Tiller Dispersion in Populations of the Bunchgrass Schizachyrium scoparium: Implications for Herbivory Tolerance

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Abstract
We tested the hypothesis that a population structurally modified by herbivory, into a configuration consisting of a high density of small plants, possesses greater herbivory tolerance than a population composed of a low density of large plants. Three levels of plant tiller dispersion were established by transplanting Schizachyrium scoparium plants with basal areas of 30, 60 and 120 cm2 at densities of 2, 4 or 8 plants per 0.25 m2 plot, respectively, while maintaining constant tiller densities of approximately 250 per plot. Tiller dispersion did not significantly affect aboveground production in either defoliated or nondefoliated populations. However, populations constructed with maximum tiller dispersion (8 plants with 30 cm2 basal areas) displayed greater relative increases in tiller density and basal area. These responses were apparently mediated through a mechanism regulating tiller recruitment, as opposed to the efficiency of resource acquisition, because annual aboveground production was comparable across the range of tiller dispersion evaluated. Neither morphometric variables nor water relations of tillers were significantly affected by the pattern of tiller dispersion. However, defoliated tillers had more favourable water potentials and stomatal conductances than nondefoliated tillers irrespective of dispersion treatment. The initial hypothesis was rejected indicating that the spatial arrangement of tillers in populations of S. scoparium was inconsequential in mediating plant responses to herbivory.