Sagebrush ecosystems occur in the cold deserts of the Western United States. These deserts are found in the Intermountain region which lies between the Pacific Coast mountain ranges and the Rocky Mountains. 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. 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.
Emblematic of this region are the sagebrush ecosystems of the Great Basin, northern Colorado Plateau and Columbia River plateau. Sagebrush ecosystems constitute a vast area of the western US covering 62 million ha (153 million acres). Numerous species of sagebrush occur, but big sagebrush (A. tridentata) is the most abundant species of sagebrush in the region, and Wyoming (A. tridentata wyomingensis) and mountain big sagebrush (A. tridentata vaseyana) are the most common varieties of big sagebrush. At low elevations and on more xeric sites Wyoming big sagebrush is common, while at higher elevations and on more mesic sites mountain big sagebrush becomes more common. Sagebrush cover ranges from 10 to 90% and can account for over 70% of the phytomass in these systems. Sagebrush cover in mountain big sagebrush communities is typically 2.4 times greater than in Wyoming big sagebrush. Herbaceous cover in mountain big sagebrush is also higher due to increased productivity at higher elevations and the prevalence of large perennial bunchgrasses like bluebunch wheatgrass (Pseudoroegneria spicata) and Thurber’s needle grass (Achnatherum thurberianum). Small perennial bunch grasses like Sandberg bluegrass (Poa secunda) and forbs are more important in Wyoming big sagebrush communities but these species occur in both communities. Depending on grazing history and site potential, understory cover can be extremely variable ranging from 0 to 100%. Although perennial grasses once dominated the understory of lower elevation sagebrush communities, exotic annual grasses, especially cheatgrass (Bromus tectorum), now often dominate.
Sagebrush ecosystems are experiencing widespread degradation due to a variety of causes and are now some of the most endangered ecosystems in the United States. Excessive and inappropriate grazing at the turn of the last century and well into the 1950s significantly reduced palatable grasses and forbs and resulted in increases in shrubs and trees. Grazing capacity was decreased by as much as 60-90% in some areas. Despite improved grazing management, the legacy effects of grazing and continued over grazing in some areas increases the potential for non-native grass invasion and contributes to a decline in ecological site condition. The invasion of non-native plants into sagebrush ecosystems, notably cheatgrass and medushead, is altering fire regimes, nutrient cycling and successional dynamics. And the expansion of piñon and juniper trees into sagebrush ecosystems is reducing the abundance of sagebrush species, elevating fuel loads, and increasing the risk of large and severe fires. Also sagebrush historically was viewed as an undesirable species on the rangelands and managers used various methods to remove the species from large areas and reseed these areas with pasture grasses. Now, large tracts are used for suburban and exurban development in this rapidly growing region, and other areas are being considered for development of solar and wind energy. All of these factors have led to a decline in biodiversity and threatened species dependent on sagebrush such as sage-grouse, sagebrush sparrow and pygmy rabbit.