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 lowest in the valleys, such as where this association is found, typically only 18 cm (7 inches). There is a peak during May and June, when convective showers are common, but winters are relatively dry. Summers are warm, and winters cold.

The region is mountainous with high, massive ranges, dissected by deep valleys. The topography is the result of fault-block activity, with a linear basin-and-range topography. Intervening valleys are typically broad and gentle. Coalescing alluvial fans have been deposited at the base of many of the ranges. Most of these are of older, Quaternary materials which are now being dissected and planed by present-day streams. This association occurs on alluvial fans at elevations from approximately 1890 to 1980 m (6200-6500 feet). These fans are a complex of interlayered materials consisting of boulders, cobbles, and pebbles mixed with sand, silt, and clay.

The soils are highly variable, in some sites derived from sand and in others derived from clay. Most of the ground surface is covered by a cobble or boulder pavement, resulting from the removal of fine-grained materials by wind and surface runoff. The soils associated with this vegetation type occur on alluvial flats, and are dry gravelly loams, with shallow rooting depths, and a windswept surface. Achnatherum swallenii is known to be associated with calcareous soils. Additionally, Caicco and Wellner (1983l) suggest that the vegetation mosaic on these alluvial fans appears to be related to depth of winter snowpack, and presumably timing of spring snowmelt.