Biomass and nitrogen responses to grazing of upland steppe on Yellowstone's northern winter range.
The responses of herbaceous vegetation on upland steppe of Yellowstone National Park's N. winter rangeland to winter grazing by elk were investigated during 1987-88 using exclosures erected at 4 sites in 1958-62. The rangeland was dominated by Agropyron spicatum [Elymus spicatus], Festuca idahoensis, Koeleria cristata [K. macrantha] and Artemisia tridentata. Elk winter-grazing on plants reduced standing dead material and litter biomass at most but not all sites and dates. Plant productivity was not reduced. Live aboveground grass biomass was reduced by winter grazing at 1 of 4 sites in 1987 and at 3 sites in 1988. Live grass was more abundant in grazed areas than in ungrazed areas at 2 sites in 1987. Live forb biomass was approx. 33% of grass biomass, and was not affected by grazing in 1987, but increased slightly outside exclosures in 1988, perhaps because of the dry summer. Root biomass was not affected by grazing at any site. Winter grazing increased the N concn of live grass, dead grass and Artemisia frigida. Total N flow to herbivores in 1987 would consequently have been stimulated by herbivory, and in 1988 would have been depressed by herbivory, only one third as much as biomass. The net effects of winter grazing appeared to be a redirection of biomass flow from decomposers into elk, accompanied by an enhanced rate of N recycling back to elk. The fact that perennial root biomass had persisted best indicated the net consequences of winter grazing for these plants.