Although the plant community composition of tidal marshes has long been known to shift across estuarine landscapes, only a few studies have examined the mechanisms responsible for such plant distribution patterns. In Georgia tidal marshes at the southeast Atlantic coast, we used a combination of monitoring and experimental transplants to study the mechanisms mediating plant distribution at 12 sites across the estuarine landscape. We hypothesized that abiotic conditions would be most severe in the saltiest marsh sites, and competition strongest in the freshest marsh sites.
Results/Conclusions
The results were only partly consistent with our hypotheses. High salinity levels in brackish and salt marshes were extremely stressful to freshwater marsh plants, but, with fairly high water logging intensity and very low salinity levels (and/or some other abiotic factors), freshwater marsh habitats also were stressful to some salt marsh plant species. Relative competition intensity between background vegetation and transplanted plants generally peaked at one of the brackish sites, where the background vegetation had the highest aboveground productivity, rather than at the freshest sites. A strong correlation between competition intensity and light interception by background vegetation across sites indicated that light might be the main resource that plants competed for in more productive marshes. Background vegetation facilitated some transplanted plants in the saltiest site, showing that positive biotic interactions may play a role in maintaining plant species richness in the most stressful environments. These results illustrate the complexity of natural gradients in mediating plant distributions, since multiple factors, both abiotic and biotic, may vary in different ways across the landscape, and different plant species may respond to these factors differently.