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Shrub-Steppe

Shrub-SteppeEveryone likes sagebrush. Whether it's the ancient look they have, with their gnarled branches and gray-green visage, or the way they smell after a rainstorm or when leaves are crushed in your hand, sagebrush are, well, cool. But hidden below the tops of the more impressive sagebrush, rabbitbrush and other desert shrubs, the bunchgrasses—the "steppe"—have a fascinating ecological story to tell.

History and Rarity

 

The Monument is characterized as a shrub-steppe ecosystem. Such ecosystems are typically dominated by a shrub overstory with a grass understory. At 195,777 acres, the Monument, along with the Department of Defense's Yakima Training Center, retain the largest remaining blocks of relatively undisturbed shrub-steppe in the Columbia Basin Ecoregion.

When settlers arrived, the vegetation in the ecoregion consisted primarily of shrubs, perennial bunchgrasses, and a variety of forbs. The dominant plants in the area were big sagebrush underlain by perennial Sandberg's bluegrass and bluebunch wheatgrass. With the advent of settlement, livestock grazing and agricultural production contributed to colonization by non-native plant species that currently dominate large portions of the landscape. An estimated 60% of shrub-steppe in Washington has been converted to agriculture or other uses. Much of what remains is in small parcels in shallow rocky soils or has been degraded by historic land uses (mostly livestock grazing). This conversion of land extends even into the Monument; the Monument encompasses undeveloped land interspersed with industrial development along the southern shoreline of the Columbia River, and human-made intrusions—such as roads, power lines, irrigation canals, communications structures, and remnant domestic plants—are evident throughout the Monument.

The Monument contains some of the best remaining large-scale examples of the shrub-steppe vegetation type in the Pacific Northwest, supporting habitat for many species of native wildlife (including shrub-steppe obligate species), a diverse array of native plant communities (including many threatened and endangered taxa) and microbiotic crusts, and a unique invertebrate fauna that is still being catalogued. Many places in the Monument are relatively free of non-native species and are extensive enough to retain characteristic populations of shrub-steppe plants and animals that are absent or scarce in other areas.

The Monument’s importance as a refuge for the shrub-steppe ecosystem is not solely related to size, however. The presence of a large diversity of physical features and examples of rare, undeveloped, deep and sandy soil has led to a corresponding diversity of plant and animal communities. Because it is located within the hottest and driest part of the ecoregion, the Monument also retains some of its own uniqueness and fragility.

Want to know about sagebrush? Read on.

Sagebrush

 

Sagebrush is a particularly important plant in eastern Washington and throughout the Intermountain West. Mature stands of big sagebrush (where plants are large enough to have developed a woody branching structure) provide important habitat for species that rely on sagebrush to survive. The loggerhead shrike, sage thrasher, and sage sparrow all require mature shrubs for nesting. Other species of conservation concern in Washington, such as the greater sage grouse, require the leaves of big sagebrush as a food source. Black-tailed jackrabbits also rely heavily on big sagebrush for food, protective cover, and shade.

There are three species of sagebrush found on the Monument. The overwhelming majority is big sagebrush. The other two species are stiff sagebrush, which is deciduous, and threetip sagebrush. Big sagebrush and threetip sagebrush are not deciduous, and their leaves can be an important food source for some wildlife in winter months when other shrubs are leafless. Stiff sagebrush, which is less common on the Hanford Site because it is restricted to rock outcrops and rocky, shallow soils, is a winter deciduous shrub. The sagebrush communities of the Hanford Site represent a particularly important natural resource for the state because other lands within eastern Washington have been developed for agriculture and industry.

And a bit about our grasses next.

Grasses

 

On the Monument, bluebunch wheatgrass, Sandberg's bluegrass, a variety of flowering plants (forbs), and a cryptogamic soil crust form the understory of the big sagebrush/bluebunch wheatgrass zone. The understory vegetation comprises those species that grow below the tallest shrubs or trees in a plant community, that is the forbs and grass species. Most native perennial grass species commonly found on the Hanford Site are cool-season bunchgrasses, including needle-and-thread grass, Indian ricegrass, Cusick's bluegrass, and Idaho fescue.

The grasses native to the Monument are primarily what are known as bunchgrasses. The reason they are called that is simple—they grow in bunches, or clumps. Part of this is an adaptation to the climate—there isn't enough moisture during the summer to support a thick carpet of plants, and bunchgrasses can draw moisture from the area around the clump—and part of it is an adaptation to fire. When a fire comes through an area of bunchgrass, it has to 'hop' from bunch to bunch. This results in a slower, less intense fire leaving root systems intact from which to resprout. It also means that some sagebrush is likely to survive the fire.

