The role of dispersal in shaping plant community composition of wetlands within an oldgrowth forest

1. Dispersal ability can influence the importance of dispersal relative to other processes organizing metacommunities, such as species sorting among habitats along environmental gradients. 2. We compare plants with different dispersal modes and habitat affinities, evaluating the roles of environmental and spatial controls on plant community composition in 128 wetlands within 10 km of old-growthmaple–beech forest in southern Québec, Canada. 3. We address two hypotheses. First, we ask whether species with short-distance dispersal mechanisms are more dispersal-limited than species with adaptations for long-distance dispersal. Second, because wetland habitats are more fragmented than upland habitats in this forested area, we test the hypothesis that wetland species are more dispersal-limited than upland species growing in the same wetlands (e.g. on hummocks within a swamp). 4. Variation partitioning based on constrained ordinations showed that the distributions of species with short-distance dispersal mechanisms related more strongly to spatial factors than the distributions of long-distance dispersers, supporting the interpretation that these species are more dispersallimited. The distributions of short-distance dispersers also showed finer-scale spatial patterns than the distributions of long-distance dispersers. 5. Distributions of wetland species related more strongly to environmental conditions than the distributions of upland species growing in the same wetlands, suggesting that wetland species are actually less dispersal-limited than upland species. Wetland and upland species had similar patterns of spatial variation in community composition. 6. Synthesis. The processes of dispersal limitation and species sorting along environmental gradients have differential importance to plants that growwithin the same communities, but differ in dispersal ability and habitat affinity. This result emphasizes the impact of dispersal ability on the organization ofmetacommunities.