No. 2. Westside Oak and Dry Douglas-fir Forest and Woodlands

Christopher B. Chappell and Jimmy Kagan

Geographic Distribution. This forest and woodland habitat is primarily found in the Willamette Valley, Puget Lowlands, and Klamath Mountains ecoregions. In the Puget Lowlands, it is common in and around the San Juan Islands and parts of Thurston, Pierce, and Mason counties. In southwestern Oregon, it is now restricted mainly to the valleys of the Rogue and Umpqua rivers. Minor occurrences can also be found in the northeastern Olympic Mountains and western Cascades.

This habitat is comprised of several geographic variants: California black oak and ponderosa pine are important only in southwestern Oregon and the southern Willamette Valley. The latter is also found in a small area of Pierce County, Washington. Shore pine is only important in the Puget Lowland, mainly in San Juan and Mason counties, and is often absent near saltwater shores. Dry Douglas-fir forests (without oak or madrone) are mainly in the Puget Lowland and rarely in the Olympic Mountains, west Cascades, and Willamette Valley. Pacific madrone and Douglas-fir/Pacific madrone stands without oak are limited to the Puget Lowland and the southern Willamette Valley foothills. Mixed oak-madrone stands occur primarily in Oregon, especially southwestern Oregon.

Physical Setting. This habitat typically occupies dry sites west of the Cascades. Annual mean precipitation ranges from 17 to 60 inches (43 to 152 cm), occasionally higher. Elevation ranges from sea level to about 3,500 ft (1,069 m) in the Olympic Mountains, but is mainly below 1,500 ft (457 m). Topography ranges from nearly level to very steep slopes, where aspect tends to be southern or western. Soils on dry sites are typically shallow over bedrock, very stony, or very deep and excessively drained. Willamette Valley soils are typically much older and have more moderate drainage and water availability. Parent materials include various types of bedrock, shallow or very coarse glacial till, alluvium, and glacial outwash.

Landscape Setting. This habitat is found in a mosaic with, or adjacent to, Westside Grasslands, Westside Lowlands Conifer-Hardwood Forest, Westside Riparian-Wetlands, Southwest Oregon Mixed Conifer-Hardwood Forest, Urban, and Agriculture. Inclusions of Open Water or Herbaceous Wetlands sometimes occur. In the Puget Lowland, this habitat is sometimes found adjacent to Puget Sound (Nearshore Marine). Land use of this habitat includes forestry (generally small scale), livestock grazing, and low-density rural residential.

Structure. This is a forest or woodland dominated by evergreen conifers, deciduous broadleaf trees, evergreen broadleaf trees, or some mixture of conifers and broadleaf trees. Canopy structure varies from single- to multi-storied. Large conifers, when present, typically emerge above broadleaf trees in mixed canopy stands. Large snags and logs are less abundant than in other westside forest habitats, but can be prominent, especially in unlogged old stands. Understories vary in structure: grasses, shrubs, ferns, or some combination will typically dominate. Deciduous broadleaf shrubs are perhaps most typical as understory dominants in the existing landscape. Early successional stand structure varies depending on understory species present and if initiated following logging or fire.

Composition. The canopy is typically dominated by 1 or more of the following species: Douglas-fir (Pseudotsuga menziesii), Oregon white oak (Quercus garryana), Pacific madrone (Arbutus menziesii), shore pine (Pinus contorta var. contorta), or California black oak (Q. kelloggii). Ponderosa pine (Pinus ponderosa) is important in southwestern Oregon and the southern Willamette Valley as a subordinate or co-dominant with oak. Grand fir (Abies grandis) is occasionally co-dominant with Douglas-fir in the northern Puget Lowland or in the Willamette Valley. Oregon ash (Fraxinus latifolia) is occasionally co-dominant with white oak in riparian oak stands. Several other tree species may be present, but western hemlock (Tsuga heterophylla) and western redcedar (Thuja plicata) generally cannot regenerate successfully because of dry conditions. This lack of shade-tolerant tree regeneration, along with understory indicators like tall Oregongrape (Mahonia aquifolium) and blue wildrye (Elymus glaucus), help distinguish dry Douglas-fir forests from mid-seral Douglas-fir stands on more mesic sites, which are part of the Westside Lowlands Conifer-Hardwood Forest. Tree regeneration, when present, is typically Douglas-fir, less commonly grand fir. Sweet cherry (Prunus avium) and/or English hawthorn (Crataegus monogyna) have invaded and now dominate a subcanopy layer in many oak forests of the Willamette Valley.

Deciduous shrubs that commonly dominate or co-dominate the understory are oceanspray (Holodiscus discolor), baldhip rose (Rosa gymnocarpa), poison-oak (Toxicodendron diversiloba), serviceberry (Amelanchier alnifolia), beaked hazel (Corylus cornuta), trailing blackberry (Rubus ursinus), Indian plum (Oemleria cerasiformis), snowberries (Symphoricarpos albus and s. mollis), wedge-leaf ceanothus (Ceanothus cuneatus), and oval-leaf viburnum (Viburnum ellipticum). Evergreen shrubs or vines that sometimes are dominant where conifers are important in the canopy include salal (Gaultheria shallon), dwarf Oregongrape (Mahonia nervosa), Pacific rhododendron (Rhododendron macrophyllum), hairy honeysuckle (Lonicera hispidula), evergreen huckleberry (Vaccinium ovatum), and Piper’s barberry (Mahonia piperiana).

