The effects of dominant bunchgrass species on sandhill longleaf pine savanna ecosystem function: a comparison of wiregrass to the bluestems.
West, Jason Brossard
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A key question in understanding plant-soil relationships is to what degree are these relationships species-specific? The research presented in this dissertation is a comparison of several bunchgrass species native to, and dominant in, the fall-line sandhill Pinus palustris savannas. I compared relevant plant traits, as well as components of C and N cycling to determine whether these species differ in their effects on those processes and what traits may account for those differences. In a greenhouse experiment I demonstrated performance differences between Aristida stricta (wiregrass) and Schizachyrium scoparium (little bluestem), as well as differences in their responses to added N. Aristida stricta grew more slowly and responded less to N addition than did S. scoparium, suggesting differences in nutrient use strategies. In a field study differences in plant tissue quality as well as species-specific influences on C and N cycling were found at the scale of an individual plant, especially between A. stricta and the "bluestems" (S. scoparium and Andropogon spp.). In a study of planted monocultures of these species, I showed species-specific root proliferation responses, as well as species-specific depletion of inorganic N pools following N addition. Aristida stricta surprisingly showed the greatest proliferation response to N addition. The ability of the bluestems to deplete the available N pool depended on its size and to a certain extent its form (ammonium versus nitrate). In an observational study of fine root dynamics and soil resources I showed a clear gradient in resource availability that corresponded with a gradient in soil type, as well as vegetation dominance. Finally, a comparison of patch types within the middle of the gradient dominated by either wiregrass or a mix of bluestem species showed little difference in fine root dynamics or soil resource availability between these patches. An interesting pattern in root production did emerge, however, that is consistent with the root production phenology of S. scoparium versus A. stricta. Although these relationships can be complex, I conclude generally that species-specific effects on belowground ecosystem function are common and may be related to differences among species in adaptation to soil conditions.