The high salt marsh ecotone: A study of its structure and function and the influence of grazing and nitrogen addition on its community dynamics.

摘要

Ecotones are important landscape features, often harboring rich faunal and floral assemblages and serving as filters for materials across the landscape. The goal of my research was to describe the structure and function of the high salt marsh ecotone and to evaluate the influences of grazing and nitrogen addition. My objectives were to answer the following questions: (1) can a community of high marsh plants be identified? Are there any plants that indicate it? (2) What environmental conditions best describe the high marsh? (3) Does the high marsh function as an ecotone by serving as a place of increased nutrients, especially nitrogen, and increased diversity in plant and spider richness? (4) Are marshes that are dominated by salt grass, Distichlis spicata, structured by grazers: either directly by herbivory or indirectly by excess nitrogen input?; To answer the first 3 questions I conducted a survey of 12 marshes in the Pt. Reyes area of California. To address the last question, I conducted manipulative studies to test the hypotheses: (1) grazer exclusion will result in increased vegetation with species rich, homogeneous communities; and (2) Distichlis spicata is a better competitor for nitrogen and will dominate in N-addition plots.; The high marsh appears to be a unique ecotonal community responding to elevational, salinity, and moisture gradients and harboring increased pools of nitrogen. The ecotone had increased plant and spider richness as well. Plant richness was greatest in wider areas of the high marsh, and spider richness was enhanced as vegetation complexity increased.; Excluding herbivory in these nutrient-limited high marsh communities increased vegetation cover and enhanced diversity. Grazing in high marsh shifts plant community structure, in support of the grazer reversal hypothesis of reduced diversity in resource-limited ecosystems. There was no significant effect on spider and ant diversity.; Nitrogen addition did not shift dominance in plots towards Distichlis spicata, as predicted, and plant richness and evenness did not differ significantly between treatments. Although several species thrived (e.g. Salicornia, Distichlis, Triglochin), competitive exclusion did not occur. Instead, nitrogen addition increased overall biomass and tissue nitrogen accumulation, especially in Triglochin concinna and Jaumea carnosa.