RESPONSES OF BLUEBUNCH WHEATGRASS, IDAHO FESCUE, AND NEMATODES TO UNGULATE GRAZING IN YELLOWSTONE-NATIONAL-PARK

We sampled above and below ground biomass of Idaho fescue (Festuca ida hoensis) and bluebunch wheatgrass (Agropyron spicatum) plants and nema tode densities under these species inside and outside a 2-year old exc losure on the northern range of Yellowstone National Park in May throu gh September, 1990. Native ungulates grazed the site primarily in wint er and early spring. Grazing during this period removed essentially al l the standing dead plant material. In early May, green biomass of pla nts of both species outside the exclosure was significantly lower than plants in the exclosure but off-take by ungulates accounted for only 18-51% of this initial difference. Indirect effects of grazing, such a s the effects of removing standing dead material on microclimatic cond itions, likely influenced early growth. By the end of the growing seas on, both species had similar biomass to ungrazed plants despite an inc rease in root-feeding nematodes early in the growing season. Lower roo t biomass, higher densities of bacteria-feeding nematodes (no./g root biomass), and higher concentrations in foliar nitrogen (N) with grazin g suggested that root mortality, due to spring grazing, provided a sho rt-term source of carbon for microbial activity and that microbial-fee ding nematodes increased the turnover rates of microbial bound N. Dens ities (no./g root biomass) of root-feeding nematodes increased rather than decreased with grazing. Because N concentration of roots did not differ between grazed and ungrazed plants, we suggest that there was a reduction in secondary chemicals or an increase in root hairs which a re preferred sites for nematode feeding.