However, there is a problem—an introduced problem. Cheatgrass (downy brome) is native to temperate regions of Eurasia and northern Africa. Carried over by accident from these areas, cheatgrass has become so ingrained in the landscape that it is no longer even classified as a weed—there is no hope of getting rid of it, only of controlling it.  Although first detected in North America in 1790 in Lancaster County, Pennsylvania, it was still rare until at least the 1860s. As it spread west, though, it found the perfect conditions to thrive. On the Monument and other areas of the West, plants are adapted to low annual precipitation (6.8 inches on the Monument), low water-holding capacity of the rooting substrate (sand), dry summers, and cold winters—situations that are ideal for cheatgrass.

Cheatgrass is an aggressive colonizer. Where established bunchgrass communities exist, they do well in holding off cheatgrass. However, when there is a disturbance, such as fire, cheatgrass moves in with a vengeance. Cheatgrass germinates earlier than native grasses, so it establishes itself after a fire before other grasses can. Once established, it takes work and usually chemicals to get it under some semblance of control.

Since cheatgrass germinates in late fall, winter and early spring, it cures (drys out) earlier than native grasses, usually by early June. As the cheatgrass cures, it becomes an available and abundant fuel. Often, fires start within cheatgrass stands and spread to other adjacent communities. Subsequently, other plants are exposed to burning earlier in the fire season than they historically would have been. This weakens native plants because they are burned during the peak of their growing cycle, which allows cheatgrass to spread even further into native plant communities. This reduces biodiversity and accelerates the fire cycle.

Adding to the problem is that the highly flammable cheatgrass grows in a continuous carpet, taking advantage of the wetter seasons on the Monument. The fires come earlier, but they also come faster and much hotter. The discontinuous fuel that native bunchgrasses provided were invaded by thick, continuous fuels that would carry fires over large areas. The hotter fires kill root systems, so recolonization by native grasses depends on windblown seeds from outside the area.

And if you really want to learn about plant communities, read on.

Plant Communities

 

Throughout the Columbia Basin Ecoregion, a number of different plant association zones occur as climatic climaxes (i.e., the plant association or community expected to occur in typical sites in the absence of disturbance). The largest and driest of these zones is the big sagebrush (Artemisia tridentata) / bluebunch wheatgrass (Pseudoroegneria spicata [=Agropyron spicatum]) association. This association occupies the center of the Columbia Basin Ecoregion, which includes the Hanford Reach National Monument. In general, the big sagebrush / bluebunch wheatgrass association is characterized by four layers of vegetation—an overstory layer composed mostly of big sagebrush up to two meters tall, a tall understory layer of bluebunch wheatgrass, a short understory dominated by Sandberg's bluegrass (Poa sandbergii [included within Poa secunda]), and a layer of algae, lichens and mosses on the soil surface (i.e., the microbiotic crust).

The microbiotic crust is a critical component of native grasslands and shrub-steppe communities. This diminutive community of mosses, lichens, liverworts, algae and bacteria stabilizes the soils and fills the interstitial space between bunchgrass clumps. Perennial forbs are a minor constituent of the tall understory layer, whereas most annual forbs occur in the short understory layer. Other shrubs that may be present include rabbitbrush (Chrysothamnus spp.), bitterbrush (Purshia tridentata), spiny hopsage (Grayia spinosa), and three-tip sagebrush (Artemisia tripartita). Additional locally abundant bunchgrasses include needle-and-thread (Stipa comata), Indian ricegrass (Oryzopsis hymenoides), Cusick's bluegrass (Poa cusickii [included within Poa secunda]) and Idaho fescue (Festuca idahoensis).

Other associations, such as big sagebrush / Idaho fescue, bluebunch wheatgrass / Sandberg's bluegrass, and bluebunch wheatgrass / Idaho fescue can occur as topographic climaxes on moister sites within the big sagebrush / bluebunch wheatgrass association. Certain edaphic (soil-related) plant associations also are of ecological importance within the ecoregion. On deep soils dominated by gravel, sand, or strongly weathered volcanic ash, needle-and-thread and/or Indian ricegrass replaces bluebunch wheatgrass as the dominant grass in several associations. The dominant shrub in these associations can be either big or three-tip sagebrush or bitterbrush. On stony soils or extremely shallow soils over bedrock (lithosols), various species of buckwheat (Eriogonum) and/or stiff sage (Artemisia rigida) dominate the shrub layer and Sandberg's bluegrass dominates the understory. As the hottest, driest, and lowest elevation part of the ecoregion, the Hanford Site also possesses a series of three plant associations found on reasonably deep, loamy (but dry) soils. These are the big sagebrush / Sandberg's bluegrass, spiny hopsage / Sandberg's bluegrass, and winterfat (Atoides [=Eurotia] lanata) / Sandberg's bluegrass associations. Each of these associations is characterized by the lack of large, perennial bunchgrasses (Sandberg's bluegrass is relatively small) and low overall plant diversity.

Page Photo Credits — Shrub-Steppe - Matt Lavin
Last Updated: May 03, 2013
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