Seed dispersal, habitat suitability and the distribution of halophytes across a salt marsh tidal gradient

Abstract

Summary 1 The relative importance of seed availability and the post‐dispersal environment in causing the distribution and abundance patterns of five halophytic forbs and a shrub was investigated across a New England salt marsh tidal gradient. Seed traps and soil samples were used to assess the spatial pattern of seed availability across the marsh, and experimental seed additions were performed to examine the effects of tidal elevation and interspecific competition with dominant grasses and rushes on seedling emergence and survivorship. 2 Seed distributions strongly paralleled adult plant abundance patterns across the marsh, suggesting localized dispersal with limited movement out of parental environments. 3 Adding seeds typically increased seedling densities by at least an order of magnitude, thus lack of seed availability may be important in limiting plant abundance within marsh zones. 4 Post‐dispersal factors were primarily responsible for determining species distribution patterns across zones. Lower limits to the distribution of species typically found at high‐marsh elevations were determined by intolerance to abiotic conditions in the lower marsh zones. In contrast, species typically found at low‐marsh elevations were precluded from the high marsh due to competitive suppression by dominant plants. Patterns of post‐dispersal success were strongly reinforced by limited dispersal. 5 Seed dispersal patterns and post‐dispersal factors may therefore interact to generate distribution and abundance patterns in salt marsh plant communities.