16 – Shrub-steppe
Rex. C. Crawford & Jimmy Kagan
Geographic Distribution. Shrub-steppe habitats are common across
the Columbia Plateau of Washington, Oregon, Idaho, and adjacent Wyoming, Utah,
and Nevada. They extend up into the cold, dry environments of surrounding
mountains.
Basin big sagebrush shrub-steppe occurs along stream channels,
in valley bottoms and flats throughout eastern Oregon and Washington.
Wyoming sagebrush shrub-steppe is the most widespread habitat in
eastern Oregon and Washington, occurring throughout the Columbia
Plateau and the northern Great Basin. Mountain big sagebrush shrub-steppe
occurs throughout the mountains of the eastern Oregon and Washington.
Bitterbrush shrub-steppe appears primarily along the eastern slope
of the Cascades, from north-central Washington to California and
occasionally in the Blue Mountains. Three-tip sagebrush shrub-steppe
occurs mostly along the northern and western Columbia Basin in Washington
and occasionally appears in the lower valleys of the Blue Mountains
and in the Owyhee Upland ecoregions of Oregon. Interior shrub dunes
and sandy steppe and shrub-steppe is concentrated at low elevations
near the Columbia River and in isolated pockets in the Northern
Basin and Range and Owyhee Uplands. Bolander silver sagebrush shrub-steppe
is common in southeastern Oregon. Mountain silver sagebrush is more
prevalent in the Oregon East Cascades and in montane meadows in
the southern Ochoco and Blue Mountains.
Physical Setting. Generally, this habitat is associated with dry, hot
environments in the Pacific Northwest although variants are in cool, moist areas
with some snow accumulation in climatically dry mountains. Elevation range is
wide (300-9,000 ft [91-2,743 m]) with most habitat occurring between 2,000 and
6,000 ft (610-1,830 m). Habitat occurs on deep alluvial, loess, silty or
sandy-silty soils, stony flats, ridges, mountain slopes, and slopes of lake beds
with ash or pumice soils. Landscape Setting. Shrub-steppe
habitat defines a biogeographic region and is the major vegetation on average
sites in the Columbia Plateau, usually below Ponderosa Pine Forest and Woodland,
and Western Juniper and Mountain Mahogany Woodland habitats. It forms mosaic
landscapes with these woodland habitats and Eastside Grasslands, Dwarf
Shrub-steppe, and Desert Playa and Salt Scrub habitats. Mountain sagebrush
shrub- steppe occurs at high elevations occasionally within the dry Eastside
Mixed Conifer Forest and Montane Mixed Conifer Forest habitats. Shrub-steppe
habitat can appear in large landscape patches. Livestock grazing is the primary
land use in the shrub-steppe although much has been converted to irrigation or
dry land agriculture. Large areas occur in military training areas and wildlife
refuges.
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Structure. This habitat is a shrub savanna or shrubland
with shrub coverage of 10-60%. In an undisturbed condition, shrub cover varies
between 10 and 30%. Shrubs are generally evergreen although deciduous shrubs are
prominent in many habitats. Shrub height typically is medium-tall (1.6-3.3 ft
[0.5-1.0 m]) although some sites support shrubs approaching 9 ft (2.7 m) tall.
Vegetation structure in this habitat is characteristically an open shrub layer
over a moderately open to closed bunchgrass layer. The more northern or
productive sites generally have a denser grass layer and sparser shrub layer
than southern or more xeric sites. In fact, the rare good-condition site is
better characterized as grassland with shrubs than a shrubland. The bunchgrass
layer may contain a variety of forbs. Good-condition habitat has very little
exposed bare ground, and has mosses and lichens carpeting the area between
taller plants. However, heavily grazed sites have dense shrubs making up >40%
cover, with introduced annual grasses and little or no moss or lichen cover.
Moist sites may support tall bunchgrasses (>3.3 ft [1 m]) or rhizomatous
grasses. More southern shrub-steppe may have native low shrubs dominating with
bunchgrasses. Composition. Characteristic and dominant mid-tall
shrubs in the shrub-steppe habitat include all three subspecies of big
sagebrush, basin (Artemisia tridentata ssp. tridentata), Wyoming
(A. t. ssp. wyomingensis) or mountain (A. t. ssp.
vaseyana), antelope bitterbrush (Purshia tridentata), and two shorter
sagebrushes, silver (A. cana) and three-tip (A. tripartita). Each
of these species can be the only shrub or appear in complex seral conditions
with other shrubs. Common shrub complexes are bitterbrush and Wyoming big
sagebrush, bitterbrush and three-tip sagebrush, Wyoming big sagebrush and
three-tip sagebrush, and mountain big sagebrush and silver sagebrush. Wyoming
and mountain big sagebrush can codominate areas with tobacco brush (Ceanothus
velutinus). Rabbitbrush (Chrysothamnus viscidiflorus) and short-spine
horsebrush (Tetradymia spinosa) are common associates and often dominate
sites after disturbance. Big sagebrush occurs with the shorter stiff sagebrush
(A. rigida) or low sagebrush (A. arbuscula) on shallow soils or
high elevation sites. Many sandy areas are shrub-free or are open to patchy
shrublands of bitterbrush and/or rabbitbrush. Silver sagebrush is the dominant
and characteristic shrub along the edges of stream courses, moist meadows, and
ponds. Silver sagebrush and rabbitbrush are associates in disturbed areas.
