The Intermountain Grasslands vegetation type takes form as canyon grasslands along the Snake and Columbia Rivers, on the mountain benches and valleys of the Rocky Mountains, and on vast rolling plains including the Palouse, Zumwalt, and Camas Prairies in Washington, Oregon, and Idaho. Though these grasslands occur in many places under a wide range of climatic conditions, they all have a similar appearance because they are dominated by native bunchgrasses and herbaceous wildflowers with shrubs mostly restricted to creek beds and rocky hillsides. These treeless plains, valleys, and hillsides are important foraging areas for cattle and sheep and for native ungulates like elk and deer.
The Intermountain Grassland region is a prairie dominated by cool-season bunchgrasses including bluebunch wheatgrass, Idaho fescue, and Sandberg bluegrass. These grasslands include an abundant array of spring wildflowers with few shrubs. This grassland type extends across large portions of the northern Intermountain West stretching from the forested Cascade Mountains in the east to the Rocky Mountains in Montana, and from south-central Oregon into Alberta and British Columbia.
Historically, this landscape covered 29 million acres with an array of types of grasslands including the Palouse Prairie, Canyon Grasslands, Zumwalt Prairie, and Intermountain Bunchgrass. Though they are known by different names, all these grasslands are characterized by bunchgrasses with spring wildflowers in the spaces between bunchgrasses.
The grassy landscapes that characterize the late-seral, or climax, communities of this region are or were maintained primarily by long dry summer periods that slowed invasion by shrubs and trees. Wildfires were also important in maintaining these grasslands; however, large thunderstorms that might come with summer rain are not common and thus lightning-caused wildfires are not as common in this northern Intermountain region as they are in the Great Plains.
A vast majority of these Intermountain Grasslands were rapidly converted to croplands during the homestead era. The deep, rich grasslands soils were well-suited to cultivation and the mild dry summers were conducive to crop growth and maturation. Today the Intermountain Grasslands cover only 1.6 million hectares (4 million acres), less than 15% of its original extent. For example, the true Palouse Prairie is unique grassland that occurred on wind-blown soils, or loess, in eastern Washington and western Idaho. Very little (< 1%) of the original prairie exists today because the soils and climate were so will suited for grain production. The Palouse region today is almost completely farmed and is one of the most productive dryland wheat-and pea/lentil-producing regions on the globe.
The climate of the Intermountain Grassland is arid to semi-arid due to the rain shadow effect from the Cascade mountain range. Precipitation ranges between 30–64 cm (12″-25″) and falls mostly as snow or rain in the winter. Temperatures are milder than the Great Basin or Desert Southwest, with summers rarely over 35° C (95′ F). This area experiences a shorter growing season that other areas west of the Rocky Mountains, with cold winter nights and occasional frost in the summer, especially at high elevations.
Historic Plant Communities
Historically, the Intermountain Grassland was a large stretches of cool season perennial bunchgrasses including bluebunch wheatgrass (Pseudoroegneria spicata) ,Idaho fescue (Festuca idahoensis), and Sandberg bluegrass (Poa secunda). The spaces between grasses are punctuated periodically by the brightly colored wildflowers such as arrowleaf balsamroot (Balsamorhiza sagittata), wild geranium(Geranium maculatum), biscuitroot (Lomatium spp.) and a variety of lupines. Shrubs may be present, especially in rocky areas, but dry summers and fires limit any dominant establishment of shrubs in the grasslands.
Introduced Plant Community
Several exotic plants can invade and dominate landscapes of the Intermountain Grassland. Many of the most problematic invasive plants include cheatgrass (Bromus tectorum), medusahead (Taeniatherum caput-medusae), and yellow starthistle (Centaurea solstitialis). These are winter annuals that can take advantage of the mild, wet winters and secure space and resources before the perennial natives start significant growth in the spring.
Many large ungulates call the Intermountain Grassland home including mule deer, elk, and pronghorn. Small rodents including meadow voles, pocket gophers, ground squirrels and shrews are also abundant in these Intermountain prairies. The many herbivores on these grasslands are a good prey source that creates a diversity of predators, including coyotes, badgers, lynx, bobcat, foxes, wolves and mountain lions. Several avian species make their home in the prairie including red-tailed hawks, great-horned owls, and golden eagles.
Natural forces that influence changes to Intermountain Grassland vegetation dynamics include summer drought, occasional wildfire, and invasive plants. Like most rangeland types, several years of low precipitation can create droughty conditions that limit plant growth. This is also true of the Intermountain Grassland where native plants may produce little biomass and become dormant early in the season as the long dry summer ensues.
Wildfire is also an influential force in maintaining the Intermountain Grasslands. The lack of late summer precipitation limits the number of fires naturally ignited by lightning, but when fires are started they may burn for miles on the historic prairies. Native Americans also spread fire across the landscape, removing shrubs and maintaining the native prairie vegetation with particular interest in promoting native food plants such as camas and biscuitroot, which are stimulated by fire.
Large herds of grazing herbivores like bison are largely absent from the Intermountain Bunchgrass region, but seasonal grazing by deer and elk, and defoliation by small mammals have resulted in a prairie that can sustain period and early season grazing.
The system was dramatically altered by the arrival of homesteaders and explorers. The landscape became fragmented as the best lands became cultivated in the late 1800s. Heavy grazing by cattle and sheep more than 150 years ago likely reduced the competitive ability of the bunch grasses, resulting in an invasion of shrubs and trees, including juniper, sagebrush, and rabbitbrush into these grasslands. Cheatgrass, and other annual grasses, began to dominate areas in the Intermountain region in the mid-1900s, and today areas are near mono-cultures of exotic annual grasses. As a result of all these influences, today’s remnant Intermountain Grassland is a mix of shrubs, forbs, and grasses with a strong representation of non-native vegetation.
The vast majority of the original Intermountain Grassland is used for wheat production and other agricultural uses. The remaining habitat is highly fragmented, with some hunting, grazing, and few recreational activities.
Major restoration efforts are underway throughout much of the Intermountain Grasslands to suppress invasive annuals and re-establish dominance by perennial bunchgrasses and native forbs. Reduced grazing pressure and improved management in the 20th century had positive effects on the grazed areas of the prairie. Targeted grazing with livestock has been employed in recent years to suppress the exotic plants and give a competitive advantage back to perennial bunchgrasses. Woody plants have also been suppressed by prescribed fire or spraying.
Many of the farmed areas in the Intermountain Bunchgrass occur on productive soils that are susceptible to erosion. Many of these acres had been planted to perennial grasses through the Conservation Reserve Program (CRP) and Wildlife Habitat Incentive Program (WHIP). Programs such as these pay farmers to plant perennial grasses and forbs to preserve soil and convert farmland to wildlife habitat.
The greatest challenges in conserving and restoring the Intermountain Bunchgrass region will continue to be a battle against invasive plants, and re-establishing perennial grasslands. Increasingly frequent wildland fires are also a threat to these landscapes. These wildfires are promoted by exotic annual grasses that invade spaces between the bunchgrasses, creating an exceptional fine fuel bed. In addition, human activities and development in these wildlands increase the number of human-caused wildfires.
The remaining grasslands are largely too steep or too rocky to be converted to farmlands. Thus, these wildlands have economic value for grazing by livestock, hunting, and other forms of outdoor recreation including hiking, biking, and horseback riding. The ecological values of these landscapes include wildlife habitat, native plants, and watershed health. The threats to these landscapes will be managed with carefully managed grazing, strategic use of herbicides, and efforts to reduce fragmentation and wildfire.