Native graminoids that commonly dominate or co-dominate the understory are western fescue (Festuca occidentalis), Alaska oniongrass (Melica subulata), blue wildrye, and long-stolon sedge (Carex inops). Kentucky bluegrass (Poa pratensis) is a major non-native dominant in oak woodland understories. Swordfern (Polystichum munitum) or, less commonly, bracken fern (Pteridium aquilinum) sometimes co-dominates the understory, especially on sites that formerly supported grasslands and savannas. Forbs, many of which are characteristic of these dry sites, are often abundant and diverse, but typically do not dominate. Common camas (Camassia quamash), cleavers (Galium aparine), or other forbs are occasionally co-dominant with graminoids.

Other Classifications and Key References. This habitat has been described as oak groves and dry site Douglas-fir forest in the Tsuga heterophylla zone of western Washington and northwestern Oregon as well as oak woodland in the interior valleys of western Oregon ⁸⁸. Also referred to as Oregon oakwoods No. 22 and a minor part of Cedar-hemlock-Douglas-fir forest No. 2 ¹³⁶; The Oregon Gap II Project ¹²⁶ and Oregon Vegetation Landscape-Level Cover Types ¹²⁷ that would represent this type are Oregon white oak forest and Douglas-fir/white oak forest. The Washington Gap Project represents this habitat as part of hardwood forest, mixed hardwood/conifer forest, and conifer forest in the Woodland/Prairie Mosaic, Puget Sound Douglas-fir, and, to a minor degree, Cowlitz River, and Willamette Valley zones of Washington ³⁷. Other references also describe elements of this habitat ^{13, 17, 40, 41, 86, 111, 115, 202, 210}.

Natural Disturbance Regime. Fire is the major natural disturbance in this habitat. In presettlement times, fire frequency probably ranged from frequent (every few years) to moderately frequent (once every 50-100 years), and reflected low-severity and moderate-severity fire regimes ¹. Fire frequency has been much lower in the last 100 years. Windstorms are an occasional disturbance, most important in the San Juan Islands and vicinity. Understories are sometimes browsed heavily by deer in the San Juan Islands, thus preventing dominance by deciduous shrubs and favoring grasses and forbs.

Succession and Stand Dynamics. Many of these forests and woodlands were formerly either grasslands or savannas that probably burned frequently, thus preventing dominance by trees ^{41, 54}. Some portions of this habitat in the central Puget Lowlands may have formerly been dominated by shrubs (salal, beaked hazel, evergreen huckleberry, hairy manzanita [Arctostaphylos columbiana]) for lengthy periods, probably also because of the particular combination of fire frequency and intensity. Other areas were woodlands to semi-open forests that burned moderately frequently, as evidenced by the relict stands of old-growth Douglas-fir. The dominant trees in this habitat establish most abundantly after fire. Moderate-severity fires kill many trees but also leave many alive, creating opportunities for establishment of new cohorts of trees and increasing structural complexity ¹. Oaks and madrone resprout after fire if they are top-killed. Without periodic fire, most oak-dominated stands will eventually convert to Douglas-fir forests ¹. Animal dissemination of acorns may be important in dispersal of oaks. Shore pine, where present, is an early-seral upper canopy species that grows quickly and dies out after about 100-150 years, yielding to a mature Douglas-fir stand unless another fire intervenes before the death of the pine.

Effects of Management and Anthropogenic Impacts. Clearcut or similar logging reduces canopy structural complexity and abundance of large woody debris. Dry Douglas-fir stands are well suited to alternative silvicultural practices, such as uneven-aged management or maintaining 2-storied stands. Oaks and madrone will typically resprout after logging and thus can increase in importance relative to conifers in mixed canopy stands. Selective logging of Douglas-fir in oak stands can prevent long-term loss of oak dominance. With fire exclusion, stands have probably increased in tree density and grassy understories have been replaced by deciduous shrubs ⁴¹. Moderate to heavy grazing or other significant ground disturbance, especially in grassy understories, leads to increases in non-native invader species, many of which are now abundant in stands with grassy or formerly grassy understories. Scot’s broom (Cytisus scoparius) is an exotic shrub particularly invasive and persistent in oak woodlands. Exotic herbaceous invaders include colonial bentgrass (Agrostis capillaris), common velvetgrass (Holcus lanatus), Kentucky bluegrass, tall oatgrass (Arrhenatherum elatius), rigid brome (Bromus rigidus), orchardgrass (Dactylis glomerata), hedgehog dogtail (Cynosurus echinatus), tall fescue (Festuca arundinacea), and common St. Johnswort (Hypericum perforatum).

Status and Trends. This habitat is relatively limited in area and is currently declining in extent and condition. With the cessation of regular burning 100-130 years ago, many grasslands and savannas were invaded by a greater density of trees and thus converted to a different habitat. Conversely, large areas of this habitat have been converted to Urban or Agriculture habitats. Most of what remains has been considerably degraded by invasion of exotic species or by logging and consequent loss of structural diversity. Ongoing threats include residential development, increase and spread of exotic species, and fire suppression effects (the latter especially in oak-dominated stands). Thirteen of 27 plant associations listed in the National Vegetation Classification are considered globally imperiled or critically imperiled ¹⁰.