruption in connectivity of plant populations. Copyright © 2005 John Wiley & Sons, Ltd.