Shrub Steppe: Washington's Disappearing Sea

February 20, 2009  •  Leave a Comment

Washington State is known for its incredibly old, large and lush forests, beautiful coastline, and massive volcanoes, but nearly 1/3 of the state is occupied by what was once an expansive sea of sagebrush--the Columbia Basin. This area, known locally as shrub steppe, occurs from the eastern base of the Cascade Range, the southern base of the Okanogan Highlands, western base of the Northern Rocky Mountains, and south into Oregon. The entire area slopes inward from the base of the surrounding mountains down toward the valley of the Columbia River. Actually, shrub-steppe vegetation is much more expansive, extending throughout the majority of the inter-mountain west occupying much of Washington, Oregon, Idaho, Nevada, Utah, Wyoming, and Colorado. It may seem odd that such a dry region could occur in Washington State, which also boasts the wettest areas in the lower 48 states. However, the Cascade Range has an overwhelming impact on the amount of moisture that gets carried in from the Pacific Ocean. As Pacific storms blow landward, the Cascades force moisture out of the clouds as they begin to climb up the western edge of the mountains. By the time the storms have moved across the Olympic Mountains and Cascade Crest, much of their moisture has been rung out leaving little to fall on the lands of eastern Washington. As a consequence, places on the Olympic Peninsula may receive upwards 0f 200 inches of rain/year while portions of eastern Washington may only get 5-10 inches/year!
Shrub Steppe - the Columbia Plateau is delimited by the tan color. (Created with Google Maps)

The foundation of the Columbia Basin was laid down by massive amounts of lava that issued from regional fissures. The lave poured out of these vents and developed a thick layer of basalt that is up to 6,000 feet deep in some places! During the last Ice Age, massive floods, created by the failure of large glacier dams, carved apart this landscape leaving huge coulees (dry canyons), scablands, and unique topography across the Basin's contemporary landscape. These floods not only scoured the landscape but they deposited much material leaving some areas with deep sand and gravel, other areas with fine soils, and others with very little soil at all. Wind blown silt and volcanic ash were deposited and accumulated over a vast portion of the eastern portion of the Columbia Basin. This area is known as the Palouse and supports (or did support...nearly 99% is lost) a unique and highly endangered grassland that I will ponder in a future post.

Moses Coulee: One of the many dry canyons created by glacial floods.

The physical template left by historical floods, wind-driven deposits, and in situ soil development from the underlying basalt have allowed the development of a diversity of shrub-steppe plant communities. To the incurious eye, the shrub steppe appears to be a monotonous swath of gray sagebrush and green or brown (depending on the season!) grasses and herbs. Biological diversity appears absent but is simply inconspicuous. A closer and more detailed look reveals a diversity of sagebrush and herbaceous species closely tied to specific environmental conditions.
A large portion of the Columbia Plateau is dominated by Artemisia tridentata subsp. wyomingensis / Pseudoroegernia spicata (Wyoming big sagebrush/bluebunch wheategrass) plant association. This vegetation type occurs on modal (typical) soils and environmental conditions. The diversity of other native shrubs, grasses, and forbs is moderate and rarely exceeds 30 species.

On deep and/or sandy soils Artemisia tridentata subsp. tridentata (basin big sagebrush) is the most conspicuous sagebrush species. Stipa comata (needle-and-thread grass) and Purshia tridentata (bitterbrush) are abundant in deep sandy or gravelly areas, including sand dunes which are scattered throughout the Columbia Basin. Deep soil sites also provide habitat for the state endangered pygmy rabbit (Brachylagus idahoensis). The pygmy rabbit, the smallest rabbit in the United States, needs deep soils in order to dig its burrow. This little rabbit is, unfortunately, struggling in Washington--there are thought to be only about 30 rabbits left in the state.
The aspect, or the direction in which a site faces, has a significant impact on the type of vegetation which may develop. For example, south-facing slopes often receive more intense and a longer duration of direct sunlight than north-facing slopes. As a result, south slopes are typically warmer and drier due to increased evaporation from more intense solar radiation. Conversely, north-facing slopes are slightly cooler and moister. These differences result in unique expressions of vegetation. This is true in any ecosystem and not just the shrub-steppe. The relatively harsh environments of south-facing slopes makes them more susceptible to degradation from livestock grazing or other human-induced impacts. Similarly, south-facing slopes take much longer, than north-facing slopes, to recover from these disturbances.

and Artemisa tridentata subsp. wyomingensis can be seen in the foreground.
In the background is a south-facing slope dominated by Artemisa tridentata subsp.
wyomingensis, Salvia dorrii, and a sparse cover of herbaceous vegetation, including
an abundance of nonnative species (in this case cheatgrass).
Eastern end of the Beezley Hills, near Ephrata, WA.

