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 idahoensis) and bluebunch wheatgrass (Agropyron spicatum) plants and nematode densities under these species inside and outside a 2-year old exclosure on the northern range of Yellowstone National Park in May through September, 1990. Native ungulates grazed the site primarily in winter and early spring. Grazing during this period removed essentially all the standing dead plant material. In early May, green biomass of plants 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 as the effects of removing standing dead material on microclimatic conditions, likely influenced early growth. By the end of the growing season, both species had similar biomass to ungrazed plants despite an increase in root-feeding nematodes early in the growing season. Lower root biomass, higher densities of bacteria-feeding nematodes (no./g root biomass), and higher concentrations in foliar nitrogen (N) with grazing suggested that root mortality, due to spring grazing, provided a short-term source of carbon for microbial activity and that microbial-feeding nematodes increased the turnover rates of microbial bound N. Densities (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 are preferred sites for nematode feeding.