No. 3. Southwest Oregon Mixed Conifer-Hardwood Forest

Christopher B. Chappell and Jimmy Kagan

Geographic Distribution. This upland forest and woodland habitat occurs in southwestern Oregon, northwestern California, and the Sierra Nevada. In southern Oregon, it is found at low and middle elevations in the Klamath Mountains, Cascades, Coast Range, and Eastern Cascade Slopes and Foothills ecoregions. Portions of Curry, Josephine, Jackson, Douglas, Lane, and Klamath counties are included in the range of this habitat.

Physical Setting. The climate varies from relatively dry and very warm to moderately moist and cool to slightly warm and very moist. Mean annual precipitation ranges from 20 to 140 inches (51 to 356 cm). Snow is uncommon except at the highest elevations, where a winter snow pack occurs for a few months. Summers are hot and dry. Elevation ranges from near sea level to 6,000 ft (1,829 m). Topography is mostly mountainous but also includes 2 fairly large valleys, and a corresponding variety of terrain. Soils are diverse as is the bedrock geology. Serpentine soils are common in portions of the Siskiyou Mountains, where they have a major effect on vegetation.

H03_1.JPG (351618 bytes)Landscape Setting. This habitat is typically bounded at its upper elevation limits by Montane Mixed Conifer Forest and at its lower limits, along the coast, by Westside Lowlands Conifer-Hardwood Forest. At lower elevations in the Rogue and Umpqua valleys it can be found in a mosaic with Westside Oak and Dry Douglas-fir Forest and Woodland, Ceanothus-Manzanita Shrublands, Urban, and Agriculture. Small inclusions of Open Water, Herbaceous Wetlands, Westside Riparian-Wetlands, and Ceanothus-Manzanita Shrublands occur scattered throughout this habitat. The predominant land use is forestry. Low-density residential is prominent in the Rogue and Umpqua valleys. Grazing occurs on some areas, especially at lower elevations.

Structure. Conifer trees typically dominate this forest or woodland habitat. In some generally more coastal areas, a well developed subcanopy layer of smaller evergreen broadleaf trees is present. Occasionally, deciduous broadleaf trees are co-dominant. Complex multi-layered canopies are typical, though single-layered canopies also occur, especially in areas of intensive forest management. Dominant canopy trees vary from 60 to >300 ft (18 to >91 m) tall at maturity. Large woody debris (snags and logs) is typically common, although variable. Understories are mostly dominated by shrubs, but can be dominated by forbs, graminoids, or may be largely depauperate.

Composition. The tree canopy is often diverse. Douglas-fir (Pseudotsuga menziesii), white fir (Abies concolor), sugar pine (Pinus lambertiana), ponderosa pine (P. ponderosa), or incense cedar (Calocedrus decurrens) are typically dominant or co-dominant. Port-Orford cedar (Chamaecyparis lawsoniana), tanoak (Lithocarpus densiflorus), canyon live oak (Quercus chrysolepis), and Pacific madrone (Arbutus menziesii) are locally important. Jeffrey pine (Pinus jeffreyi) is dominant on serpentine parent materials in the Siskiyou Mountains, and to a lesser degree in the southwestern Cascades.  

H03_2.JPG (410501 bytes)Douglas-fir is found in almost every area; ponderosa pine is also found in most stands, although it has been declining with fire suppression. White fir, incense cedar , and sugar pine are common in mixed stands in the Cascades and central and eastern Siskiyous on all but the driest sites. White fir dominates the canopy in only the moist, cool sites at higher elevations, although it is the major tree regeneration in most areas. Jeffrey pine and knobcone pine (Pinus attenuata) are limited primarily to serpentine soils, which they dominate. Port-Orford cedar dominates some more moist sites near the coast and riparian and wetland habitats inland. Brewer’s spruce (Picea breweri) is an uncommon dominant at high elevations in the Siskiyous. The broadleaf subcanopy is most prominent on the western sides of the Coast Range and Siskiyous, where tanoak is most abundant, with Pacific madrone, golden chinquapin (Castanopsis chrysophylla), or canyon live oak also sometimes dominating the subcanopy. Coast redwood (Sequoia sempervirens) occurs only in a very small area near the coast in far southern Oregon. 

Dominant or co-dominant evergreen shrubs include pinemat manzanita (Arctostaphylos nevadensis), green-leaf manzanita (A. patula), white-leaf manzanita (A. viscida), kinnikinnick (A. uva-ursi), Piper’s barberry (Mahonia piperiana), dwarf Oregongrape (M. nervosa), tobacco brush (Ceanothus velutinus), squawcarpet (C. prostratus), salal (Gaultheria shallon), deer oak (Quercus sadleriana), huckleberry oak (Q. vacciniifolia), snow bramble (Rubus nivalis), Pacific rhododendron (Rhododendron macrophyllum), and evergreen huckleberry (Vaccinium ovatum). Major deciduous shrubs are serviceberry (Amelanchier alnifolia), sticky currant (Ribes viscosissimum), oceanspray (Holodiscus discolor), creeping snowberry (Symphoricarpos mollis), baldhip rose (Rosa gymnocarpa), beaked hazel (Corylus cornuta), Rocky Mountain maple (Acer glabrum), vine maple (A. circinatum), poison-oak (Toxicodendron diversiloba), big huckleberry (Vaccinium membranaceum), deerbrush (Ceanothus integerrimus), and trailing blackberry (Rubus ursinus). Early seral shrublands, part of this habitat, can be difficult to distinguish from Ceanothus-Manzanita Shrublands. They are best separated by their different species composition, especially the predominance in this habitat of Ceanothus velutinus, Arctostaphylos patula, and A. nevadensis.