Scattered throughout the Columbia Basin are outcrops of basalt with minimal or shallowly developed soil. Such sites are often referred to as lithosols or scablands and support a sparse cover of vegetation. Artemisia rigida (rigid sagebrush), Poa secunda (Sandberg's bluegrass), Phlox spp. (phlox), and a variety of Eriogonum spp. (buckwheats) and Lomatium spp. (biscuit roots) are the most common occupants of such habitats. The shrub steppe also wraps its arms around many other smaller habitats such as vernal pools (which support numerous rare plants), freshwater wetlands, riparian areas, sand dunes, cliffs, and playas. The contribution these small ecosystems they make toward overall landscape and species diversity (beta and alpha diversity, respectively) is immense.

Scabland site in foreground with Artemisia rigida and Poa secunda.

Rocky habitat supporting Salvia dorrii (purple sage)

North-facing slope dominated by lush shrub steppe consisting of Festuca idahoensis

Wyoming big sagebrush and bluebunch wheatgrass plant community.This is the
dominant vegetation type in the central portion of Washington's shrub steppe.
Two Steppe Natural Area, Washington

As one heads toward the edge of the Columbia Basin, toward any of the surrounding mountains, local environments become cooler and more moist than the interior part of the shrub steppe. The change is subtle to most visitors as the domimance of bunchgrasses and sagebrush continues. However, the dominant species of typical shrub-steppe habitat shifts. Artemisia tripartita (three-tip sagebrush) and Festuca idahoensis (Idaho fescue) replace Artemisia tridenata subsp. wyomingensis and Pseudoroegenria spicata as the most abundant species. In addition, overall species richness of native plants can climb up to nearly 50 species in these moister habitats.

Cooler and moister sites support
Artemisia tripartita and Festuca idahoensis shrub-steppe
along with a higher diversity of species than other sagebrush steppe vegetation types.
An important characteristic of most shrub-steppe plant communities is the presence of a biological soil crust which is made up of fungi, mosses, lichens and algae (collectively called cryptogams). Anyone willing to get on their knees and lower their nose close to the ground can observe the beautiful colors, textures, and patterns that these tiny little creatures offer. This crust not only supports a rich diversity of cryptogams but also plays a vital role in the functional health of the shrub steppe. Intact cryptogamic crusts improve infiltration of precipitation and thus retain moisture in the soil, protect the soil from erosion and thus provide soil stability, and even provide nutrients for other plants species. All of these are vital for the sustainability of shrub steppe. When human activities destroy this crust, the site becomes vulnerable to degradation. The absence of the soil crust provides an opportunity for a highly invasive plant, Bromus tectorum (cheatgrass) to become established. Once established, cheatgrass, an annual species, can dominate a site and push out native species. This little plant has taken over much of the shrub steppe across the Inter-mountain West of the United States. It is a nasty little plant. When the crust is intact, Bromus tectorum is unable to gain a foothold as the crust effectively serves as a barrier to germination for this species. Preservation of these crusts may be one way to stop the spread of an annual species on the verge of delivering a knockout punch to sagebrush habitat throughout the West. Of course, this requires some difficult cultural decisions about the way we use the sagebrush landscape.

