GRAZING-MEDIATED DIFFERENTIATION IN AGROPYRON-SMITHII - EVIDENCE FROM POPULATIONS WITH DIFFERENT GRAZING HISTORIES

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 were grown in monocultures and in defoliated and nondefoliated interpopulation mixtures at 5 rates of N addition. Biomass yield and distribution of biomass among components of plants of the 2 populations were not significantly altered by N-addition treatments. Total biomass yield of monocultures of the populations did not differ significantly, but biomass yield and growth rate were greater in ON-colony plants in mixtures across defoliation and N-addition treatments. Defoliation produced a similar relative reduction in cumulative biomass yield of plants of the 2 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. Rapid production of small tillers probably contributes to the prevalence of ON-colony growth forms of A. smithii. -from Authors