Pacific Northwest Dry Douglas-fir Forest

This ecological system comprises much of the major lowland forests of western Washington, northwestern Oregon, eastern Vancouver Island, and the southern Coast Ranges in British Columbia. In southwestern Oregon, it becomes local and more small-patch in nature. It occurs throughout low-elevation western Washington, except on extremely dry or moist to very wet sites. In Oregon, it occurs on the western slopes of the Cascades, around the margins of the Willamette Valley, and in the Coast Ranges. These forests occur on the drier to intermediate moisture habitats and microhabitats within the Western Hemlock Zone of the Pacific Northwest. Climate is relatively mild and moist to wet. Mean annual precipitation is mostly 90-254 cm (35-100 inches) (but as low as 20 inches in the extreme rainshadow) falling predominantly as winter rain. Snowfall ranges from rare to regular, and summers are relatively dry. Elevation ranges from sea level to 610 m (2000 feet) in northern Washington to 1067 m (3500 feet) in Oregon. Topography ranges from relatively flat glacial tillplains to steep mountainous terrain. This is generally the most extensive forest in the lowlands on the west side of the Cascades and forms the matrix within which other systems occur as patches. Throughout its range it occurs in a mosaic with North Pacific Maritime Mesic-Wet Douglas-fir-Western Hemlock Forest (CES204.002); in dry areas it occurs adjacent to or in a mosaic with North Pacific Dry Douglas-fir-(Madrone) Forest and Woodland (CES204.845), and at higher elevations it intermingles with either North Pacific Dry-Mesic Silver Fir-Western Hemlock-Douglas-fir Forest (CES204.098) or North Pacific Mesic Western Hemlock-Silver Fir Forest (CES204.097).

Overstory canopy is dominated by Pseudotsuga menziesii, with Tsuga heterophylla generally present in the subcanopy or as a canopy dominant in old-growth stands. Abies grandis, Thuja plicata, and Acer macrophyllum codominants are also represented. In the driest climatic areas, Tsuga heterophylla may be absent, and Thuja plicata takes its place as a late-seral or subcanopy tree species. Gaultheria shallon, Mahonia nervosa, Rhododendron macrophyllum, Linnaea borealis, Achlys triphylla, and Vaccinium ovatum typify the poorly to well-developed shrub layer. Acer circinatum is a common codominant with one or more of these other species. The fern Polystichum munitum can be codominant with one or more of the evergreen shrubs on sites with intermediate moisture availability (mesic). If Polystichum munitum is thoroughly dominant or greater than about 40-50% cover, then the stand is probably in the more moist North Pacific Maritime Mesic-Wet Douglas-fir-Western Hemlock Forest (CES204.002). Young stands may lack Tsuga heterophylla or Thuja plicata, especially in the Puget Lowland. Tsuga heterophylla is generally the dominant regenerating tree species. Other common associates include Acer macrophyllum, Abies grandis, and Pinus monticola. In southwestern Oregon, Pinus lambertiana, Calocedrus decurrens, and occasionally Pinus ponderosa may occur in these forests. Soils are generally well-drained and are mesic to dry for much of the year. This is in contrast to North Pacific Maritime Mesic-Wet Douglas-fir-Western Hemlock Forest (CES204.002), which occurs on sites where soils remain moist to subirrigated for much of the year and fires were less frequent. Fire is (or was) the major natural disturbance. In the past (pre-1880), fires were less commonly high-severity, typically mixed-severity or moderate-severity, with natural return intervals of 100 years or less in the driest areas, to a few hundred years in areas with more moderate to wet climates. In the drier climatic areas (central Oregon Cascades, Puget Lowlands, Georgia Basin), this system was typified by a (mixed) moderate-severity fire regime involving occasional stand-replacing fires and more frequent moderate-severity fires. This fire regime would create a complex mosaic of stand structures across the landscape.

Overstory canopy is dominated by Pseudotsuga menziesii, with Tsuga heterophylla generally present in the subcanopy or as a canopy dominant in old-growth stands. Abies grandis, Thuja plicata, and Acer macrophyllum codominants are also represented. In the driest climatic areas, Tsuga heterophylla may be absent, and Thuja plicata takes its place as a late-seral or subcanopy tree species. Gaultheria shallon, Mahonia nervosa, Rhododendron macrophyllum, Linnaea borealis, Achlys triphylla, and Vaccinium ovatum typify the poorly to well-developed shrub layer. Acer circinatum is a common codominant with one or more of these other species. The fern Polystichum munitum can be codominant with one or more of the evergreen shrubs on sites with intermediate moisture availability (mesic). Young stands may lack Tsuga heterophylla or Thuja plicata, especially in the Puget Lowland. Tsuga heterophylla is generally the dominant regenerating tree species. Other common associates include Acer macrophyllum, Abies grandis, and Pinus monticola. In southwestern Oregon, Pinus lambertiana, Calocedrus decurrens, and occasionally Pinus ponderosa may occur in these forests.

