Plant responses to defoliation and resource supplementation in the Pryor Mountains

Field studies were conducted in 2 types of grasslands in the Pryer Mountain Wild Horse Range of northern Wyoming and southern Montana to examine plant biomass production and nitrogen responses to the separate and combined effects of graminoid defoliation and increased environmental resource (water or nutrients) supply. Short-term plant responses were monitored over 2 years which differed substantially in growing season precipitation. In the arid, low elevation grassland, total grass biomass was significantly lower in the dry year than the wet year in all treatments. Defoliation of the grasses did not reduce their aboveground biomass production in the wet year, but severely reduced it in the dry year, primarily because of a decrease in tiller density. Mass of remaining individual tillers increased with clipping in the dry year, and nitrogen concentrations of the grasses increased with clipping in both years. Irrigation alone increased total belowground biomass compared to the other treatments, but did not increase the aboveground biomass production of any plant functional group. Clipping plus irrigation resulted in greater total aboveground biomass production and higher nitrogen concentrations of the grasses than control or irrigated treatments. Clipping graminoids in the more mesic montane grassland did not decrease their biomass production in either year, but did increase their nitrogen concentrations and increase the collective aboveground biomass production of the other plant functional groups. Fertilization and fertilization plus clipping significantly increased total aboveground biomass production in both years, and total belowground biomass was greatest in fertilized plots.