Butler, Jack Lee (1986-04). Demography of the bunchgrass Schizachyrium scoparium Michx. in response to intraspecific competition and herbivory. Doctoral Dissertation.
Thesis
Two separate investigations were conducted to study the influence of intraspecific competition and herbivory on plant and tiller demography of the bunchgrass Schizachyrium scoparium. The investigations were conducted over two growing seasons (May 1983-October 1984) in the post-oak savannah region of east-central Texas. Intraspecific competition was reduced by altering either plant densities per unit area or tiller densities per plant. Population response to herbivory was investigated by placing two steers within replicated pastures of 0.54 ha (treatment 1) and 0.41 ha (treatment 2). The pastures were rotationally grazed with 2-day grazing periods alternated with 32-day periods of no grazing. Tiller numbers within S. scoparium were observed to be under density-dependent control from both interplant and intertiller interference. Tiller recruitment was substantially increased in plants of both thinning treatments compared to control plants within six weeks following treatment application. Tiller recruitment was greater on the periphery than in the interior of the plants in all treatments. However, individual tiller height or weight remained unaffected by the thinning treatments. Tiller mortality was unaffected by plant thinning or location within the plant. Peak tiller recruitment in plant thinned plots occurred during early spring while peak recruitment in tiller thinned plants occurred later in the growing season. This suggests a temporal separation between the two population regulation mechanisms. Herbivory influenced demographic parameters of both plants and tillers. Average basal area per plant decreased in response to herbivory while the number of live tillers per remaining plant basal area increased. Contrasting trends in plant number per unit area and tiller number per plant produced no significant difference in tiller number per unit area. Recruitment was the primary mechanism controlling tiller numbers in both grazed and ungrazed treatments. In addition to greater tiller production in grazed plants, tillers were initiated several months earlier which resulted in the establishment of a greater number of tiller cohorts...
