No. 14. Eastside (Interior) Canyon Shrublands

Rex. C. Crawford and Jimmy Kagan

Geographic Distribution. This habitat occurs primarily on steep canyon slopes in the Blue Mountains and the margins of the Columbia Basin in Idaho, Oregon, and Washington. This habitat also appears as isolated patches across Washington’s Columbia Basin.

Physical Setting. This habitat develops in hot dry climates in the Pacific Northwest. Annual precipitation totals 12-20 inches (31-51 cm); only 10% falls in the hottest months, July through September. Snow accumulation is low (1-6 inches [3-15 cm]), persisting only a few weeks. Sites are generally steep (>60%) on all aspects but most common on northerly aspects in deep, dry canyons. Columbia River basalt is the major geologic substrate although many sites are underlain with loess deposits mixed with colluvium. Steep northerly aspects in the Palouse Hills can also support this habitat. This habitat is found from 500 to 5,000 ft (152 to 1,524 m) in elevation.

Landscape Setting. This habitat is generally found in steep canyons surrounded by the Eastside Grassland Habitat and below or in a mosaic with the Ponderosa Pine Forest and Woodlands habitat. This habitat can develop near talus slopes, at the heads of dry drainages, and toe slopes in moist shrub-steppe and steppe zones. At lower elevation sites, these are more often in a mix with bluebunch wheatgrass, dry rocky grasslands, and low-elevation riparian habitats. The primary surrounding land use is livestock grazing.

Structure. The Eastside Canyon Shrubland habitat is generally a mix of tall (5 ft [1.5 m]) to medium (1.6 ft [0.5 m]) deciduous shrublands in a mosaic with bunchgrass or annual grasslands. Shrub canopies are almost always closed (>60% cover), forming a thicket of interwoven stems and branches. Shrub layers can be 1 or 2-tiered but often are so dense they restrict the herbaceous layer to shade-tolerant rhizomatous species.

Composition. Mallowleaf ninebark (Physocarpus malvaceus), a major dominant, bitter cherry (Prunus emarginata), chokecherry (Prunus virginiana), oceanspray (Holodiscus discolor) or Rocky Mountain maple (Acer glabrum) are the most common tall shrubs in this habitat. In moist areas, black hawthorn (Crataegus douglasii) may appear and can dominate some sites as a tall shrub or small tree. Other tall shrubs such as syringa (Philadelphus lewisii) or serviceberry (Amelanchier alnifolia) often dominate sites associated with talus. Common medium-tall shrubs are common snowberry (Symphoricarpos albus), rose (Rosa nutkana, R. woodsii), smooth sumac (Rhus glabra), and currants (Ribes spp.). Basin or Wyoming big sagebrush (Artemisia tridentata ssp. tridentata or A. t. ssp. wyomingensis), along with rabbitbrush (Chrysothamnus spp.), may be important members of these thickets in weedy sites, dry areas, or transitions with grasslands. Scattered ponderosa pine (Pinus ponderosa), black cottonwood (Populus balsamifera ssp. trichocarpa) and rarely Douglas-fir (Pseudotsuga menziesii) trees may be found in and adjacent to this habitat.

Idaho fescue (Festuca idahoensis), bluebunch wheatgrass (Pseudoroegneria spicata), Thurber’s needlegrass (Stipa thurberiana), and Sandberg’s bluegrass (Poa sandbergii) found in the surrounding steppe or shrub-steppe are common but never abundant in these thickets. Basin wildrye (Leymus cinereus) can be locally important. Kentucky bluegrass (Poa pratensis) is a common introduced grass and, where grazed by livestock, is a dominant undergrowth species. Annual grasses (Bromus tectorum, B. briziformis) can be abundant especially on disturbed dry sites. Cleavers (Galium aparine) is a frequent member of the herbaceous component of this habitat. Other common forbs include red avens (Geum triflorum), horsemint (Agastache urticifolia), sticky cinquefoil (Potentilla gracilis), balsamroots (Balsamorhiza spp.), and fleabanes (Erigeron spp.).

Other Classifications and Key References. This habitat is called shrub garland ⁸⁸ or talus thickets. The Oregon Gap II Project ¹²⁶ and Oregon Vegetation Landscape-Level Cover Type ¹²⁷ that would represent this type is eastside big sagebrush shrubland. Other references describe this habitat are ^{60, 122, 123, 207}.

Natural Disturbance Regime. This habitat is within the sagebrush and bunchgrass vegetation type of Barrett et al. ²² who concluded it had a fire-return interval of 25 years. Canyon shrublands associated with talus burn less frequently but are subject to talus movement. Similar shrubfields are associated with forest landscapes and are early seral stages of the Eastside Mixed Conifer Forest Habitat.

Succession and Stand Dynamics. Many of the major shrubs sprout following fire and will be maintained with moderate fire frequency. Most thickets will increase in size without fire. This habitat has increased primarily in moist steppe and shrub-steppe habitat with fire suppression and restricted grazing. Prolonged fire suppression may lead to invasions by tree species. Apparently some representatives of this habitat could potentially support Douglas-fir or ponderosa pine woodlands after a long fire-free period.

Effects of Management and Anthropogenic Impacts. Livestock grazing in adjacent grassland or shrub-steppe habitat changes the surrounding fine-fuel matrix for fire. That, combined with fire suppression, leads to a change in habitat patch size, structure, and composition. In response to fire suppression, shrub thickets on northerly aspects near lower treeline tend to increase in patch size and height and are invaded by tree species. With heavy livestock grazing, shrubs are browsed, broken, and trampled, which eventually creates a more open shrubland with a more abundant herbaceous layer.

Status and Trends. The Eastside Canyon Shrubland habitat is restricted in range and probably has increased locally in area. Johnson and Simon ¹²³ reported increases in common snowberry-rose communities as a response to fire suppression and heavy grazing that depleted bunchgrass cover. One of the three Eastside Canyon Shrubland community types in the National Vegetation Classification is considered imperiled ¹⁰.