http://www.loc.gov/mods/v3 http://www.loc.gov/standards/mods/v3/mods-3-2.xsd
Mechanisms of persistence and species replacement in populations of Bouteloua curtipendula with contrasting grazing histories
A series of experiments were conducted with two perennial grasses to evaluate the contribution of persistence mechanisms and the impact of selective defoliation on species replacement in three communities with contrasting grazing histories on the Edwards Plateau, Texas. The communities had either been ungrazed, moderate-rotationally grazed or continuous-intensively grazed by domestic livestock since 1948. Sixty Bouteloua curtipendula plants in each of these three communities were randomly assigned to four treatments, including a non-defoliated control, to evaluate the impact of selective defoliation. Reproductive tillers of the late seral species, B. curtipendula, and the mid-seral species, Hilaria belangeri, were permanently marked in the ungrazed and intensively grazed communities in June 1993 and destructively harvested in October 1993, May and October 1994 and May 1995. Maximum axillary bud longevity was 23 months which exceeded seed longevity by 1.5 to 2 times in R curtipendula and the longevity of axillary buds in the late seral species was six months longer than in the mid-seral species. However, the contribution of dormant axillary buds to persistence was minimized by low tiller initiation (<10%). Modifications in population structure induced by long-term grazing suppressed plant and tiller demographic responses more than short-term defoliation. B. curtipendula had a higher density of smaller plants in the intensively grazed community, compared to low densities of larger plants in the ungrazed and moderately grazed communities. Selective defoliation did not affect B. curtipendula tiller variables in any community and tiller per tiller recruitment and mortality rates were comparable between communities. Still, cumulative tiller recruitment of B. curtipendula in the intensively grazed community was 44% of that in the ungrazed community because of high plant mortality (50%). The minimal response of plant variables to short-term defoliation in the intensively grazed community, compared to the ungrazed and moderately grazed communities, provides evidence that B. curtipendula has passed a population threshold in the intensively grazed community. The importance of population structure in determining responses to short-term selective defoliation indicate its ecological importance may rival that of species composition as an indicator of vegetative change.
"Major Subject: Rangeland Ecology and Management".
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Includes bibliographical references: pages 86-98.