Population Structure and Tiller Demography of the Bunchgrass Schizachyrium scoparium in Response to Herbivory

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Abstract
Plant and tiller levels of population structure within the perennial bunchgrass Schizachyrium scoparium were monitored for two field seasons in response to three intensities of livestock herbivory. Herbivory decreased individual plant basal area and increased total plant density by apparently fragmenting individual large plants into smaller units with reduced basal areas but greater tiller numbers per cm2 of remaining basal area. Tiller numbers per m2 decreased in only the most severe herbivory treatment as a consequence of reductions in individual plant basal area and total basal area per m2, rather than to reductions in total plant density or tiller numbers per cm2 of remaining basal area. Herbivory also affected tiller demography by extending the season of recruitment rather than total recruitment, increasing mortality in only the most severely grazed plants and decreasing the number of reproductive tillers. Herbivore-induced modifications in plant and population structure are very likely contributory factors in the decline or perennial bunchgrass populations in response to severe herbivory by decreasing the competitive ability of the population and increasing resource availability for the more grazing resistant species within the community.