Brizuela, Miguel A. 1985. Silicon Concentration of Grasses from Sites with Different Prairie Dog and Bison Grazing Histories. M.S. Thesis. Colorado State University, Ft. Collins, Colorado. 36 p.

Abstract

It has been suggested that grass-leaf silicification is a product of natural selection and an inducible defense against herbivores (McNaughton and Tarrants 1983). According to this hypothesis, species or populations within a species that have evolved under heavy grazing pressure should have higher leaf-silicon concentrations than those evolving in lightly grazed areas.

The main goal of this research was to evaluate the this hypothesis by experimentally assessing the seasonal silicon concentration of Agropyron smithii and Schizachyrium scoparium growing at mixed-grass prairie sites with different histories of grazing by prairie dogs and bison in Wind Cave National Park (WCNP), South Dakota. The complementary goal was to determine the effect of defoliation on the seasonal silicon concentrates of S. scoparium growing in a grazing exclosure at WCNP.

Silicon concentration in shoots of A. smithii and S. scoparium tended to increase through time, reaching maximum values late in the growing season. In addition, shoots of both species from heavily grazed prairie dog colonies typically had higher silicon concentrations that those from lightly grazed, uncolonized sites. Moreover, the results suggest that the longer an area has been colonized by prairie dogs, the greater the silicon concentration in the foliage will be. In spite of having higher shoot-silicon concentrations, plants from prairie dog colonies are grazed preferentially by bison over those from nearby, uncolonized areas (Coppock et al. 1983a,b; Kruegar in prep.). This suggests that silica may not be an effective defense against these native herbivores, as has been proposed for native African grasses as herbivores.

Results of the defoliation experiment indicated that defoliated plants of S. scoparium always had lower concentrations of silicon than nondefoliated plants over the growing season. This result does not support the hypothesis that defoliation induces an increase in plant-silicon concentration of this species, as has been suggested for several African species.