The effects of defoliation on biomass distribution were studied in plants of Holcus lanatus, Lolium perenne, and Trifolium repens, collected from three different-aged pastures in British Columbia. Data for all three species illustrated that with increasing pasture age, there was a significant trend of increasing compensatory clonal growth in response to defoliation. Plants from the three pastures showed no differences in tiller numbers or stolon length in the absence of defoliation. This suggests that selection has favored those genotypes in the oldest pasture that respond to defoliation by increasing clonal propagation. Although the number of ramets increased, biomass production, in general, was not stimulated by defoliation in the two grass species, but in some cases, was actually reduced. Compensation was evident for H. lanatus from the oldest pasture. In T. repens, however, stolon biomass was stimulated by clipping, but only in plants from the oldest pasture. These results may be interpreted as a consequence of selection pressure from grazing, however, the data do not suggest that the grass-grazer relationship is a mutualism. The results reported here suggest that selection among species susceptible to grazing may favor a high potential for the plastic response of clonal expansion and hence long generation times.
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