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Ecological Dynamics on Yellowstone’s Northern Range
ally at elevations of 1,800 to 2,400 m within the 40- to 75-cm precipitation zone. It is distributed throughout the park but is most common in the Gardner and Lamar River drainages (Despain 1990). The habitat type is dominated by mountain big sagebrush (Artemisia tridentata ssp. vaseyana), although Wyoming big sage (A. tridentata ssp. wyomingensis) may also be present. Identification of big sagebrush subspecies is particularly important because of differences in palatability and preference to ungulates. Idaho fescue (Festuca idahoensis) dominates the understory with Agropyron spicatum and Koeleria macrantha also present. Forbs (broad-leaved herbaceous plants), such as Geum triflorum, are abundant.
Primary production (the amount of carbon fixed by photosynthesis) varies widely in big sagebrush-Idaho fescue habitat depending on rainfall and temperature (Mueggler and Stewart 1980). A 50% difference in production may occur on any given site over a 3-year period. Production varied across the type from 560 kg/ha (Mueggler and Stewart 1980) to 1,610 kg/ha with grasses contributing 21% to 42% of the production, forbs 38% to 56%, and shrubs 10% to 41%. Between 88% and 98% of the shrub production is from big sagebrush.
Big sagebrush-Idaho fescue habitat, which is heavily grazed in winter by ungulates, and the grassland habitat type (Idaho fescue-bearded wheatgrass) account for slightly more than half of all the nonforested vegetation in the park and on the northern range (Houston 1982). These two types probably furnish most of the forage for the large number of grazing animals in the park (Despain 1990).
Wyoming big sagebrush-bluebunch wheatgrass (A. spicatum, now Pseudoroegneria spicatum) habitat type occurs in the Gardner River canyon in small areas on southern and western slopes, often between big sagebrush-Idaho fescue and other grasslands on ridgetops and upper slopes. It occurs on shallow to moderately deep soils formed over several parent materials.
Mountain big sagebrush is the dominant shrub, although basin big sagebrush (Artemisia tridentata ssp. tridentata) may occur on deeper soils in drainages. Low shrubs, such as A. frigida and Gutierrezia sarothrae, are usually present. In addition to bluebunch wheatgrass, other conspicuous grasses include K. macrantha, Poa secunda, and Stipa comata. Production varies between 670 and 1,120 kg/ha with high variability between sites but not between years (Mueggler and Stewart 1980). This type is heavily grazed in winter by ungulates in the Gardiner area. Big sagebrush receives enough browsing to reduce the size of its canopies.