Environmental Description:  These forests and woodlands typically occur at the lowest elevations of the coniferous forest and woodland zone within their range. These are among the driest forested habitats in the western United States, but quantity and timing of precipitation vary greatly across the range of this vegetation. Generally, these woodlands occur in areas which receive 25-60 cm of precipitation annually, with at least some seasonal drought. Fire is a key factor in maintaining the relatively open canopies characteristic of these stands, but soil drought or infertility may be equally important in some areas. Within the Rocky Mountains, Pinus ponderosa grows at elevations between 1800 and 2600 m (6000-8500 feet) (Mehl 1992). In the Black Hills and northeastern Wyoming, it can be found from 1080-2100 m (3600-7000 feet). Soils are highly variable across the range of this type, and are derived from igneous, metamorphic, and sedimentary materials. The most characteristic soil features are good aeration and drainage, circumneutral to slightly acidic pH, an abundance of mineral material, and periods of drought during the growing season. It can be found on slopes of a variety of aspects and pitches, but is most often on gentle to moderate, northeast- to northwest-facing slopes. Some stands may occur as edaphic climax communities on very skeletal, infertile, and/or excessively drained soils, such as cinder or lava fields.