Silicon Concentration of Grasses Growing in Sites With Different Grazing Histories

In recent studies of three grasses from the Serengeti Plains of Africa, McNaughton and Tarrants (1983) and McNaughton et al. (1985) reported that silica concentrations were greater in leaves and other organs of species, or populations within a species, that had evolved under heavy grazing pressure than in those from more lightly grazed areas. They also observed that silica concentrations increased in leaf blades and sheaths following defoliation. These investigators inferred, therefore, that silicification of grass parts is a product of herbivore-mediated natural selection and that leaf silicification represents an inducible defense against herbivores. The objective of the author's research was to evaluate this hypothesis for two native North American grass species by determining shoot silicon concentrations through out a growing season at sites with different grazing histories. They also conducted a field experiment to determine the effect of mechanical defoliation on shoot silicon concentration in one of the species.