Biomass and Nitrogen Responses to Grazing of Upland Steppe on Yellowstone's Northern Winter Range

Abstract

(1) The responses of herbaceous vegetation on upland steppe of Yellowstone National Park's northern winter range to winter grazing by elk were investigated during 1987-88 using enclosures erected at four sites in 1958-62. (2) 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. (3) Live above-ground grass biomass was reduced by winter grazing at one of four sites in 1987 and at three sites in 1988. Live grass was more abundant in grazed areas at two sites in 1987. (4) Live forb biomass was c. 33% of grass biomass, and was not affected by grazing in 1987, but increased slightly outside enclosures in 1988, perhaps because of the dry summer. (5) Root biomass was not affected by grazing at any site. (6) Nitrogen concentrations of live grass, dead grass and Artemisia frigida were increased by winter grazing. 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. (7) 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 has persisted best indicated the net consequences of winter grazing for these plants.