15 – Eastside Grasslands
Rex. C. Crawford & Jimmy Kagan
Geographic Distribution. This habitat is found primarily in the
Columbia Basin of Idaho, Oregon, and Washington, at mid- to low elevations and
on plateaus in the Blue Mountains, usually within the ponderosa pine zone in
Oregon. Idaho fescue grassland habitats were formerly widespread in the
Palouse region of southeastern Washington and adjacent Idaho; most of this
habitat has been converted to agriculture. Idaho fescue grasslands still occur
in isolated, moist sites near lower treeline in the foothills of the Blue
Mountains, the Northern Rockies, and east Cascades near the Columbia River
Gorge. Bluebunch wheatgrass grassland habitats are common throughout the
Columbia Basin, both as modified native grasslands in deep canyons and the dry
Palouse and as fire-induced representatives in the shrub-steppe. Similar
grasslands appear on the High Lava Plains ecoregion, where they occur in a
matrix with big sagebrush or juniper woodlands. In Oregon they are also found in
burned shrub-steppe and canyons in the Basin and Range and Owyhee Uplands. Sand
dropseed and three-awn needlegrass grassland habitats are restricted to river
terraces in the Columbia Basin, Blue Mountains, and Owyhee Uplands of Oregon and
Washington. Primary location of this habitat extends along the Snake River from
Lewiston south to the Owyhee River.
Physical Setting. This
habitat develops in hot, dry climates in the Pacific Northwest. Annual
precipitation totals 8-20 inches (20-51 cm); only 10% falls in the hottest
months, July through September. Snow accumulation is low (1-6 inches [3-15 cm])
and occurs only in January and February in eastern portions of its range and
November through March in the west. More snow accumulates in grasslands within
the forest matrix. Soils are variable: (1) highly productive loess soils up to
51 inches (130 cm) deep, (2) rocky flats, (3) steep slopes, and (4) sandy,
gravel or cobble soils. An important variant of this habitat occurs on sandy,
gravelly, or silty river terraces or seasonally exposed river gravel or Spokane
flood deposits. The grassland habitat is typically upland vegetation but it may
also include riparian bottomlands dominated by non-native grasses. This habitat
is found from 500 to 6,000 ft (152-1,830 m) in elevation. Landscape
Setting. Eastside grassland habitats appear well below and in a matrix with
lower treeline Ponderosa Pine Forest and Woodlands or Western Juniper and
Mountain Mahogany Woodlands. It can also be part of the lower elevation forest
matrix. Most grassland habitat occurs in two distinct large landscapes: plateau
and canyon grasslands. Several rivers flow through narrow basalt canyons below
plateaus supporting prairies or shrub-steppe. The canyons can be some 2,132 ft
(650 m) deep below the plateau. The plateau above is composed of gentle slopes
with deep silty loess soils in an expansive rolling dune-like landscape.
Grasslands may occur in a patchwork with shallow soil scablands or within
biscuit scablands or mounded topography. Naturally occurring grasslands are
beyond the range of bitterbrush and sagebrush species. This habitat exists today
in the shrub- steppe landscape where grasslands are created by brush removal,
chaining or spraying, or by fire. Agricultural uses and introduced perennial
plants on abandoned or planted fields are common throughout the current
distribution of eastside grassland habitats.
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Structure. This
habitat is dominated by short to medium- tall grasses (<3.3 ft [1 m]). Total
herbaceous cover can be closed to only sparsely vegetated. In general, this
habitat is an open and irregular arrangement of grass clumps rather than a
continuous sod cover. These medium-tall grasslands often have scattered and
diverse patches of low shrubs, but few or no medium-tall shrubs (<10% cover
of shrubs are taller than the grass layer). Native forbs may contribute
significant cover or they may be absent. Grasslands in canyons are dominated by
bunchgrasses growing in lower densities than on deep-soil prairie sites. The
soil surface between perennial plants can be covered with a diverse cryptogamic
or microbiotic layer of mosses, lichens, and various soil bacteria and algae.
Moister environments can support a dense sod of rhizomatous perennial grasses.
Annual plants are a common spring and early summer feature of this habitat.
Composition. Bluebunch wheatgrass (Pseudoroegneria spicata)
and Idaho fescue (Festuca idahoensis) are the characteristic
native bunchgrasses of this habitat and either or both can be dominant.
Idaho fescue is common in more moist areas and bluebunch wheatgrass
more abundant in drier areas. Rough fescue (F. campestris)
is a characteristic dominant on moist sites in northeastern Washington.
Sand dropseed (Sporobolus cryptandrus) or three-awn (Aristida
longiseta) are native dominant grasses on hot dry sites in deep
canyons. Sandberg bluegrass (Poa sandbergii) is usually present,
and occasionally codominant in drier areas. Bottlebrush squirreltail
(Elymus elymoides) and Thurber needlegrass (Stipa thurberiana)
can be locally dominant. Annual grasses are usually present; cheatgrass
(Bromus tectorum) is the most widespread. In addition, medusahead
(Taeniatherum caput-medusae), and other annual bromes (Bromus
commutatus, B. mollis, B. japonicus) may be present to co-dominant.
