General information

Provisional. A provisional ecological site description has undergone quality control and quality assurance review. It contains a working state and transition model and enough information to identify the ecological site.

MLRA notes

Major Land Resource Area (MLRA): 002X–Willamette and Puget Sound Valleys

The Willamette and Puget Sound Valleys Major Land Resource Area (MLRA 2) is in western parts of Washington and Oregon. It occupies a forearc basin between the Coast Ranges and the Cascade Mountain volcanic arc. The northern part contains Pleistocene drift, outwash, and lacustrine and glaciomarine deposits associated with continental glaciers. The southern part contains Late Pleistocene deposits from glacial outburst floods (Missoula Floods).

Climate is mild and moist, and the growing season is long. Mean annual precipitation ranges from 20 to 60 inches, received mostly in fall, winter, and spring. Summers are dry. The soil temperature regime is mesic, and the soil moisture regimes are xeric and aquic.

This MLRA includes a variety of ecological sites. Many are characterized by forest vegetation but some were maintained as prairie, savanna, or oak woodland through cultural burning prior to Euro-American settlement. In the northern part of this MLRA Puget Sound has a moderating effect on temperature and humidity is generally higher. Douglas-fir (Pseudotsuga menziesii) is widespread throughout. Oregon white oak (Quercus garryana) is common on valley bottoms and surrounding slopes in the south. In the north its historic extent is more limited, occurring on warm aspects, with exposed and droughty conditions, and areas affected by rain shadowing from local ranges, particularly east and south of the Olympic Mountains. Big leaf maple (Acer macrophyllum) is common as a codominant or sub canopy tree across many sites. Pacific madrone grows in areas close to saltwater, or within drier forest sites with well-drained soils. Western hemlock (Tsuga heterophylla) and western red cedar (Thuja plicata) are common in wetter portions of the MLRA. Flood plains typically contain Brayshaw black cottonwood (Populus balsamifera ssp. Trichocarpa) and red alder (Alnus rubra). Oregon ash (Fraxinus latifolia) is typical of forested wetlands in the south.

Forestry, urban development, and cultivated agriculture are currently the most extensive land uses (USDA, Agriculture Handbook 296, 2022).

LRU notes

The Puget Sound Trough Lowlands Land Resource Unit (LRU) is bounded to the north by the Frasier River Valley at the international border with Canada and extends south to the Cowlitz River. To the west lie Puget Sound and the Strait of Juan De Fuca; to the east lie the foothills of the Cascade Range. The LRU is affected by the proximity of climate-moderating saltwater. Modest annual swings in temperature, winters that seldom experience freezing temperatures, adequate rainfall, and warm, dry summers support small-scale agriculture and forestry. This climate also supports the largest population and highest population density in the Northwest. Aside from isolated areas affected by local rain shadows and marine-influenced fog, the climate is consistent throughout the Puget Lowlands.

The LRU represents the furthest southern extent of repeated advances of continental glaciers in western Washington. Glacial drift is the predominant parent material. The LRU also includes intermittent areas of glacially modified, resistant bedrock and several alluvial systems. Volcanic ash is present but intermittent. Soil moisture varies considerably over short distances. This variability creates a mosaic of small plant communities. Soil drainage can be restricted by dense glaciomarine sediments or till. This restriction can create widespread areas of seasonal high water tables and ponding. In places, soils that developed in deep, unconsolidated, coarse-textured sandy drift or in bedrock-restricted colluvium have low available water capacity. South-facing areas near shorelines and minor outwash plains are typically some of the drier areas in the LRU. Precipitation increases with elevation and distance from Puget Sound.

Ecological site concept

In their reference state Puget Lowlands Peat Wetlands are characterized by soils with low pH, limited nutrient availability, many unusual plant species, and unique hydrologic dynamics including a high water table and water within the rooting zone during most of the year. They typically occur as distinct patches in depressions, around lakes and marshes, or less frequently on slopes. Near saltwater these organic soils can accumulate an abundance of sodium cations from oceanic precipitation.
This ecological site includes both poor fens and bogs with a pH generally below 5 (Kulzer et al. 2001). Fens are fed by a mix of surface water, ground water and rainfall. These are distinguished from bogs which are surrounded by a peripheral lag zone that is hydrologically disconnected from the bog center; thus, the bog center receives nearly all its moisture from direct rainfall. Bog profiles can be flat, raised (domed), or sloping. In Washington, however, most are flat and have only localized hummock development. The soils in this ecological site generally have a water table at or near the soil surface for much of the winter and spring. The water table is often at or within a few feet of the soil surface for the remainder of the year. The hydrologic disconnect of bogs allows the plants and upper layers of material to often become quite dry during long periods without rainfall, the sole source of saturation (Rocchio 2021). Rich fens with a pH generally above 5.5 are not included in this ecological site.
In the reference state, plants uniquely suited to these highly acidic soil conditions occupy these areas. The most common tree species are western hemlock (Tsuga heterophylla) and beach pine (Pinus contorta var. contorta), although western redcedar (Thuja plicata) and western white pine (Pinus monticola) may also occur. If present, trees usually occur at low densities and are often stunted. The shrub layer is typically composed of Labrador tea (Ledum groenlandicum), western bog laurel (Kalmia microphylla var. occidentalis), salal (Gaultheria shallon), bog blueberry (Vaccinium uliginosum) and bog cranberry (Vaccinium oxycoccos). Common herbaceous species include roundleaf sundew (Drosera rotundifolia), rusty cottongrass (Eriophorum chamissonis), white beaked sedge (Rhynchospora alba), beaked sedge (Carex utriculata), western skunk cabbage (Lysichiton americanus), and bracken fern (Pteridium aquilinum). The lag zone around bogs is often dominated by Oregon crabapple (Malus fusca), rose spirea (Spiraea douglasii), Sitka sedge (Carex aquatilis var. dives) and slough sedge (Carex obnupta). Other species typical of other types of wetlands may co-occur in fens and lags surrounding bogs. Lichens and mosses, especially Sphagnum species, are common throughout both bogs and fens (Riggs 1925, Kulzer et al. 2001, Rocchio 2021). Once peat is removed by mining or by an unusual event such as fire during a severe drought, peat wetlands may be impossible to restore or may take a very long time to rebuild the peat content that characterizes their soils.
This site can be compared to the Portland Basin Peat Wetlands site in LRU B, which is similar but has a higher summer temperature and lower amounts of summer precipitation. Drier conditions during the growing season are possible in LRU B, leading to a longer recovery between disturbances than in LRU A.

Associated sites

Similar sites

Table 1. Dominant plant species

Legacy ID

R002XA003WA