Date of Award

1996

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biological Sciences

First Advisor

James B. Grace

Abstract

Both vertebrate herbivores and fire have long been known to have dramatic and important effects on wetlands. In the first part of this study, conducted in the Pearl River Basin coastal marshes of Louisiana, the interaction between the effects of mammalian herbivores, especially nutria and wild boar, and fire was examined in three marsh community types: those dominated by Sagittaria lancifolia, Panicum virgatum, or Spartina patens/Scirpus americanus. Overall, above-ground biomass was reduced by burning but increased by fencing. Richness only increased in plots that were both burned and fenced. In the three communities, only Scirpus americanus cover was enhanced by fencing. Fencing reduced Spartina patens cover in the Panicum and Sagittaria marshes. Cover of Panicum virgatum in the Panicum marsh and Sagittaria lancifolia and Vigna luteola in the Sagittaria marsh were all enhanced by burning. Therefore, burning shifts the communities from S. patens to S. lancifolia, V. luteola. In the second part of this study, exclosures were used in conjunction with feldspar and benchmark techniques to measure the effects of herbivore exclusion on vertical soil accretion, marsh elevation changes, and litter production. Overall, vertical soil accretion appears to be contributing sufficient material to offset the effects of subsidence in both the grazed and ungrazed plots. However, the examination of the root zone over time indicates that not only is this layer expanding and contracting, but it appears that it also is contributing to soil elevation change. The third part of this study was conducted in order to assess the impact of herbivores on plant biomass, cover, and species richness. Biomass was four times as great in exclosed plots for both Spartina patens and Scirpus americanus compared to the control plots. A detrended correspondence analysis revealed that exclusion of grazers resulted in overall shifts in community composition. Species richness increased in the grazed plots after fifteen months and decreased in the ungrazed plots, with a total difference in richness of two species. Overall, the results suggest that grazing in the system selectively alters species composition and increases species diversity.

ISBN

9780591133530

Pages

120

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