When this habitat is in good or better ecological condition a
bunchgrass steppe layer is characteristic. Diagnostic native bunchgrasses that
often dominate different shrub-steppe habitats are (1) mid-grasses: bluebunch
wheatgrass (Pseudoroegneria spicata), Idaho fescue (Festuca
idahoensis), bottlebrush squirreltail (Elymus elymoides), and Thurber
needlegrass (Stipa thurberiana); (2) short grasses: threadleaf sedge
(Carex filifolia) and Sandberg bluegrass (Poa sandbergii); and (3)
the tall grass, basin wildrye (Leymus cinereus). Idaho fescue is
characteristic of the most productive shrub-steppe vegetation. Bluebunch
wheatgrass is codominant at xeric locations, whereas western needlegrass
(Stipa occidentalis), long-stolon (Carex inops) or Geyer’s
sedge (C. geyeri) increase in abundance in higher elevation shrub-steppe
habitats. Needle-and-thread (Stipa comata) is the characteristic native
bunchgrass on stabilized sandy soils. Indian ricegrass (Oryzopsis
hymenoides) characterizes dunes. Grass layers on montane sites contain
slender wheatgrass (Elymus trachycaulus), mountain fescue (F.
brachyphylla), green fescue (F. viridula), Geyer’s sedge, or
tall bluegrasses (Poa spp.). Bottlebrush squirreltail can be locally
important in the Columbia Basin, sand dropseed (Sporobolus cryptandrus)
is important in the Basin and Range and basin wildrye is common in the more
alkaline areas. Nevada bluegrass (Poa secunda), Richardson muhly
(Muhlenbergia richardsonis), or alkali grass (Puccinella spp.) can
dominate silver sagebrush flats. Many sites support non-native plants, primarily
cheatgrass (Bromus tectorum) or crested wheatgrass (Agropyron
cristatum) with or without native grasses. Shrub-steppe habitat, depending
on site potential and disturbance history, can be rich in forbs or have little
forb cover. Trees may be present in some shrub-steppe habitats, usually as
isolated individuals from adjacent forest or woodland habitats.
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Other
Classifications and Key References. This habitat is called Sagebrush steppe
and Great Basin sagebrush by Kuchler.136 The Oregon
Gap II Project126 and Oregon Vegetation
Landscape-Level Cover Types127 that would represent
this type are big sagebrush shrubland, sagebrush steppe, and bitterbrush-big
sagebrush shrubland. Franklin and Dyrness88 discussed
this habitat in shrub-steppe zones of Washington and Oregon. Other references
describe this habitat. 60, 116, 122, 123, 212, 224, 225
Natural Disturbance Regime. Barrett et al.22
concluded that the fire-return interval for this habitat is 25 years. The native
shrub-steppe habitat apparently lacked extensive herds of large grazing and
browsing animals until the late 1800s. Burrowing animals and their predators
likely played important roles in creating small-scale patch patterns.
Succession and Stand Dynamics. With disturbance, mature stands of big
sagebrush are reinvaded through soil-stored or windborne seeds. Invasion can be
slow because sagebrush is not disseminated over long distances. Site dominance
by big sagebrush usually takes a decade or more depending on fire severity and
season, seed rain, postfire moisture, and plant competition. Three-tip sagebrush
is a climax species that reestablishes (from seeds or commonly from sprouts)
within 5-10 years following a disturbance. Certain disturbance regimes promote
three- tip sagebrush and it can out-compete herbaceous species. Bitterbrush is a
climax species that plays a seral role colonizing by seed onto rocky and/or
pumice soils. Bitterbrush may be declining and may be replaced by woodlands in
the absence of fire. Silver sagebrush is a climax species that establishes
during early seral stages and coexists with later arriving species. Big
sagebrush, rabbitbrush, and short-spine horsebrush invade and can form dense
stands after fire or livestock grazing. Frequent or high-intensity fire can
create a patchy shrub cover or can eliminate shrub cover and create Eastside
Grasslands habitat. Effects of Management and Anthropogenic
Impacts. Shrub density and annual cover increase, whereas bunchgrass
density decreases, with livestock use. Repeated or intense disturbance,
particularly on drier sites, leads to cheatgrass dominance and replacement of
native bunchgrasses. Dry and sandy soils are sensitive to grazing, with
needle-and-thread replaced by cheatgrass at most sites. These disturbed sites
can be converted to modified grasslands in the Agriculture habitat.
Status and Trends. Shrub-steppe habitat still dominates most of
southeastern Oregon although half of its original distribution in the Columbia
Basin has been converted to agriculture. Alteration of fire regimes,
fragmentation, livestock grazing, and the addition of >800 exotic plant
species have changed the character of shrub-steppe habitat. Quigley and
Arbelbide181 concluded that Big Sagebrush and Mountain
Sagebrush cover types are significantly smaller in area than before 1900, and
that Bitterbrush/Bluebunch Wheatgrass cover type is similar to the pre-1900
extent. They concluded that Basin Big Sagebrush and Big Sagebrush-Warm potential
vegetation type’s successional pathways are altered, that some pathways of
Antelope Bitterbrush are altered and that most pathways for Big Sagebrush-Cool
are unaltered. Overall this habitat has seen an increase in exotic plant
importance and a decrease in native bunchgrasses. More than half of the Pacific
Northwest shrub-steppe habitat community types listed in the National Vegetation
Classification are considered imperiled or critically
imperiled.10
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