Cryptogamic crust on shrub-steppe soil.
In addition to the pygmy rabbit mentioned above, there are numerous other critters which entirely depend on sagebrush habitats for survival. Such species are called sagebrush obligates and include Spizella breweri (Brewer's Sparrow), Amphispiza belli (Sage Sparrow), Oreoscoptes montanus (Sage Thrasher), Tympanuchus phasianellus subsp. columbianus (Columbian Sharp-tailed Grouse), and Centrocercus urophasianus (Sage Grouse). The latter two are listed as Threatened by the Washington Department of Fish & Wildlife. Both species once occurred throughout the Columbia Basin but loss of shrub-steppe habitat, degradation of existing habitat, and fragmentation caused by the network of roads, powerlines, development, agricultural fields, etc. have led to a sharp decline in both species. Sage Sparrows, Sage Thrashers, Athene cunicularia (Burrowing Owls), Spemophilus washingtoni (Washington ground squirrels), and Spemophilus townsendii (Townsend's ground squirrel) are all listed as State Candidate Species, meaning that they are currently being considered by Washington's Department of Fish & Wildlife for listing as either Sensitive, Threatened, or Endangered. Many rare plants are also limited to the Columbia Basin's shrub steppe such as Erigeron piperianus (Piper's daisy).

Erigeron piperianus (Piper's daisy), a plant only found in the Columbia Basin's shrub steppe.

On top of all the geologic, climatic, and soil factors that influence the distribution of vegetation, human activities have a strong influence on vegetation patterns. Some human acitivity has resulted in complete loss of the shrub steppe. For example, along with development, conversion to fields of wheat, orchards, hops, potatoes, and other crops has resulted in the loss of>55% of the original acreage of Washington's shrub steppe. Throughout the Intermountain West, overgrazing by sheep and cattle has degraded most of the remaining shrub-steppe with only about 10% thought to be left in good ecological condition. Overgrazing can break up the cryptogamic crust which can increase erosion and provide an opportunity for cheatgrass to gain a foothold. Grazing can also stress-out native plants which did not evolve with native grazers. In contrast to many other grasslands which evolved with grazers such as buffalo and antelope, most researchers believe that the shrub steppe in the Columbia Basin did not support significant populations of grazing animals and consequently is not highly resilient to grazing. The spread of exotic species, which is associated with all of the above human activities, also has its own unique impact on the ecological quality of shrub-steppe. Bromus tectorum (cheatgrass), Centaurea solstitialis (yellow starthistle), Sisymbrium altissimum (tumble mustard) are some of the nastier nonnative species which are quickly displacing native plants and altering key ecological processes such as fire regimes.

Red areas indicate places where human activities have eliminated or severely
degraded native ecosystems. Green areas are relatively intact. Notice the color
of the Columbia Basin. From 'The Human Footprint in the West.
Ecological Applications, 18(5), 2008, pp. 1119–1139'

The shrub steppe is disappearing. Despite the fact that it still occurs across much of the western U.S., incompatible land uses continue to push the sagebrush ecosystem toward the edge of extinction, especially in the Columbia Basin. Whenever anything becomes rare or unique, it becomes much easier to convince others of its importance. But, by then it is often too late. Why must we wait to lose something before recognizing the pain of its absence? The flora and fauna which evolved with this widespread habitat are suffering. We have choices. We can rearrange our footprint on the landscape in order to make room for other critters or we can continue to make decisions which achieve short-term gain but long-term loss of what has sometimes been referred to as Washington's inland sea.

Sources: Daubenmire, R. 1970. Steppe Vegetation of Washington. Washingon Agricultural Experimental Station. Technical Bulletin 62. 131 pp.
Chappell, C.B., R.C. Crawford, C. Barrett, J. Kagan, D.H. Johnson, M. O'Mealy, G.A. Green, H.L. Ferguson, W.D. Edge, E.L. Greda, and T.A. O'Neal. 2001. Chapter 2. Wildlife Habitats: Descriptions, Status, Trends, and System Dynamics. In Wildlife Habitat Relationships in Washington and Oregon (D.H. Johnson and T.A. O'Neal, editors). Oregon State University Press, Corvallis, OR.
Vander Haegen, W.M., S.M. McCorquodale, C.R. Peterson, G.A. Green, and E. Yensen. 2001. Chapter 11. Wildlife of Eastside Shrubland and Grassland Habitats. In Wildlife Habitat Relationships in Washington and Oregon (D.H. Johnson and T.A. O'Neal, editors). Oregon State University Press, Corvallis, OR.


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