Variation in response to defoliation between populations of Bouteloua curtipendula var. caespitosa (Poaceae) with different livestock grazing histories.

Distinctive plant growth and development have been associated with resistance to defoliation by herbivores. Descriptions of resistance (tolerance/avoidance) strategies are common, although little is known about genetic variation affecting their expression in natural plant communities. This research compared response to three frequencies of defoliation in populations of Bouteloua curtipendula var. caespitosa from Arizona with different histories of exposure to domestic herbivores. One population occurs on a site that has likely been unaffected by large herbivores including cattle, while the other has been regularly grazed by cattle. Mixed-model analysis of variance was used to examine variation in traits associated with defoliation resistance. Data were collected in a 168-d greenhouse experiment using ramets of 15 genets from each population. Mortality was lower in the cattle-impacted population, suggesting improved defoliation resistance. This was associated with increased production of tillers with lower mass following defoliation. Plasticity was observed for all traits in response to different defoliation frequencies. The cattle-free population exhibited significant genetic variation for plasticity affecting leaf blade angle, while little variation was noted for this trait in the cattle-impacted population. Evidence of reduced genetic variation within the cattle-impacted population was noted for traits associated with defoliation resistance. In general, these observations are consistent with expectations following selection for resistance to defoliation.