This system occurs throughout low-elevation western Washington, except on extremely dry or moist to very wet sites. These forests occur on the drier to intermediate moisture habitats and microhabitats within the Western Hemlock Zone of the Pacific Northwest. Climate is relatively mild and moist to wet. Mean annual precipitation is mostly 90-254 cm (35-100 inches) (but as low as 50 cm [20 inches] in the extreme rainshadow) falling predominantly as winter rain. Snowfall ranges from rare to regular, and summers are relatively dry. Elevation ranges from sea level to 610 m (2000 feet) in northern Washington to 1067 m (3500 feet) in Oregon. Topography ranges from relatively flat glacial tillplains to steep mountainous terrain. This is generally the most extensive forest in the lowlands on the west side of the Cascades and forms the matrix within which other systems occur as patches.

Fire is (or was) the major natural disturbance process. In the past (pre-1880), fires were high-severity or, less commonly, moderate-severity, with natural return intervals of 100 years or less in the driest areas, to a few hundred years in areas with more moderate to wet climates. In the drier climatic areas (central Oregon Cascades, Puget Lowlands, Georgia Basin), this system was typified by a (mixed) moderate-severity fire regime involving occasional stand-replacement fires and more frequent moderate-severity fires. This fire regime would create a complex mosaic of stand structures across the landscape. Landfire VDDT models: #RDFHEdry Douglas-fir Hemlock dry mesic describes general successional stage relationship with bias to OR (Landfire 2007a).

Conversion of this type has commonly come from logging and urban development. Development, timber harvest, road building, fire suppression, tree plantations and introduced diseases have all impacted natural disturbance regimes, forest structure, composition, landscape patch diversity, and tree regeneration. Development has fragmented the landscape changing fire regime and connectivity of this small patch system particularly in lowlands. Timber harvest operations change canopy structural complexity and abundance of large woody debris of individual stands and has altered whole landscape patch pattern, age and structural complexity (Van Pelt 2007, as cited in WNHP 2011). Plantation forestry has changed local tree gene pools, horizontal arrangement of trees and homogenized the diversity of tree sizes. Other effects include loss of early-seral shrub species, advanced stand development, increased stand density, and increased tree mortality. Older logged areas can support dense, stagnating second growth with root rot (Arno 2000, as cited in WNHP 2011). Ohlman and Waddel (2002) (as cited in WNHP 2011) speculated that snag abundance more likely reflect recent disturbance and forest succession, whereas down wood amounts more are strongly reflect long-term stand history and site productivity (WNHP 2011).

In the Pacific Northwest, regionally downscaled climate models project increases in annual temperature of, on average, 3.2°F by the 2040s. Projected changes in annual precipitation, averaged over all models, are small (+1 to +2%), but some models project wetter autumns and winters and drier summers. Increases in extreme high precipitation (falling as rain) in the western Cascades and reductions in snowpack are key projections from high-resolution regional climate models (Littell et al. 2009). Warmer temperatures will result in more winter precipitation falling as rain rather than snow throughout much of the Pacific Northwest, particularly in mid-elevation basins where average winter temperatures are near freezing (Littell et al. 2009).

Drier overall climate may drive this ecosystem to a drier Douglas-fir-dominated type with the loss of western hemlock, as this species is limited to humid climate, and in subhumid regions with relatively dry growing seasons, and in the southern part of its range it is confined to northerly aspects or moist stream bottoms (Packee 1990). However, regional climate model simulations generally predict increases in extreme high precipitation over the next half-century, particularly around Puget Sound (Spittlehouse 2008, Littell et al. 2009, Werner 2011). In many coastal regions, the interaction between oceanographic and terrestrial air masses may be ecologically important. Intensifying upwelling along the California coast under climate change may intensify fog development and onshore flows in summer months, leading to decreased temperatures and increased moisture flux over land (Snyder et al. 2003, Lebassi et al. 2009, as cited in PRBO Conservation Science 2011). Coastal terrestrial ecosystems could benefit from these changes. However, current trends in fog frequency along the Pacific coast from 1901-2008 have been negative (Johnstone and Dawson 2010, as cited in PRBO Conservation Science 2011), thus the effect of climate change on coastal fog remains uncertain. (From PRBO Conservation Science 2011). However, affect on coastal fog is not addressed in the Washington Climate Change Impacts Assessment (Littell et al. 2009) nor is it currently a factor in Georgia Basin (C. Cadrin pers. comm. 2013). Summer-time fog and its associated fog-drip and cooling effect may increase with warmer inland air temperatures (PRBO Conservation Science 2011), but this will depend on oceanic circulations and he complex interaction of the El Niño-Southern Oscillation and the Pacific Decadal Oscillation makes prediction of land/ocean interaction difficult and increases the uncertainty of regional climate modeling outcomes (Karl et al. 2009). Increased fire frequency due to warmer temperatures resulting in drier fuels the area burned by fire regionally is projected to double by the 2040s and triple by the 2080s (Littell et al. 2009). In the Georgia Depression, increases of up to 10% in fire severity are reported (Haughian et al. 2012).

This system comprises the major lowland and low montane forests of western Washington, northwestern Oregon, and southwestern British Columbia. In British Columbia and Washington, it is uncommon to absent on the windward side of the coastal mountains where fire is rare. It also occurs locally in far southwestern Oregon (Klamath ecoregion) as small to large patches.