Determinants of Pattern in a New England Salt Marsh Plant Community

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Abstract
In New England salt marshes, Spartina alterniflora dominates the low–marsh habitat, which is covered daily by tides. The high–marsh habitat, which is not flooded daily, is dominated on its seaward border by Spartina patens, and on its terrestrial border by Juncus gerardi. Each of these vegetation zones has a characteristic suite of physical factors associated with differences in tidal inundation. In particular, substrate redox increases and salinity decreases with decreasing marsh elevation. Although correlations between physical factors and the occurrence of specific marsh plants have been suggested to be causal, a 5–mo transplant experiment suggested that the distribution of perennials across the marsh does not correspond to their potential performance across the marsh in the absence of surrounding vegetation. While the high–marsh perennials appear to be restricted to the high–marsh habitat by harsh physical conditions in the low–marsh habitat, the low–marsh dominant, S. alterniflora, is capable of vigorous growth across the entire marsh and appears to be excluded from the high–marsh habitat by the high–marsh perennials. Throughout the high marsh, two other plant species, Distichlis spicata and Salicornia europaea, are found associated with areas that have been disturbed recently. Physical disturbance, in the form of mats of dead plant material (wrack) rafted by tides onto the marsh, is most severe in the spring and early summer, and decreases with increasing marsh elevation. Differential plant mortality results from short–term disturbance events. D. spicata and S. alterniflora are more tolerant of wrack burial than are the other marsh plants, and short–term disturbance increases the relative abundance of these species in the community. Longer lasting disturbance events kill all the underlying vegetation, leaving discrete bare patches throughout the high marsh. D. spicata rapidly colonizes these patches with vegetative runners, while S. alterniflora and Sa. europaea recruit to these patches by seed. The relative abundance of these plants in recently created bare patches exceeds greatly their relative abundance in the surrounding vegetation. Over time, however, these early colonizers are overgrown and displaced in high–marsh patches by S. patens and J. gerardi, which grow slowly, as dense turfs of roots, rhizomes, and tillers. Physical disturbance and interspecific competition appear to be major determinants of the spatial pattern of marsh plant communities. These processes will need to be considered in relation to edaphic factors in elucidating the underlying mechanisms of salt marsh plant zonation.