Over this 18-week experiment in South Dakota, Cid et al. examined two morphologically distinct growth forms of Agropyron smithii, cloned from material collected from a prairie dog colony (PDC) and a nearby exclosure (EX), by comparing their silicification patterns. At week 18, A. smithii plants from the PDC population were 60% the height of EX plants, produced 38% more tillers per plant and had leaves which were about 80% the length, 85% as wide, and less erect than leaves of EX plants. Shoot Si concentration of nondefoliated plants was greater in the PDC population (2.2%) than in the EX population (1.9%). For both populations, Si concentration was significantly lower in shoots of defoliated plants than of nondefoliated plants. Although intensive, long-term grazing appears to have selected for plants with higher leaf Si concentrations, this may not be a defense against subsequent grazing. Plant silica concentrations may vary considerably depending on factors such as species, plant age, microclimate, and soil properties, as well as grazing history.
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