Variation in tidal wetland plant diversity and composition within and among coastal estuaries: assessing the relative importance of environmental gradients

Questions What is the relative importance of topographic (elevation), edaphic (soil salinity, nitrogen and particle size) and hydrologic (estuarine river flow) gradients for variation in tidal wetland plant composition and diversity? Location Four Oregon estuaries: a marine-dominated lagoon, two tidal-driven bays, and a river-dominated site. Methods We surveyed species presence, cover and richness; and environmental factors (soil salinity, grain size, soil nitrogen and elevation) in plots in marsh and swamp. We assessed patterns of community structure and the relative importance of environmental gradients with hierarchical partitioning, ordination, species accumulation curves and path analysis. Results The relative importance of measured environmental gradients on plant occurrence differed by species. Soil salinity or elevation explained the most variation in the majority of common species. Estuarine hydrology, soil nitrogen and soil clay content were usually of secondary or minor importance. Assemblage composition and species richness varied most strongly with tidal elevation. Local soil salinity also affected composition, but differences in estuarine hydrology had comparatively less effect on composition and richness. Higher-elevation wetlands supported larger species pools and higher plot-level richness; fresher wetlands had larger species pools than salt marsh but plot-level richness was relatively invariant to differences in soil salinity. Conclusions Elevation and salinity tended to exert more influence on the vegetation structure of tidal wetlands than estuarine hydrology or other edaphic variables. With relative sea-level rise expected to increase both flooding intensity and salinity exposure in future wetlands, global climate change may lead to changes in species distributions, altered floristic composition and reduced plant species richness.