Anderson, Val Jo (1989-04). Species replacement in response to herbivory : influence of grazing history, competitive interactions and population structure. Doctoral Dissertation.
Thesis
Although the concept of range condition classification has served as a basic tenant of range management in the United States for nearly 40 years, the mechanisms associated with species replacement have received remarkably little investigation. Two major viewpoints which address the causal mechanisms of species replacement are: (1) differential utilization among species based primarily on growth-form and (2) differential regrowth ability among species based on meristematic availability and physiological attributes. The potential mechanisms facilitating species composition changes in response to long-term herbivory were investigated in a series of three experiments within a transplant garden. The research approach involved the establishment of the climax dominant, Schizachyrium scoparium, into three neighborhoods (Schizachyrium scoparium, Bothriochloa saccharoides, and Stipa leucotricha) representing herbivore-induced species composition changes as herbaceous retrogression progresses in the southern true prairie. Disproportionate utilization of the climax dominant, as opposed to differential regrowth capabilities among species, was the predominate mechanism contributing to the replacement of the climax dominant by lower seral species. Intraspecific neighborhoods provided a more competitive environment for the climax dominant than neighborhoods of the two lower seral species indicating that the regrowth capability of the climax dominant equaled or exceeded that of the lower seral species which replace it under intensive herbivory. Climax dominants having a prior history of herbivory did not express significant architectural differences or increased competitive abilities in the presents of interspecific competition in comparison with plants without a history of herbivory. Tiller dispersion into a configuration represented by a large number of small plants did not increase production following defoliation in comparison to a similar number of tillers in fewer, larger plants. The mechanism inducing species replacement in the southern true prairie is largely under managerial control as it relates to the frequency, intensity and seasonality of herbivory.
