Although many studies have described relationships between diversity and productivity, less is known about the mechanisms producing these relationships. In Georgia tidal marshes, we found a hump-shaped relationship between productivity and species richness across the estuarine salinity gradient. In salt marshes, both plant species richness and biomass were low; in brackish marshes, species richness was intermediate and biomass was high; and in fresh marshes, species richness was high and biomass intermediate. We hypothesized that physical stress would decrease species richness at higher salinities, but that biomass was potentially greatest at intermediate salinities due to beneficial effects of salt and sulfate on nutrient availability. We are conducting reciprocal transplant experiments to test these hypotheses.

Results/Conclusions

The freshwater plant Zizaniopsis miliacea died when it was transplanted into fresh and brackish marshes. The brackish marsh plant Juncus roemerianus survived in all salinity zones but grew best at lower salinities. In contrast, the saltmarsh plant Spartina alterniflora grew best in brackish marshes when neighbors were removed, suggesting that conditions other than salt stress were best at intermediate salinities. Competitive effects of neighbors were weak in salt marshes but strong in brackish and fresh marshes. These results suggest that physical stress imposes a strong filter limiting species richness along the salinity gradient, that the productivity of the habitat peaks at intermediate rather than low salinities, and that competition does not reduce species richness as severely as physical stress does. We are transplanting additional species to test the generality of these results.