ZONATION OF SPARTINA-PATENS AND SPARTINA-ALTERNIFLORA IN A NEW-ENGLAND SALT-MARSH

In New England salt marshes the boundary between frequently flooded low marsh habitats and less frequently flooded high marsh habitats is characterized by striking plant zonation. Spartina alterniflora monocultures dominate low marsh habitats while the seaward border of high marsh habitats is generally dominated by Spartina patens. In this paper I examine the role of interspecific competition in maintaining this zonation pattern. Spartina patens turfs and tillers transplanted into the low marsh were severely stunted with or without S. alterniflora neighbors, and low marsh bare patches bordering S. patens monocultures were not significantly colonized by S. patens in three growing seasons. The limited ability of S. patens to oxygenate its rhizosphere in anoxic soils appears to limit S. patens to high marsh habitats. In contrast, S. alterniflora transplants grew vigorously in the high and low marsh when buffered from neighbors, but were excluded from the high marsh in 2-3 yr when S. patens was present. S. alterniflora also rapidly invaded the high marsh in the absence of S. patens. These results support the hypothesis that S. alterniflora is restricted to low marsh habitats by competitive displacement. S. alterniflora thrives in anoxic low marsh habitats due to its ability to oxygenate its roots and rhizosphere. Rhizosphere oxidation by S. alterniflora, however, is not evident in seedlings and small colonizing patches and both seedlings and small colonizing patches of S. alterniflora are stunted in anoxic low marsh substrates. This suggests that the success of S. alterniflora in anoxic habitats is size dependent and may be driven by group benefits of rhizosphere oxidation. These results suggest that the maintenance of intertidal zonation in rocky beach and marsh plant communities is very similar. In both assemblages, competitive dominants monopolize physically benign habitats and displace competitive subordinates to physically stressful habitats.