Pacific Northwest Oak Woodland

This ecological system is limited to the southern portions of the North Pacific region. It occurs primarily in the Puget Trough and Willamette Valley but trickles down into the Klamath ecoregion and into California. This system is associated with dry, predominantly low-elevation sites and/or sites that experienced frequent presettlement fires. In the Willamette Valley, soils are mesic yet well-drained, and the type is clearly large patch in nature. In the Puget Lowland and Georgia Basin, this system is primarily found on dry sites, typically either shallow bedrock soils or deep gravelly glacial outwash soils. It occurs on various soils in the interior valleys of the Klamath Mountains, and on shallow soils of "bald hill" toward the coast. Even where more environmentally limited, the system is strongly associated with a pre-European settlement, low-severity fire regime. Succession in the absence of fire tends to favor increased shrub dominance in the understory, increased tree density, and increased importance of conifers, with the end result being conversion to a conifer forest. The vegetation ranges from savanna and woodland to forest dominated by deciduous broadleaf trees, mostly Quercus garryana. Codominance by the evergreen conifer Pseudotsuga menziesii is common, and Pinus ponderosa is important in some stands. In the south, common associates also include Quercus kelloggii and Arbutus menziesii. This system merges into Mediterranean California Lower Montane Black Oak-Conifer Forest and Woodland (CES206.923) on sites that support more conifer cover, and into Mediterranean California Mixed Oak Woodland (CES206.909) in the southern portion of its distribution. This system is borderline between small patch and large patch in its dynamics.

This type is associated with dry, predominantly low-elevation sites and/or sites that experienced frequent presettlement fires. In the Willamette Valley, soils are mesic yet well-drained, and the type is clearly large patch in nature. In the Puget Lowland and Georgia Basin, this system is primarily found on dry sites, typically either on shallow bedrock soils or deep gravelly glacial outwash soils. It occurs on various soils in the interior valleys of the Klamath Mountains, and on shallow soils of "bald hills" toward the coast.

Even where more environmentally limited, the system is strongly associated with a pre-European settlement, low-severity fire regime. Succession in the absence of fire tends to favor increased shrub dominance in the understory, increased tree density, and increased importance of conifers, with the end result being conversion to a conifer forest. Landfire (2007a) model: Fire Regime I, primarily short-interval (e.g., <10 years) surface fires. Surface fires every 3-10 years maintained an open savanna-like structure. Fires can be mixed-severity especially when closed-canopy conditions or additional species such as conifers and shrubs are present. Native American burning was a significant factor in fire frequency of this type, but fire frequency may have decreased significantly with a little distance from native settlements and valley bottoms. Landfire VDDT models: #R OWOA Oregon White Oak applies to southern occurrences. Dissemination of acorns by squirrels and chipmunks is thought to be the most important long-distance dispersal mechanism for the oaks (WNHP 2011).

Conversion of this type has commonly come from urbanization or conversion to agriculture, but also conversion to conifer-dominated woodlands or forest due to lack of fire. Ongoing threats include residential development, increase and spread of exotic species, and fire suppression effects. With the cessation of regular burning 100-130 years ago, many oak woodlands have been invaded by a greater density and cover of oak and conifer trees. Fire suppression has also increased shrub cover in many oak woodlands. Removal of Quercus garryana trees for firewood, fence posts, and other lumber products has and continues to occur in some area. Selective logging of Pseudotsuga menziesii in oak stands can prevent long-term loss of Quercus garryana dominance. Oaks typically resprout after logging. Moderate to heavy grazing can lead to an increase in non-native species, many of which are now abundant. Cytisus scoparius is invasive and persistent in many oak woodlands. Prunus avium and Crataegus monogyna have invaded and now dominate the subcanopy in Willamette Valley oak woodlands. Poa pratensis is a major non-native dominant in the understory.

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%), and 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. This change will result in less winter snow accumulation, higher winter streamflows, earlier spring snowmelt, earlier peak spring streamflow and lower summer streamflows in rivers that depend on snowmelt (as do most rivers in the Pacific Northwest) (Littell et al. 2009). Potential climate change effects could include: reduction in freshwater inflows through the further reduction in summer flows (Littell et al. 2009); but models also predict increases in extreme high precipitation over the next half-century, particularly around Puget Sound (Littell et al. 2009), which may provide freshwater pulses that are intermittent, less predictable; drop in groundwater table; and 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).

This system occurs primarily in the Puget Trough and Willamette Valley and extends southward at low elevations in the Klamath Mountains on both sides of the Oregon/California stateline.