Wyoming Basin shrub steppe

Category: Grassland, Tundra, and Human Biomes.

Geographic Location: North America.

Summary: This rolling “sagebrush sea” extends over high plains in vast, cold desert basins, providing a transition from northern Great Plains grasslands into the semiarid steppe of the northern intermountain west.

The Wyoming Basin Shrub Steppe biome comprises some 50,000 square miles (129,500 square kilometers) of intermountain basins, high plains, and cold desert centered on the state of Wyoming. The area is a veritable sea of sagebrush and salt desert scrub, interspersed with rock outcroppings, badlands, and sand dunes. This classic western landscape known for its roaming antelope has set the scene for Plains Native American cultures and westward migrants, provided a rancher’s or sportsman’s paradise, witnessed the rise of railroads and coal mining, and most recently has transformed into a modern economy based mostly on energy development.

This region is defined by major watersheds of the Bighorn River and Upper Green River, the Red Desert, the Great Divide Basin, and other basins as far east as the Laramie Basin. Most of the area lies at 5,900–7,800 feet (1,800–2,380 meters) elevation. Mountains rising from the margins of each basin support woodlands and forests of Douglas fir, pine, and aspen.

Weather in the Wyoming basins is harsh. Climate here is defined as arid to semiarid. Although annual rainfall may reach 16 inches (40 centimeters) along the base of the mountains, the basins often fall within their rain shadow, resulting in precipitation from rain and snow of just 6–10 inches (15–25 centimeters) per year. Temperatures vary from bitter cold winters to hot summers, but freezing temperatures are possible in any month of the year. Snowfall is heavier in surrounding mountains than the basins, but across these semiarid basins, snowmelt enables soils to retain moisture longer than sudden rainfall events.

Flora and Fauna

Forming a transition from the northern Great Plains grasslands to the arid intermountain region, vegetation in the Wyoming Basins is mainly shrub-steppe. Shrub-steppe occurs in temperate latitudes, typically where semiarid climates support a mix of grass cover and scattered shrubs. Most of the Wyoming basins are sagebrush steppe, one of the most abundant and widespread types of vegetation in North America. Soils are typically deep, often with a thin, fragile crust of algae, lichen, and moss.

This shrub-steppe is dominated by perennial grasses, including western wheatgrass and bluebunch wheatgrass, with several distinctive subspecies of big sagebrush, silver 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, they have mostly been plowed under. Drier soils tend to support Wyoming big sagebrush. In places of shallow soil and on windswept ridges, Wyoming big sagebrush may be replaced by black sagebrush or communities of cushion plants.

Historically, natural wildfire and grazing by bison probably maintained a patchy mosaic of open and closed shrubland among the grassland. Shrubs may increase in density following heavy grazing or with suppression of wildfire. In many areas where surface disturbance has occurred, the invasive annual cheatgrass or other annual brome grasses can be abundant. At somewhat higher elevations, mountain big sagebrush becomes dominant, extending up among woodlands and forests surrounding the basins. Other low sagebrush, snowberries, juneberries, and currants, along with a greater diversity of grass and forb plants, are common in these areas.

In the semiarid basin bottoms, water evaporation often leads to salt accumulation on the soil surface, limiting the plant species that can survive here. Throughout this region, saline basins often include open shrubland composed of one or more saltbush species, such as four-wing saltbush, shadscale, cattle saltbush, and spinescale. Other shrubs may include Wyoming big sagebrush, greasewood, rubber rabbitbrush, and winterfat. Some areas of the biome contain soils rich in clay. These areas expand and contract as they become wet and dry, preventing much plant growth. Derived from mudstones and shales, these zones are highly erodible, forming the characteristic buttes and cliffs called badlands.

Historically, this shrub-steppe was home to massive herds of bison. Today, it retains numerous birds such as greater sage-grouse, Brewer’s sparrow, grasshopper sparrow, mountain plover, and prairie falcon. Increasingly intense cattle grazing in mixed-grass prairie leads to a shift in grass species toward needle-and-thread grass and, ultimately, lawns of blue grama grass.

These grasslands are also home to prairie dogs, whose range has been reduced to less than 2 percent of the area occupied just 150 years ago. Prairie dog colonies in turn are important to critically endangered black-footed ferrets, ferruginous hawks, swift foxes, mountain plovers, and burrowing owls. Elk, pronghorn, and coyote still inhabit the area, as do many smaller mammals, such as ground squirrel, mice, and shrew.

Human Impact

The cultural history of the Wyoming basins has been largely influenced by grazing animals. For the Plains Indians, these were the bison lands. Throughout the 1800s, massive migrations of European-American settlers passed through this region along the Oregon, Mormon, and Overland trails. Most of the area was homesteaded by the turn of the twentieth century.

At first, large herds of cattle were brought into the area to fatten over summer and fall, trailed to markets in Denver and Cheyenne, and later shipped by rail to markets in the Midwest. With homesteads limited in size, grazing of cattle and sheep extended over the open range, leading in places to severe overgrazing and soil erosion. In most of the region, large roundup districts were created to move livestock throughout the landscape. Sheep were almost universally herded and moved to higher grasslands in the summer.

Traditional roundups and herding of livestock became more challenging during World War II, as men enlisted for the military. Fencing of common rangeland caused the landscape to be fragmented into smaller, intensively grazed units. It was not until the late 1940s and early 1950s that rotational grazing was first implemented among grazing allotments. Today, to remedy past environmental effects and to manage multiple values and uses, public land managers and private landowners work in an increasingly collaborative fashion here.

Another major influence shaping the face of the Wyoming basins has been industrial development, first via the railroads and then through mining and energy development. Nineteenth-century land ownership patterns were determined in part by the Union Pacific Railroad routing along the edge of southern Wyoming. This railroad route was influenced by the important coal seams at Hannah, Rawlins, and Rock Springs, Wyoming.

Most major basins in this region are important centers for oil, gas, coal, coal-bed methane, and soda-ash production. Additionally, recent years have seen the rapid expansion of renewable energy production. Extensive wind farms harness that abundant energy source along prominent low ridge lines across this windswept plain. Roads, power lines, and pipelines needed to support these industries continue to fragment and affect this regional landscape. Despite all these developments, this region remains one of the least affected and least populated in the United States.

Bibliography

Cutright, Paul R. Lewis and Clark: Pioneering Naturalists. Lincoln: University of Nebraska Press, 1969.

Dobkin, David S. and Joel D. Sauder. Shrubsteppe Landscapes in Jeopardy: Distributions, Abundances, and the Uncertain Future of Birds and Small Mammals in the Intermountain West. Bend, OR: High Desert Ecological Research Institute, 2004.

Knight, Dennis H. Mountains and Plains: The Ecology of Wyoming Landscapes. New Haven, CT: Yale University Press, 1994.

McPhee, John. Basin and Range. New York: Farrar, Straus and Giroux, 1982.

McPhee, John. Rising From the Plains. New York: Farrar, Straus and Giroux, 1986.

Noss, Reed. "Wyoming Basin Shrub Steppe." One Earth, 2022, www.oneearth.org/ecoregions/wyoming-basin-shrub-steppe/. Accessed 17 Aug. 2022.

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