Effect of grazing history, experimental defoliation, and genotype on patterns of silicification in Agrostis tenuis Sibth.

We examined the effect of grazing exclosures in reducing the concentration of silica, the effect of experimental defoliation on silica accumulation, and the existence of genotypic variability in silica content in plants of Agrostis tenuis. Silica content of plants was higher in heavily grazed areas than within exclosures during the summer, but no significant differences in content were found in winter. Laboratory experiments indicated that there were significant differences among genotypes in their silica content. Some genotypes increased their silica content after clipping at intervals of 14 days but most of them reduced their silica content when clipped weekly. The existence of a significant genotype-clipping treatment interaction term showed that silica accumulation as a response to clipping varied among genotypes, and may provide the basis for microevolutionary changes in natural populations of A. tenuis. However, the possibility of silica acting as a defence was not supported when caterpillars of the graminivorous Lepidoptera, Pararge aegeria, fed on plants. The caterpillars fed more extensively on plants from grazed areas, which had a higher silica content. We conclude that there is some genetic variation in patterns of silica accumulation in plants of A. tenuis and that different grazing histories lead to differences in silica content of plants. Although the effect of silica concentration on large herbivores remains uncertain, herbivores can exert an effect on the silica content of the leaf blades of A. tenuis.