Grazing-Mediated Differentiation in Agropyron smithii: Evidence from Populations with Different Grazing Histories

Abstract

Agropyron smithii Rydb. (western wheatgrass) was collected from a heavily grazed area colonized by black-tailed prairie dogs (Cynomys ludovicianus) and from an adjacent lightly grazed, noncolonized site (ON- and OFF-colony populations, respectively). Plants of the two populations were grown in monocultures and in defoliated and nondefoliated interpopulation mixtures at five rates of nitrogen (N) addition. We predicted that the biomass yield of ON- relative to OFF-colony plants would be increased by defoliation and elevated soil-N availability, potentially selective factors encountered by plants on heavily grazed prairie dog colonies. Biomass yield and distribution of biomass among components of plants of the two populations were not significantly altered by N-addition treatments, indicating that plants were N saturated at all N-additions employed. Total biomass yield of monocultures of the two populations did not differ significantly, but biomass yield and growth rate were greater in ON- than OFF-colony plants in mixtures across defoliation and N-addition treatments. Defoliation produced a similar relative reduction in cumulative biomass yield of plants of the 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. We suggest that rapid production of small tillers contributes to the prevalence of ON-colony growth forms of A. smithii under intensive grazing.