Polley and Detling collected western wheatgrass (Agropyron smithii) from a heavily grazed area in South Dakota, colonized by black-tailed prairie dogs (Cynomys ludovicianus), and from an adjacent lightly grazed, noncolonized site. Total and residual biomass yields and biomass yields of components of both aboveground and belowground structures were greater in on- than off-colony plants of A. smithii in mixtures across defoliation regimes. Thus, growth rate was greater in on- than off-colony plants in mixtures, as there were no significant differences between populations in initial biomass per plant. Defoliation produced a similar relative reduction in cumulative biomass yield of plants of two populations. Proportional loss of biomass to defoliation was greater in off-colony individuals, which had fewer and larger tillers and greater relative investment of biomass in aboveground components under defoliation, than in on-colony plants. It is suggested that rapid production of small tillers contributes to the prevalence of on-colony growth forms of A. smithii under intensive grazing.
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