The impact of cattle grazing on salt marsh and elevated hummock vegetation communities of a Texas barrier island

Abstract

To assess the effects of cattle herbivory on vegetation community structure and composition in a Texas coastal salt marsh, data measuring several vegetation parameters were collected in four distinct habitats within a heavily grazed marsh over an eighteen month period. Grazing reduced total biomass and total cover in the intertidal zone, mainly due to reductions in Spartina alterniflora Loisel. Biomass and cover of the non-palatable Salicornia virginica L., the other common species in the zone, was relatively unaffected. Grazing had a negative effect on average height of both species. Grazing effects increased with elevation for all tested variables. Substantial shoreline erosion was measured, but marsh loss was unattributable to grazing. Grazing also strongly reduced cover and average height of both species in the intertidal marsh/salt flat boundary ecotone. The S. virginica vegetation line in ungrazed plots advanced landward while the line in grazed plots receded. Results for S. alterniflora vegetation line migration in response to cattle activity were unclear. Data tracking average distances between the vegetation line and nearest landward individual of the two species also suggest that cattle presence may have inhibited the natural expansion of the vegetation line. The negative impacts of cattle in the intertidal marsh and the marsh flat ecotone on S. alterniflora performance were almost certainly due to consumption of plants by cattle, while reduced S. virginica performance was attributed mainly to deterioration of edaphic conditions and other associated effects of trampling. In the more diverse hummocks, grazing effect was directly related to elevation for three plant categories, with greater reductions in cover and average height observed in grazed plots in higher elevation zones. These results were attributed to the increased palatability of higher zone grasses compared to the less palatable halophytic succulents and forbs occurring in the highly saline lower zones. The results also showed a temporal increase in grazing effect for most species over the course of the study. Grazing also resulted in decreased species richness in the hummocks. Grazing generally inhibited growth and accelerated mortality of transplants in the salt flats, although ultimate survival was probably due more to physical factors.