Abstract - Effects of Grazing Exposure History and Defoliation on Silicon Concentration of Agropyron smithii
Cid, Maria S. 1985. Effects of Grazing Exposure History and Defoliation on Silicon Concentration of Agropyron smithii. M.S. Thesis. Colorado State University, Fort Collins, CO. 34+ p.
McNaughton and Tarrants (1983) reported that plants native to heavily grazed areas accumulate more silica in leaf blades than plants from lightly grazed areas and that silica concentration in leaf blades is higher in defoiliated than in nondefoliated plants, indicating that silicification is an inducible defense against herbivores.
The goal of this research was to evaluated, under controlled conditions, the hypothesis of McNaugton and Tarrants (1983) by determining whether shoot silicon concentrations are different in two morphologically distinct populations of Agropyron smithii Rydb. plants - one from a heavily grazed prairie dog (Cynomys ludovicianus) colony, the other froman uncolonized exclosure - Wind Cave National Park, South Dakota, and whether defoliation affects shoot silicon concentration.
The higher shoot-silicon concentration in plants from the heavily grazed site suggests that previous grazing exposure influences silicon concentration; however, the fact that plants from the two populations did not differ in total silicon content indicates that the differences in shoot-silicon concentration are indirectly and partly related to previous grazing history through morphological grazing and partly related to previous grazing history through morphological grazing adaptation that could cause plants from sites with high herbivore pressure to have higher blade-to-culm biomass ratios. In addition, the fact that silicon concentration of leaf blades of plants from the two sites were different suggests that the ability to accumulate silicon may be a characteristic selected for by intense grazing pressure. Whether the higher silicon concentrations confer long-term defense against herbivores remains to be demonstrated, but plants on heavily grazed prairie dog colonies are preferentially selected for food by other herbivores, such as bison (Coppock et al. 1983, Krueger in prep.).
Under the experimental conditions of this study, silicification in A. smithii was not induced by defoliation in plants from either heavily grazed or ungrazed sites. On the contrary, regrowth folliage had lower silicon concentration that foliage of nondefoliated controls, especially at the end of the experiment. Therefore, silicification does not appear to be an inducible defense against herbivores in A. smithii, McNaughton and Tarrants (1983) suggested for the African grasses.