Moist environments, including riparian bottomlands, are often co-dominated
by Kentucky bluegrass (Poa pratensis).
A dense and diverse forb layer can be present or
entirely absent; >40 species of native forbs can grow in this habitat
including balsamroots (Balsamorhiza spp.), biscuitroots (Lomatium
spp.), buckwheat (Eriogonum spp.), fleabane (Erigeron spp.),
lupines (Lupinus spp.), and milkvetches (Astragalus spp.). Common
exotic forbs that can grow in this habitat are knapweeds (Centaurea
solstitialis, C. diffusa,
C. maculosa), tall tumblemustard (Sisymbrium altissimum), and
Russian thistle (Salsola kali). Smooth sumac (Rhus glabra)
is a deciduous shrub locally found in combination with these grassland species.
Rabbitbrushes (Chrysothamnus nauseosus, C. viscidiflorus) can
occur in this habitat in small amounts, especially where grazed by livestock. In
moist Palouse regions, common snowberry (Symphoricarpos albus) or Nootka
rose (Rosa nutkana) may be present, but is shorter than the bunchgrasses.
Dry sites contain low succulent pricklypear (Opuntia polyacantha). Big
sagebrush (Artemisia tridentata) is occasional and may be increasing in
grasslands on former shrub-steppe sites. Black hawthorn (Crataegus
douglasii) and other tall shrubs can form dense thickets near Idaho fescue
grasslands. Rarely, ponderosa pine (Pinus ponderosa) or western juniper
(Juniperus occidentalis) can occur as isolated trees.
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Other Classifications and Key References. This habitat
is called Palouse Prairie, Pacific Northwest grassland, steppe vegetation,
or bunchgrass prairie in general ecological literature. Quigley
and Arbelbide181 called this habitat Fescue-Bunchgrass
and Wheatgrass Bunchgrass and the dry Grass cover type. The Oregon
Gap II Project126 and Oregon Vegetation
Landscape-Level Cover Types127 that would
represent this type are northeast Oregon canyon grassland, forest-grassland
mosaic, and modified grassland; Washington Gap37
types 13, 21, 22, 24, 29-31 , 82, and 99 map this habitat. Kuchler136
includes this within Fescue-wheatgrass and wheatgrass-bluegrass.
Franklin and Dyrness88 include this habitat
in steppe zones of Washington and Oregon. Other references describe
this habitat.1, 28, 60, 159, 166, 206, 207
Natural
Disturbance Regime. The fire-return interval for sagebrush and bunchgrass is
estimated at 25 years.22 The native bunchgrass 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. Currently fires burn less frequently in the Palouse grasslands
than historically because of fire suppression, roads, and conversions to
cropland.159 Without fire, black hawthorn shrubland
patches expand on slopes along with common snowberry and rose. Fires covering
large areas of Shrub-steppe habitat can eliminate shrubs and their seed sources
and create Eastside Grasslands habitat. Fires that follow heavy grazing or
repeated early season fires can result in annual grasslands of cheatgrass,
medusahead, knapweed, or yellow star-thistle. Annual exotic grasslands are
common in dry grasslands and are included in modified grasslands as part of the
Agriculture habitat. Effects of Management and Anthropogenic
Impacts. Large expanses of grasslands are currently used for
livestock ranching. Deep soil Palouse sites are mostly converted to agriculture.
Drier grasslands and canyon grasslands, those with shallower soils, steeper
topography, or hotter, drier environments, were more intensively grazed and for
longer periods than were deep-soil grasslands.207
Evidently, these drier native bunchgrass grasslands changed irreversibly to
persistent annual grass and forblands. Some annual grassland, native bunchgrass,
and shrub-steppe habitats were converted to intermediate wheatgrass, or more
commonly, crested wheatgrass (Agropyron cristatum)-dominated areas.
Apparently, these form persistent grasslands and are included as modified
grasslands in the Agriculture habitat. With intense livestock use, some riparian
bottomlands become dominated by non-native grasses. Many native dropseed
grasslands have been submerged by dam reservoirs.
Status and Trends. Most of the Palouse prairie of southeastern
Washington and adjacent Idaho and Oregon has been converted to agriculture.
Remnants still occur in the foothills of the Blue Mountains and
in isolated, moist Columbia Basin sites. The Palouse is one of the
most endangered ecosystems in the U.S.166
with only 1% of the original habitat remaining; it is highly fragmented
with most sites <10 acres. All these areas are subject to weed
invasions and drift of aerial biocides. Since 1900, 94% of the Palouse
grasslands have been converted to crop, hay, or pasture lands. Quigley
and Arbelbide181 concluded that Fescue-Bunchgrass
and Wheatgrass bunchgrass cover types have significantly decreased
in area since before1900, while exotic forbs and annual grasses
have significantly increased since pre-1900. Fifty percent of the
plant associations recognized as components of Eastside Grasslands
habitat listed in the National Vegetation Classification are considered
imperiled or critically imperiled.10
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