Environmental Description:  This association occurs on valley bottoms and toeslopes of valley sides in a glaciated, mountainous region. The valleys are flanked by rugged mountains of granitic, quartzitic, calcareous sedimentary, and Challis volcanic origins, and have been modified by glacial activity. Valley floors are generally wide, with gentle gradients in the lower reaches, while middle reaches are somewhat narrower or U-shaped. Glacial and alluvial deposits comprise the valley floors. The climate of these montane to subalpine valleys is cool and semi-arid. Summers are warm, but winters are very cold, and morning frost in low-lying pockets is common throughout the warm season. Average annual precipitation is 25 cm (10 inches) in the lowest valleys, much of which falls during November to April in the form of snow. Valley bottoms are typically snow-covered from November through late April or May.

The association is found most commonly from 2060 to 2350 m (6750-7700 feet) elevation, in middle to upper reaches of tributaries. Examples of it have been observed up to 2740 m (9000 feet). It is associated with valley bottoms wider than 30 m, and often greater than 100 m wide. Slopes vary from 0 to 25%. Most often, the association occurs on gently sloping sides of valley bottoms, on convex terrace brows, or on toeslopes, perched above and away from the central stream. Subsurface water flow is important, primarily from adjacent sideslopes, and small rivulet channels are dispersed throughout. The soils are typically composed of organic materials, in some cases over subsurface mineral layers, and are nearly saturated during the growing season. They are poorly drained. Parent materials of the mineral subsurface horizons are predominantly calcareous sedimentary and Challis volcanic rocks.