Environmental Description:  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 in the Custer County location 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 this 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 2.4 cm (8 inches) of precipitation annually, combined with low-elevation occurrences of andesites and tuffs of the Challis Volcanics. In the Lemhi Range, the bedrock is limestone, with some interbedding of sandy limestone and calcareous siltstone and sandstone. This plant association occurs on southerly slopes composed of volcanic tuffs and on canyon bottoms where soils are derived from limestone alluvium and colluvium. Elevations are low, from roughly 1586 to 1891 m (5200-6200 feet). The tuff substrate is typically unstable, with constant downslope movement of soil surface layers, and a high shrink-swell ratio. Chronic soil disturbance is therefore a major factor in the species composition of this association, which includes several of the Challis Endemics.