In I970 a xeric form of mountain big sagebrush (A&m& tridcntata ssp. wmeyunu (Rydb.) Beetle) was reported in west central Idaho. Observations of morphology, habitat, and ecology, and analyses of foliage chemical components, clearly indicate these plants represent a new subspecies (xericensk) in the big sagebrush complex. It grows at lower elevations, 762-1,524 m (2,500-5,000 ft) and drier environments, 305-560 mm (12-22 in) precipitation, than most mountain big sagebrush, and is found on

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... c foothill soils often in association with bluebunch wheatgrass (Agropyron spicatum (Pursh) Scribn. & Smith). In addition to soil type, the radiate growth form and a more branched paniculate inflorescence are 2 morphological characteristics useful in separating ssp. xeri-cc&s from ssp. vuseyunu. It contains higher concentrations of crude protein (10.4%), phosphorus (0.3%), and total volatiles, and lower concentrations of tannins and total phenols than mountain big sagebrush. Distinct cbromatograms were obtained for both subspecies when extracts were analyzed by gas and high performance liquid chromatography. Leaf morphology and fluorescence of leaf water extracts are useful characters for separating ssp. xericend from ssp. tri&~&&. The chemical data, in combination with morphology and ecology, suggest this new subspecies was initially derived by hybridization of ssp. tridcntoto and ssp. vaseyana.