Interspecific Interactions among High Marsh Perennials in a New England Salt Marsh

Abstract

High marsh habitats in New England exhibit conspicuous zonation of vascular plants. Spartina patens and Juncus gerardi dominate the seaward and terrestrial borders of the high marsh, respectively, whereas Distichlis spicata is common in disturbed habitats. In this paper I examine the role of interspecific interactions among these marsh perennials in maintaining marsh plant zonation. Removal and transplant experiments were performed to examine species interactions in dense stands. In both types of experiments J. gerardi competitively dominated S. patens and D. spicata, while S. patens competitively dominated D. spicata. Each of these perennials showed maximum growth when transplanted into the J. gerardi zone without neighbors. J. gerardi appears to dominate the terrestrial border of the marsh by competitively excluding S. patens to lower marsh levels, while D. spicata is competitively restricted to disturbed habitats by boty J. gerardi and S. patens. Species interactions during the colonization of bare patches were strikingly different than those found in dense vegetation. Colonization of bare patches by boty S. patens and D. spicata facilitated the colonization of J. gerardi, the competitive dominant in dense vegetation. Bare patches in the high marsh are typically hypersaline due to increased surface evaporation in the absence of plant cover. Patch colonization by competitive subordinates, which are relatively salt tolerant, appears to reduce substrate salinity by passively shading the substrate and, as a by—product, facilitate the invasion of a superior competitor. With interspecific competition in dense vegetation dictates the zonation of New England perennials in the high marsh, secondary succession in this assemblage may commonly be driven by facilitations. This scenario may not be uncommon among assemblages in harsh physical environments where plants or sessile animals ameliorate their physical environment.