Snake-Columbia Shrub Steppe

Category: Grassland, Tundra, and Human Biomes.

Geographic Location: North America.

Summary: This vast sagebrush steppe provides a northern transition from the cold Great Basin desert to the semiarid steppe of the northern intermountain west.

The Snake-Columbia Shrub Steppe biome comprises tablelands, intermountain basins, dissected lava plains, and scattered low mountains of the northern Great Basin. Extending from the Teton Range along the Idaho and Wyoming borderland west to the foothills of the Cascade Mountains, the area is a veritable sea of sagebrush steppe punctuated by concentrated areas of row-crop agriculture.

Wildlife here includes pronghorn antelope, greater sage-grouse, Brewer’s sparrow, sage thrasher, and long-billed curlew. This classic western landscape has set the scene for plateau Native American cultures and European immigrants, and provided a ranching and farming heartland. The massive river systems here have been harnessed for irrigation and electricity, and now wind farms are becoming common sights.

Geography and Climate

The Snake-Columbia Shrub Steppe biome is located generally along the leeward side of the Cascade Mountains in Washington and Oregon, extending east across southern Idaho along the Snake River plain. Geologically, the Snake River traces the path of the North American Plate over the Yellowstone hot spot, located under Yellowstone National Park, a relatively thin portion of the Earth’s crust known for volcanic activity. Most of the area is underlain by Miocene-age basalt flows covered with loess and volcanic ash—generating highly fertile soils. This is a region of smooth plains and deeply dissected terrain, with river-cut terraces along the Snake River.

Average annual temperatures vary from 40 to 55 degrees F (4 to 13 degrees C), with a growing season of approximately 160 days. Annual precipitation across the plateaus averages 6 to 20 inches (152 to 508 millimeters), but is much higher in isolated mountain areas. The least precipitation falls in the lowest valleys, and summers overall tend to be dry. However, surface water is abundant, largely from the Snake and Columbia River systems.

Crop irrigation is by far the largest use of water in the region. Groundwater is pumped from basalt aquifers and from deep soils along river valleys. The dominant soils in the region include the black, organic-rich soils called mollisols that are typical of Midwestern prairies. Aridisols, soils with relatively little organic material and characteristic of cool deserts, are also fairly common here.

Diverse Biota

Shrub steppe occurs in temperate latitudes, typically where semiarid conditions support a mix of grass cover and scattered shrubs. Most of the Columbia plateau is a steppe of sagebrush (Artemesia spp.), one of the most abundant and widespread types of vegetation in North America. Soils supporting sagebrush steppe in this region tend to be deep, often with a thin, fragile crust of algae, lichen, and moss. This shrub steppe is dominated by perennial grasses, including western wheatgrass (Agropyron spp.) and bluebunch wheatgrass, with several distinctive subspecies of big sagebrush and antelope brush. Areas with the deepest soils commonly support basin big sagebrush, but because these lands were among the only areas suitable for row-crop agriculture, most have been plowed.

Drier, windswept, or rockier sites tend to support Wyoming big sagebrush, often mixed with black sagebrush, low sagebrush, budsage, and spiny hopsage. More moist, or even poorly drained, sites might include silver sagebrush, bluegrass, muhly, wildrye, tufted hairgrass, and various sedges. Historically, the action of natural wildfire, as well as grazing by bison, probably maintained a patchy mosaic of open and closed shrubland among the grasslands. Shrubs may increase in density following heavy grazing and/or with suppression of wildfire. In many areas where surface disturbance has occurred, the invasive annual cheatgrass or other annual brome grasses are abundant.

On shallow, stony, or poorly drained clay soil, around valley margins, and along gentle slopes, dwarf-shrub steppe is commonly dominated by low sagebrush and close relatives such as early sagebrush and black sagebrush. On shallow basalt flows, scabland occurs in rocky patches, dominated by rigid sagebrush or buckwheat, or occasionally only by grasses and forbs. Because of poor drainage through basalt, these soils are often saturated from fall to spring by winter precipitation but typically dry out completely by midsummer. Total vegetation cover is typically low; annual plants may be seasonally abundant, but cover of moss and lichen is often high in undisturbed areas. On highly eroded volcanic ash and tuff, the harsh soil and high rate of erosion support sparse vegetation that often includes spiny hopsage, needlegrass, buckwheat, and bitterbrush.

At higher elevations, mountain big sagebrush steppe becomes dominant, extending up among woodlands and forests on ridgetops and mountain slopes. Other low sagebrushes, along with snowberries, juneberries, and currants, and mixed with a greater diversity of grass and forb plants are common in these areas.

Western juniper savannas pick up in abundance across a range of elevations on the western margins of the Columbia plateau, from southwestern Idaho, along the eastern foothills of the Cascades, and south into California. Where this vegetation grades into relatively moist forest or grassland habitats, these savannas become restricted to rock outcrops or escarpments with excessively drained soils. Throughout much of this range, fire exclusion and removal of fine fuels by grazing livestock have reduced fire frequency and allowed western juniper to expand into adjacent shrub steppe and grasslands.

The Columbia River system at one time sustained one of the largest salmon runs in the world. Today, the salmon have declined to less than a tenth of their former population, as a result of the effects of dams, diversions, overfishing, and degradation in surrounding uplands. More than 200 vulnerable plants and animals, including some 70 endemic (found nowhere else) plant species, are found here. The sagebrush steppe continues to support extensive herds of pronghorn antelope that still have seasonal migrations; some of the remaining core habitat for greater sage-grouse; and numerous birds of prey that nest here at higher densities than anywhere else on Earth.

Human Activity

The cultural history of the Snake River plain and Columbia plateau has been largely influenced by the main river systems. To the plateau Indian cultures such as the Nez Perce, salmon were central to life, although with the introduction of horses by the Shoshone several centuries ago, hunting of bison became increasingly important. Throughout the early 1800s and before the development of railroads, explorers, trappers, and European-American settlers passed through this region along what became the Oregon Trail. Most of the area was homesteaded by the turn of the 20th century, at the point when dams began to be built throughout the region’s river systems.

Today, this region is primarily a mixture of cropland and rangeland. Irrigated agriculture is most significant, with crops ranging from potatoes and peas to wheat and alfalfa. Grazing is the major land use in the drier parts of the region, with about one-third of the land being federally managed.

Environmental Concerns

Water erosion, wind erosion, surface compaction, and invasion of undesirable plant species are major resource-management concerns today. The recent growth in wind-energy development has increased the urgency for action to limit further fragmentation of wildlife habitat. With this in mind, the Western Governors Association and member states have begun to proactively provide wildlife habitat information to inform the planning decisions for renewable energy.

Climate change impacts may be felt in the arid region; nitrogen deposits, atmospheric carbon dioxide, and other changes will impact the grasslands. Climate change may impact the size of the sagebrush areas, for example, which in turn constricts grouse and other birds and mammals living in these habitats.

Bibliography

Cutright, Paul Russell. Lewis and Clark: Pioneering Naturalists. Lincoln: University of Nebraska Press, 2003.

Lorenz, K., and R. Lal. "Terrestrial Land of the United States of America." Soil Organic Carbon Sequestration in Terrestrial Biomes of the United States. Springer, Cham, 2022. DOI: 10.1007/978-3-030-95193-1‗1. Accessed 1 Sept. 2022.

Petersen, Keith C. River of Life, Channel of Death: Fish and Dams on the Lower Snake. Corvallis: Oregon State University Press, 2001.

Waring, Gwendolyn. L. A Natural History of the Intermountain West: Its Ecological and Evolutionary Story. Salt Lake City: University of Utah Press, 2011.