Environmental Description:  This plant association is reported from widely separated sites in the southern end of the Lemhi Range in Idaho and in the Colorado Plateau of western Colorado. In Idaho, the association occurs on gentle lower slopes composed of volcanic tuffs, at approximately 1680 to 1890 m (5500-6200 feet) elevation. Constant downslope soil movement and a high shrink-swell ratio create a highly unstable substrate. In Colorado, this association is known from gentle to moderate slopes on somewhat barren ridges, benches and hills of Morrison Formation shales between 1960 and 2089 m (6430-6854 feet). Bare soil and gravel cover much of the unvegetated ground surface.

In Idaho, the region where the association occurs is to the east of a major mountain mass which strongly affects the climate of the region. The high mountains to the west create a rainshadow, allowing only occasional incursions of moisture-laden winter storms from the Pacific. Average annual precipitation is low, ranging from 17.8 cm (7 inches) in the valleys, such as along the Salmon River, to roughly 114.3 cm (45 inches) at the highest elevations. There is a peak during May and June, when convective showers are common, but winters are relatively dry. Summers are cool and winters cold. The region is mountainous with high, massive ranges, dissected by deep valleys. The bedrock is a heterogeneous formation of volcanic and sedimentary rock, with only minor amounts of granitics, known as Challis Volcanics. At lower elevations along the Salmon River the most common rock types of this formation are tuffs and andesites. A flora endemic to the Lemhi region, known collectively as Challis Endemics, occurs on these substrates and is joined by a flora of species disjunct from drier regions of the Great Basin. Much of the Challis endemic region roughly coincides with an area receiving less than 20.3 cm (8 inches) of precipitation annually, combined with low-elevation occurrences of andesites and tuffs of the Challis Volcanics.