No. 5. Eastside (Interior) Mixed Conifer Forest

Rex C. Crawford

Geographic Distribution. The Eastside Mixed Conifer Forest habitat appears primarily the Blue Mountains, East Cascades, and Okanogan Highland Ecoregions of Oregon, Washington, adjacent Idaho, and western Montana. It also extends north into British Columbia.

Douglas-fir-ponderosa pine forests occur along the eastern slope of the Oregon and Washington Cascades, the Blue Mountains, and the Okanogan Highlands of Washington. Grand fir-Douglas-fir forests and western larch forests are widely distributed throughout the Blue Mountains and, lesser so, along the east slope of the Cascades south of Lake Chelan and in the eastern Okanogan Highlands. Western hemlock-western redcedar-Douglas-fir forests are found in the Selkirk Mountains of eastern Washington, and on the east slope of the Cascades south of Lake Chelan to the Columbia River Gorge.

Physical Setting. The Eastside Mixed Conifer Forest habitat is primarily mid-montane with an elevation range of between 1,000 and 7,000 ft (305-2,137 m), mostly between 3,000 and 5,500 ft (914-1,676 m). Parent materials for soil development vary. This habitat receives some of the greatest amounts of precipitation in the inland northwest, 30-80 inches (76-203 cm)/year. Elevation of this habitat varies geographically, with generally higher elevations to the east.

Landscape Setting. This habitat makes up most of the continuous montane forests of the inland Pacific Northwest. It is located between the subalpine portions of the Montane Mixed Conifer Forest habitat in eastern Oregon and Washington and lower tree line Ponderosa Pine and Forest and Woodlands.

Structure. Eastside Mixed Conifer habitats are montane forests and woodlands. Stand canopy structure is generally diverse, although single-layer forest canopies are currently more common than multilayered forests with snags and large woody debris. The tree layer varies from closed forests to more open-canopy forests or woodlands. This habitat may include very open stands. The undergrowth is complex and diverse. Tall shrubs, low shrubs, forbs or any combination may dominate stands. Deciduous shrubs typify shrub layers. Prolonged canopy closure may lead to development of a sparsely vegetated undergrowth.

Composition. This habitat contains a wide array of tree species (9) and stand dominance patterns. Douglas-fir (Pseudotsuga menziesii) is the most common tree species in this habitat. It is almost always present and dominates or co-dominates most overstories. Lower elevations or drier sites may have ponderosa pine (Pinus ponderosa) as a co-dominant with Douglas-fir in the overstory and often have other shade-tolerant tree species growing in the undergrowth. On moist sites, grand fir (Abies grandis), western redcedar (Thuja plicata) and/or western hemlock (Tsuga heterophylla) are dominant or co-dominant with Douglas-fir. Other conifers include western larch (Larix occidentalis) and western white pine (Pinus monticola) on mesic sites, Engelmann spruce (Picea engelmannii), lodgepole pine (Pinus contorta), and subalpine fir (Abies lasiocarpa) on colder sites. Rarely, Pacific yew (Taxus brevifolia) may be an abundant undergrowth tree or tall shrub.

Undergrowth vegetation varies from open to nearly closed shrub thickets with 1 to many layers. Throughout the eastside conifer habitat, tall deciduous shrubs include vine maple (Acer circinatum) in the Cascades, Rocky Mountain maple (A. glabrum), serviceberry (Amelanchier alnifolia), oceanspray (Holodiscus discolor), mallowleaf ninebark (Physocarpus malvaceus), and Scouler’s willow (Salix scouleriana) at mid- to lower elevations. Medium-tall deciduous shrubs at higher elevations include fools huckleberry (Menziesia ferruginea), Cascade azalea (Rhododendron albiflorum), and big huckleberry (Vaccinium membranaceum). Widely distributed, generally drier site mid-height to short deciduous shrubs include baldhip rose (Rosa gymnocarpa), shiny-leaf spirea (Spiraea betulifolia), and snowberry (Symphoricarpos albus, S. mollis, and S. oreophilus). Low shrubs of higher elevations include low huckleberries (Vaccinium cespitosum, and V. scoparium) and five-leaved bramble (Rubus pedatus). Evergreen shrubs represented in this habitat are chinquapin (Castanopsis chrysophylla), a tall shrub in southeastern Cascades, low to mid-height dwarf Oregongrape (Mahonia nervosa in the east Cascades and M. repens elsewhere), tobacco brush (Ceanothus velutinus), an increaser with fire, Oregon boxwood (Paxistima myrsinites) generally at mid- to lower elevations, beargrass (Xerophyllum tenax), pinemat manzanita (Arctostaphylos nevadensis) and kinnikinnick (A. uva-ursi).

