Biogeography of Sagebrush Steppe

Solar Radiation And Geographic Location

Sagebrush Steppe Ecosystems are geographically located in the Northern Mid-Latitude(30 to 45 degrees North) region of approximately 40 million hectare of the Western UnitedStates (3) (Oregon, California, Idaho, Wyoming, Utah, Nevada), providing habitat for 350 vertebrae species (1). Solar radiation reaches Earth as insolation, and due to distance and angle of Earth from the Sun, insolation intensity decreases as you move farther from the equator.Declination is the point where insolation is at a maximum, and moves from 23.45 degrees North to 23.45 degrees South, which results in seasonal change (1). This also results in more variation between day/night length and maximum/minimum daily temperatures as you move away from the Equator, due to Earth’s rotation around the sun and declination (position of sun with respect to the equator). Northern Mid-Latitude regions have the warmest temperatures from June-August and the coldest temperatures from December to February, with a variation below 0 degreesCelsius to above 30 degrees celsius. Sagebrush Steppe ecosystems occur at elevations from 150to 2000 meters (500-6550 ft), with an average elevation of 1235 meters (4052 ft) (1). Vegetation is abundant Sagebrush with other shrubs, grasses, and flowering plants. Terrain is typically flat valleys and plains or gently rolling hills located in the Sierra-Nevada Mountains, CascadeMountains, and Rocky Mountains. There are 3 physiographic regions in the Intermountain West(2) including the Colorado Plateau, Great Basin, and Columbia-Snake River Plateau. Sagebrush steppe does not make up the Colorado Plateau (1).

Weather can be defined as the day to day state of the atmosphere (1). Climate can be defined as the patterns of precipitation, temperature, humidity, barometric pressure and wind over time (1). The main inputs influencing climate are solar radiation, Earth’s atmosphere, andTopography. Solar radiation heats the Earth’s surface producing wind patterns and the water cycle, influencing the distribution of weather patterns. Precipitation is the main determinant of vegetation production (1). In Sagebrush Steppe Ecosystems, precipitation falls in low amounts at9.84 inches or 200-500 mm per year, with an average of 250 mm per year (1). Due to the northern latitude location, elevation, and topography; precipitation falls mainly in the form of snow. A Mid-latitude location results in uneven distribution of solar radiation; influencing variation in photoperiod, which effects growing season of plants, daily duration of
photosynthesis, produces semi-arid plant communities, and an Orographic effect due to theCascades and Sierra-Nevada Mountains to the West and Rockys to the East. Due to the*Orographic effect, Precipitation is driven by global wind patterns and warm air flows inland off of the Ocean and crashes into a mountain. The warm air rises, cools, and the moisture condenses to fall as rain. As the air mass moves over the mountain and loses moisture as rain, the air becomes dry on the other side. Therefore, as warm air moves inland off of the Pacific Ocean, it hits the westward side of the Cascades and Sierra Nevada Mountains, rises and cools and falls as rain on the Westward side of the Mountains, then becomes dry as it rests on the Eastward side of the Mountains. Therefore, the dry air produced by the Mountains settles and produces a semi-arid environment that supports a Sagebrush Steppe Ecosystem.The Sagebrush Steppe Ecosystem land uses include forage for livestock, habitat for wildlife, aesthetic value of open space, cultural heritage of the West, energy development like wind and solar energy, and a high quality water source (4). Landscape considerations include habitat degradation such as fragmentation, erosion, decreased water quality, reduced forage and habitat, fire (suppression), converting land to agriculture, and drought (3).


Climate influences vegetation communities, water cycling, and Sagebrush Steppe ecosystem function because of temperature and the amount of precipitation. Temperature affects the rate of evaporation, degree growing days, and frost period; influencing the type, species, and abundance of a plant community. Sagebrush Steppe Ecosystems have temperatures from 34.6 to100.4 degrees F (1), and are therefore warm and dry in the summer, and colder with frost in the winter. Since precipitation is the driving factor of vegetation production (5); the low amount of precipitation in Sagebrush Steppe Ecosystems results in enough water to support grasses and shrubs, but not enough to support large amounts of trees. In Sagebrush Steppe Ecosystems; water flows through shallow creeks and contributes to the water cycle through evaporation, and is absorbed by plants and contributes to the water cycle through transpiration. A warming climate means more evaporation and less precipitation; which results in less vegetation, increased runoff,and erosion.


Topography influences vegetation communities, water cycling, and Sagebrush Steppe ecosystem function because it influences climate and geology. For example, temperature decreases as elevation increases, precipitation falls as snow (1). The main topography of Sagebrush Steppe is the formation of basins (valleys) and range (Mountains) (1). SagebrushSteppe topography includes flat valley, rolling hills, or foothills. The Orographic Effect (*refer toSolar Radiation and Geographic location) is the main reason that the Topography (Mountains
and Valleys) of the region influences the semi-arid environment of the Sagebrush SteppeEcosystem.


Soil is the primary determinant of the type of plant species in any given area. It’s influence is determined by parent material, soil organic matter, texture and structure, pH and salinity, and physical/biological crusts. Sagebrush Steppe Ecosystems have parent material that is volcanically derived with sand and clay particles. The Soil Orders and Suborders in this ecosystem include Aridisols ((Arigids and Cambids)) but also Mollisol (Ustolls and Xerolls) and Andisol (Xerands) (1). Aridisols occur in arid regions, Mollisols are dark in color because ofOrganic matter and rich in nutrients, and Andisols are derived from volcanic ash. The ColumbiaRiver region contains Loess Soils, while the Great Basin has deep alluvial soil (1). Soil is formed through primary succession of parent material such as Sedimentary, Igneous, and MetamorphicRock. Soil horizons build up as parent material mixes with Organic matter. Soil Organic Matter(SOM) reflects the fertility of a system and influences nutrient cycling, carbon sequestration, soil structure, plant rooting, water infiltration, water holding capacity, and microbe habitat. SOM amount and rate is affected by precipitation and temperature. Sagebrush Steppe has moderate levels of SOM. Soil texture and structure of a Sagebrush Steppe ecosystem is greatly affected by degradation. For example, human activities, wind, and water can put pressure on soil aggregate sand contribute to degradation. However, aggregate stability is important for erosion control,water infiltration, nutrient availability, and facilitating root growth.

(1)Stewart, Kandy. “Sagebrush Steppe Module”. Oregon State University. 2017.
(2)National Park Service. “Sagebrush Steppe”. 2017.
(3)McIver, J.D.; Brunson, M.; Bunting, S.C., and others. 2010. “The Sagebrush SteppeTreatment Evaluation Project (SageSTEP): a test of state-and transition theory.” Gen.Tech. Rep. RMRS-GTR-237. Fort Collins, CO: U.S. Department of Agriculture, ForestService, Rocky Mountain Research Station.
(4)U.S Fish and Wildlife Service. “Why Care About America’s Sagebrush”. USFWS. 2014.
(5)J.D. Bates, T. Svejcar, R.F. Miller, R.A. Angell. “The effects of precipitation timing on sagebrush steppe vegetation”. Journal of Arid Environments. 2005.

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