Spatial Variation in Process and Pattern in Salt Marsh Plant Communities in Eastern North America

Abstract

While we have learned a great deal about the structure and organization of salt marsh plant communities in the past two decades, this understanding is based on experimental studies conducted at just a handful of study sites. How general are these results and how far can we extrapolate from them to understand other marsh systems? In this paper, we argue that the zonation of eastern North American marsh plant communities may be strongly influenced by both eutrophication and climate, and that spatial variation in these factors may limit our ability to uncritically generalize between marshes. The striking zonation of marsh plant communities has been explained to be the product of competitively superior plants dominating physically mild habitats and displacing competitively subordinate plants to physically harsh habitats. At higher latitudes, this typically results in competitively dominant plants monopolizing high marsh elevations while competitively subordinate plants are limited to lower elevations. Recent studies, however, have suggested that both nutrient supply and thermal stress can influence this simple scenario. Increased nutrient availability, a typical consequence of eutrophication, may alleviate below ground competition for nutrients and lead to above ground competition for light dictating competitive dominance among marsh plants. In marsh systems that historically have been nutrient limited, this may influence plant competitive dominance hierarchies and lead to major shifts in plant zonation patterns. Similarly, climate may have important, but largely unrecognized effects on marsh plant community organization. In cool temperate marshes, low soil salinities result in salinity playing only a minor role in maintaining marsh plant distributional patterns. In contrast, at lower latitudes, hotter climates lead to salt accumulation, elevated soil salinities, and marsh zonation patterns that are strongly driven by soil salinity patterns. We suggest that our current understanding of marsh zonation patterns is oversimplified, and that the processes creating these patterns may vary in importance between marshes. Systematic experimental studies of this spatial variation will be necessary to provide a general understanding of the forces influencing marsh plant community structure.