In the Intermountain region between the Pacific Coast mountain ranges and the Rocky Mountains lie the cold deserts of the Western United States. The climate of the region is arid to semi-arid and is characterized by long and cold winters and hot and dry summers. Most precipitation falls in winter as snow, although summer afternoon thunderstorms are common in the southern part of the region. Temperatures generally increase from north to south, while the amount of precipitation falling as summer rain increases from west to east. High topographic variability strongly influences local climate and weather patterns.
Piñon (Pinus monophylla) and juniper (Juniperus osteosperma with scattered J. occidentalis) tree species co-occur with sagebrush ecosystems throughout the region. Tree dominated areas or what have been referred to as “woodlands” are most closely associated with mid-elevation mountain big sagebrush (Artemisia tridentata vaseyana) ecological types, but span an environmental gradient ranging from lower elevation black sagebrush (A. nova) and Wyoming sagebrush (A. tridentata wyomingensis) types to higher elevation mountain brush types. Areas with abundant piñon and juniper occupy about 18 percent (7.1 million ha, 17.6 million acres) of the land area of the Great Basin.
Piñon generally dominates the upper and intermediate elevations, while juniper becomes more abundant at lower elevations. Understory species composition typically reflects that of the associated sagebrush ecological type. As tree density increases the understory becomes sparser and dense stands of piñon and juniper have very little understory. Bunchgrasses may occur on the peripheries of the tree crowns.
Areas historically dominated by piñon and juniper trees or “woodlands” represent important ecological types in the Great Basin. These types include old-growth and open shrub savanna woodlands that have occurred for much of the last several hundred years. Recent expansion of piñon and juniper trees into surrounding sagebrush ecosystems and subsequent infilling has placed many sagebrush ecosystems and the species that depend on them, like sage grouse and pygmy rabbit, at risk. Since approximately 1860, the area and density of trees has increased from three- to ten-fold due to fire exclusion, over-grazing, favorable climate, and recovery from settlement-era harvesting. The expansion and infilling of piñon and juniper trees increases the risk of larger and more severe wildfires and, at low to mid-elevations, cheatgrass (Bromus tectorum) invasion. Land managers across the region are using fire and fire surrogate treatments (e.g., cut and leave, mastication) in an attempt to both increase the ecological resilience of sagebrush ecosystems and decrease the risk of high severity fires.