Graminoids that are most prominent are long-stolon sedge (Carex inops), Idaho fescue (Festuca idahoensis), and California fescue (F. californica). Forbs that are indicative of site conditions or dominate understories include common whipplea (Whipplea modesta), twinflower (Linnaea borealis), sidebells (Orthilia secunda), rattlesnake plantain (Goodyera oblongifolia), vanillaleaf (Achlys triphylla), beargrass (Xerophyllum tenax), and starry false solomonseal (Maianthemum stellata).

H03_3.JPG (370198 bytes)Other Classifications and Key References. This habitat includes the conifer-dominated forests and their successional seres within the Interior Valley, Mixed-Conifer, Mixed-Evergreen, and Abies concolor zones of southwestern Oregon, plus Redwood forests in the Picea sitchensis Zone 88. It is also referred to as Klamath Mountains mixed evergreen forests and Sierran-type mixed conifer forests 87; Pseudotsuga menziesii/hardwood forests and Abies concolor forests 1; Mixed conifer forest No. 5, Redwood forest No. 6, California mixed evergreen forest No. 29, and Montane chaparral No. 34 136. The Oregon Gap II Project 126 and Oregon Vegetation Landscape-Level Cover Types 127 that would represent this type are the southwestern portion of the Douglas-fir dominant-mixed conifer forest, Jeffrey pine forest and woodland, serpentine conifer woodland, Douglas-fir-Port Orford cedar forest, Douglas-fir mixed deciduous forest, Douglas-fir-white fir/tanoak-madrone mixed forest, and Siskiyou Mountains mixed deciduous forest. Other references also describe this habitat 13, 15, 17, 111, 117.

Natural Disturbance Regime. Fire is the predominant natural disturbance. Fire regime varies depending on environmental conditions.  Drier, hotter sites within this area have a low-severity fire regime. Cooler and/or moister sites typically have a moderate-severity fire regime. Presettlement mean fire return intervals vary from £10 years to about 80 years 1, 98, 154. Lightning ignitions are more frequent here than anywhere else in the region and Native Americans probably burned some areas intentionally 1. Wind is a somewhat important disturbance at higher elevations. Root rot fungi and insects are other important disturbances in some forests, mostly operating at small-scales.

Succession and Stand Dynamics. Most evergreen broadleaf trees, when present, are top-killed by moderate-severity fires but resprout vigorously to dominate or co-dominate after most fires 14, 152. Mature Oregon white oak and canyon live oak can survive fairly hot fires if the fuels do not extend into the canopy. Conifers are at a disadvantage in regeneration following stand replacement fires because of dependence on local seed-fall. Many conifers of this habitat are able to survive moderate-severity fire well, including, in decreasing order of fire resistance, Ponderosa pine, Jeffrey pine, Douglas-fir, sugar pine, coast redwood, incense cedar, and Port-Orford cedar. These species are fairly well represented throughout the successional sequence, unless a high-severity fire was closely followed by another, in which case the subcanopy broadleaf species are likely to dominate 1. Development of complex multi-layered canopies of conifers and broadleaf evergreens are typical under a moderate-severity fire regime.

H03_4.JPG (440029 bytes)Where hardwoods are absent and white fir is prominent, succession differs from that described above. Under a low-severity fire regime with frequent fires, white fir is relatively unimportant and fire-resistant conifers, especially Douglas-fir and ponderosa pine, dominate. White fir increases in the absence of fire 171. With a moderate-severity fire regime, i.e., less frequent fires, white fir can dominate or co-dominate, especially on cooler sites 1. Small gaps created by moderate-severity fires, blowdown, or disease afford opportunities for regeneration of less shade-tolerant tree species, thus maintaining a diverse tree canopy for lengthy periods. Evergreen shrubs, especially tobacco brush, often dominate after high-severity fire and may persist as a cover type for decades, especially if they are reburned 51, 88. On the driest, hottest sites in this habitat, white fir does not grow and tree regeneration is limited to Douglas-fir and ponderosa pine, with the former tending to increase in the absence of fire.

Effects of Management and Anthropogenic Impacts. Clearcut logging where hardwoods are present favors post-disturbance dominance of tanoak or madrone. Control of this competing vegetation has been a major focus of timber management in this habitat. Fire control over the last 100 years has decreased fire frequencies and altered stand structure through increases in small tree density and heavy fuels, especially where low-severity fire regimes were prevalent. As a result, most of these areas are more susceptible to stand-replacement fires. White fir has increased dramatically on drier sites where it occurs, creating dense subcanopy thickets 1, 128. Evergreen shrubs often dominate after clearcut logging and in some cases hinder the establishment of conifers 140. Clearcut logging tends to decrease tree species diversity, coarse woody debris loads, and structural diversity. The non-native species white pine blister rust (Cronartium ribicola) and Phytophthora lateralis, a root rot disease, have had significant negative impacts on the abundance of sugar pine and Port-Orford cedar, respectively.

Status and Trends. This habitat covers most of southwestern Oregon and has declined little in areal extent. Conditions of most communities and stands have been degraded by forestry practices and by fire suppression. The low-elevation, driest communities have been altered by grazing and invasion of exotic species. Port-Orford cedar has declined dramatically in extent from logging and Phytophthora lateralis 230. Effects of fire suppression and logging-related impacts continue to be threats. Twenty-one percent of 68 plant associations representing this habitat listed in the National Vegetation Classification are listed as imperiled 10.

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