Herbaceous broadleaf plants are important indicators of site productivity and disturbance. Species generally indicating productive sites include western oakfern (Gymnocarpium dryopteris), vanillaleaf (Achlys triphylla), wild sarsparilla (Aralia nudicaulis), wild ginger (Asarum caudatum), queen’s cup (Clintonia uniflora), goldthread (Coptis occidentalis), false bugbane (Trautvetteria caroliniensis), windflower (Anemone oregana, A. piperi, A. lyallii), fairybells (Disporum hookeri), Sitka valerian (Valeriana sitchensis), and pioneer violet (Viola glabella). Other indicator forbs are dogbane (Apocynum androsaemifolium), false solomonseal (Maianthemum stellata), heartleaf arnica (Arnica cordifolia), several lupines (Lupinus caudatus, L. latifolius, L. argenteus ssp. argenteus var laxiflorus), western meadowrue (Thalictrum occidentale), rattlesnake plantain (Goodyera oblongifolia), skunkleaf polemonium (Polemonium pulcherrimum), trailplant (Adenocaulon bicolor), twinflower (Linnaea borealis), western starflower (Trientalis latifolia), and several wintergreens (Pyrola asarifolia, P. picta, Orthilia secunda).

Graminoids are common in this forest habitat. Columbia brome (Bromus vulgaris), oniongrass (Melica bulbosa), northwestern sedge (Carex concinnoides) and western fescue (Festuca occidentalis) are found mostly in mesic forests with shrubs or mixed with forb species. Bluebunch wheatgrass (Pseudoroegneria spicata), Idaho fescue (Festuca idahoensis), and junegrass (Koeleria macrantha) are found in drier more open forests or woodlands. Pinegrass (Calamagrostis rubescens) and Geyer’s sedge (C. geyeri) can form a dense layer under Douglas-fir or grand fir trees.

Other Classifications and Key References. This habitat includes the moist portions of the Pseudotsuga menziesii, the Abies grandis, and the Tsuga heterophylla zones of eastern Oregon and Washington ⁸⁸. This habitat is called Douglas-fir (No. 12), Cedar-Hemlock-Pine (No. 13), and Grand fir-Douglas-fir (No. 14) forests in Kuchler ¹³⁶. The Oregon Gap II Project ¹²⁶ and Oregon Vegetation Landscape-Level Cover Types ¹²⁷ that would represent this type are the eastside Douglas-fir dominant-mixed conifer forest, ponderosa pine dominant mixed conifer forest, and the northeast Oregon mixed conifer forest. Quigley and Arbelbide ¹⁸¹ referred to this habitat as Grand fir/White fir, the Interior Douglas-fir, Western larch, Western redcedar/Western hemlock, and Western white pine cover types and the Moist Forest potential vegetation group. Other references detail forest associations for this habitat ^{45, 59, 117, 118, 123, 122, 144, 148, 208, 209, 212, 221, 228}.

Natural Disturbance Regime. Fires were probably of moderate frequency (30-100 years) in presettlement times. Inland Pacific Northwest Douglas-fir and western larch forests have a mean fire interval of 52 years ²². Typically, stand-replacement fire-return intervals are 150-500 years with moderate severity-fire intervals of 50-100 years. Specific fire influences vary with site characteristics. Generally, wetter sites burn less frequently and stands are older with more western hemlock and western redcedar than drier sites. Many sites dominated by Douglas-fir and ponderosa pine, which were formerly maintained by wildfire, may now be dominated by grand fir (a fire sensitive, shade-tolerant species).

Succession and Stand Dynamics. Successional relationships of this type reflect complex interrelationships between site potential, plant species characteristics, and disturbance regime ²²⁸. Generally, early seral forests of shade-intolerant trees (western larch, western white pine, ponderosa pine, Douglas-fir) or tolerant trees (grand fir, western redcedar, western hemlock) develop some 50 years following disturbance. This stage is preceded by forb- or shrub- dominated communities. These early stage mosaics are maintained on ridges and drier topographic positions by frequent fires. Early seral forest develops into mid-seral habitat of large trees during the next 50-100 years. Stand replacing fires recycle this stage back to early seral stages over most of the landscape. Without high-severity fires, a late-seral condition develops either single-layer or multilayer structure during the next 100-200 years. These structures are typical of cool bottomlands that usually only experience low-intensity fires.

Effects of Management and Anthropogenic Impacts. This habitat has been most affected by timber harvesting and fire suppression. Timber harvesting has focused on large shade-intolerant species in mid- and late-seral forests, leaving shade-tolerant species. Fire suppression enforces those logging priorities by promoting less fire-resistant, shade-intolerant trees. The resultant stands at all seral stages tend to lack snags, have high tree density, and are composed of smaller and more shade-tolerant trees. Mid-seral forest structure is currently 70% more abundant than in historical, native systems ¹⁸¹. Late-seral forests of shade-intolerant species are now essentially absent. Early-seral forest abundance is similar to that found historically but lacks snags and other legacy features.

Status and Trends. Quigley and Arbelbide ¹⁸¹ concluded that the Interior Douglas-fir, Grand fir, and Western redcedar/Western hemlock cover types are more abundant now than before 1900, whereas the Western larch and Western white pine types are significantly less abundant. Twenty percent of Pacific Northwest Douglas-fir, grand fir, western redcedar, western hemlock, and western white pine associations listed in the National Vegetation Classification are considered imperiled or critically imperiled ¹⁰. Roads, timber harvest, periodic grazing, and altered fire regimes have compromised these forests. Even though this habitat is more extensive than pre-1900, natural processes and functions have been modified enough to alter its natural status as functional habitat for many species.