Nutrient effects on the composition of salt marsh plant communities along the Southern Atlantic and gulf coasts of the United States

Abstract

Nutrient availability is known to mediate plant community structure in many systems, but relatively few studies of nutrient effects have been done in systems where strong gradients in physical stress might constrain the effects of nutrients. Recent studies in New England, United States, salt marshes indicate that nutrients may strongly mediate plant community composition by increasing the competitive ability of stress-tolerant species that are normally displaced by competition to recently-disturbed or low-intertidal habitats. It is unknown whether these results can be generalized to salt marshes in other geographic regions that experience different climates, tidal regimes, and edaphic conditions. To address the generality of these results from New England, we fertilized seven different mixtures of salt marsh plants at study sites on the southeast and Gulf coasts of the U.S. Two of these mixtures were studied in both geographic regions. Consistent with results from New England, fertilization always increased the biomass of the low-marsh dominantSpartina alterniflora and usually led to it increasing in dominance at the expense of high-marsh species. Fertilization also led to increased community dominance byDistichlis, but only in a mixture where it was already common. Fertilization led to changes in plant dominance patterns in four of the seven types of mixtures that we studied. Results were not a function of edaphic conditions, at least within the range included in our study, and were consistent between the southeastern and Gulf coasts, which experience markedly different tidal regimes. The broad similarity of these results suggests that changes in nutrient input may lead to predictable changes in the composition of similar salt marsh plant communities across large geographic areas despite site to site variation in the abiotic environment.