WINTER HABITAT USE BY LARGE UNGULATES FOLLOWING FIRE IN NORTHERN YELLOWSTONE-NATIONAL-PARK

The effect of fire and habitat heterogeneity on winter foraging by ungulates was studied in northern Yellowstone National Park (YNP). Grazing was monitored at 15 study sites for 14 wk during the winters of 1991 and 1992. The location and intensity of grazing activity within each site were recorded on topographic maps and digitized into a geographic information system. Maps of grazing intensity were compared to map layers of grassland habitat type, elevation, slope, aspect, annual precipitation, and the spatial pattern of fires that occurred in 1988. Burned areas were used by ungulates more often than expected based on their availability, especially during mid- to late winter, but the spatial pattern of burned areas (i.e., fragmented or clumped) was not related to grazing intensity. Ungulate grazing, as measured by minimum cumulative grazing intensity (MCGI), was greatest at low-elevation drier sites across the northern range and on steep southerly slopes. The influence of environmental characteristics on MCGI was evaluated at four spatial scales (1, 9, 81, and 255 ha). Grazing intensity was best predicted by environmental heterogeneity, especially the presence of burned areas, and topography (slope and aspect), at broader scales (81 and 255 ha) rather than on a per-hectare basis. The explanatory power of broad-scale features suggests that wintering ungulates in YNP respond strongly to coarse-grained variation in these landscapes. Interpreting or predicting ungulate grazing at a specific location requires understanding of environmental heterogeneity in the surrounding landscape.