Pacific Northwest Rainforest Cedar-Hemlock Forest

These forests occupy the outer coastal portions of British Columbia, southeastern Alaska, and northwestern Washington. Their center of distribution is the central coast of British Columbia, as Thuja plicata approaches its northernmost limit in the southern half of southeastern Alaska. These forests occur mainly on islands but also fringe the mainland. They are generally less than 25 km from saltwater; elevation ranges from 0 to 600 m, and below 245 m in Alaska (above 200 m, Callitropsis nootkatensis (= Chamaecyparis nootkatensis) replaces Thuja plicata). The climate is hypermaritime, with cool summers, very wet winters, abundant fog, and without a major snowpack. Fire is absent from this system in Alaska and rare throughout the rest of the range. These forests are influenced by gap disturbance processes and intense windstorms and not much by fire. The terrain is mostly gentle to rolling, of low topographic relief, and often rocky. Soils typically have a distinct humus layer overlying mineral horizons or bedrock; where the system is best developed in central British Columbia, the humus layers are very thick (mean 17-35 cm). Soils are often imperfectly drained, but this is not a wetland system. Thuja plicata and Tsuga heterophylla are the dominant tree species throughout, and Callitropsis nootkatensis joins them from northern Vancouver Island north. Canopy cover of trees is typically over 60%. Pinus contorta and Tsuga mertensiana can be present in some locations in the central and northern portion of the range. Abies amabilis occurs in British Columbia and northern Washington stands but is not typically found in southeastern Alaska. In Washington, nearly pure stands of Tsuga heterophylla are common and seem to be associated with microsites most exposed to intense windstorms. A shrub layer of Gaultheria shallon, Vaccinium ovalifolium, and Menziesia ferruginea is usually well-developed. The fern Blechnum spicant in great abundance is typical of hypermaritime conditions. Oxalis oregana (absent in Alaska) is important in the understory of moist sites in Washington. Polystichum munitum occurs at the northern end of its range in southeastern Alaska on well-drained sites. The abundance of Thuja plicata in relation to other conifers is one of the diagnostic characters of these forests; the other is the low abundance of Pseudotsuga menziesii (absent in Alaska) and Picea sitchensis. Where these forests are best developed, they occur in a mosaic with forested wetlands, bogs, and Sitka spruce forests (the latter in riparian areas and on steep, more productive soils).

These forests occur mainly on islands but also fringe the mainland and coastal fjords. They are generally less than 25 km from saltwater; elevation ranges from 0 to 600 m, and below 245 m in Alaska (above 200 m, Callitropsis nootkatensis replaces Thuja plicata). Climate is characterized by moist mild air from the Pacific. Frequent winter storms produce abundant precipitation as they encounter rising mountain slopes. In summer, large high-pressure areas off the coast produce prolonged spells of fine weather (Taylor 1997). The climate is classified as hypermaritime, with cool summers, very wet winters, abundant fog, and without a major snowpack (Meidinger and Pojar 1991). Rainfall is relatively high for the region at 254-380 cm (100-150 inches) rain annually, rarely as snow (Landfire 2007a). The terrain is mostly gentle to rolling, of low topographic relief, and often rocky. This type generally occurs on relatively old, acidic, humic soils with a distinct humus layer overlying mineral horizons or bedrock; where the system is best developed in central British Columbia, the humus layers are very thick (mean 17-35 cm) (Banner et al. 1993, Green and Klinka 1994, Steen and Coupe 1997). Soils are often imperfectly drained, but this is not a wetland system. Where these forests are best developed, they occur in a mosaic with forested wetlands, bogs, and Sitka spruce forests (the latter in riparian areas and on steep, more productive soils). This system represents the upper end of the productivity gradient within the Cedar-Hemlock Ecological Zone and the lower end of the Western Hemlock Ecological Zone (DeMeo et al. 1992).

Fire is absent from this system in Alaska and rare throughout the rest of the range, e.g., British Columbia's north coast (Banner et al. 1993, Landfire 2007a). These forests are primarily influenced by gap disturbance processes (gaps created by the death of individual trees, or small patches due to disease, insect damage and treefall following mortality). On the most exposed areas of the coastline, occasional hurricane force winds and severe storms result in major windthrow events. Less severe winds may cause breakage or early blowdown of diseased trees. The ground surface often has pit-and-mound microtopography that is formed by windthrow events. Storms are generally from the southwest and sweep across the low country of southwestern Washington, and strike either the front range of the Cascades or the southwest face of the Olympics. Wind damage tends to repeat at certain locations either due to direct exposure or due to the funneling of winds around topographic features. Wind damage tends to be more significant on the coast than further inland. Studies by USFS in southeastern Alaska show lots of broken boles as cause of tree mortality (Hennon 2008).

Conversion of this type has commonly come from clearcutting, selective logging and urban development (WNHP 2011). Timber harvest, tree plantations and introduced species and diseases have impacted forest structure, composition, landscape patch diversity, and tree regeneration. For essentially all but Tsuga heterophylla, the understory shrub and herb layers are severely degraded on the Queen Charlotte Islands by the browsing of coast blacktail deer (Odocoileus hemionus) introduced in the early 1900s. Other stressors limited in scope are development, road building and pipelines, hydroelectric operations, and tree plantations. Development has fragmented the landscape changing connectivity of this small-patch system particularly in lowlands Washington, while limited recreational development has more of an impact in British Columbia. 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). Restocking and plantation forestry (more in Washington than British Columbia) have 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 reflects recent disturbance and forest succession, whereas downed wood amounts more 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. In British Columbia's central and north coast, projections into the 2050s are 2.1° to 2.3°C annual increase that is 7-12% relative to 1961-1990 annual temperatures (Werner 2011). 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 and as much as 55% decline in coastal mountain snowpack in British Columbia (Littell et al. 2009, Rodenhuis et al. 2009). More intense wind storms are projected for Haida Gwaii, British Columbia North Coast and Alaska Panhandle (Haughian et al. 2012). 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).

In the southern extent of the range, a drier overall climate may drive this ecosystem to a drier Douglas-fir-dominated type with the loss of western red-cedar, as this species is limited to humid climate, and in subhumid regions with relatively dry growing seasons, although it can occur much farther inland than other coastal conifer species, so coastal stands may be able to tolerate warmer and drier climates (Minore 1990). Stands may also experience the loss of western hemlock, as this species is limited to humid climate, and in subhumid regions with relatively dry growing seasons, in the southern part of its distribution it is currently 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 for the Puget Sound (Littell et al. 2009) and British Columbia (Spittlehouse 2008, Rodenhuis et al. 2009). The frequency of intense windstorms will increase from the more common light storms historically occurring along British Columbia's west coast. Increased wind speeds are anticipated for the coast and coastal mountains of British Columbia, varying by locale from slight 2% increase to up to 14% increase (Haughian et al. 2012). 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 observed 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). Affect on coastal fog is not addressed in the Washington Climate Change Impacts Assessment (Littell et al. 2009). 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).

In the southern part of the range, an 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, Haughian et al. 2012), and this may certainly occur in neighboring drier ecosystems on ridge crests, upper southern exposures and on shallow soils (Dorner and Wong 2003) which could affect this moister system as well on a landscape scale. An important factor in changes in the coastal forests will be the frequency and intensity of fire. Fires will likely increase, especially with warmer drier summers. Under such conditions Douglas-fir could expand rapidly (Hebda 1997). Preliminary studies of coastal sites on south Vancouver Island reveal much more fire activity in the early Holocene warm, dry interval than currently (Hebda 1997). In addition, current disturbance of the substrate and opening of the canopy from recent logging practices may have the same result as increased fire frequency (Hebda 1997).

This system is found in the outer coastal portions of British Columbia and southern southeast Alaska, as well as northwestern Washington.