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 or droughty conditions and areas affected by rain shadowing from local ranges, particularly 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.

Classification relationships

This community is described as Estuarine Association ED02 in:
MacKenzie, W.H. and J.R. Moran. 2004. Wetlands of British Columbia: a guide to identification. Res. Br., B.C. Min. For., Victoria, B.C. Land Manage. Handbook. No. 52.

Ecological site concept

This ecological site occurs in nearshore low-lying areas and marine estuaries where inundation by both salt water and fresh water occurs. Tides often greatly influence this site, allowing for regular periods of inundation as marine and riverine waters interact. This site is in areas that are cooler than the rest of the immediate Puget Lowlands area because the site is exposed to prevailing winds from across marine waters. Because of these influences, this site is typically dominated by graminoids, forbs and low shrubs adapted to brackish conditions. With drainage or diking to control tidal and freshwater influences, the site can be developed for agricultural use.
Typical native plant species include Tufted Hairgrass (Deschampsia caespitosa), Meadow Barley (Hordeum brachyantherum), Red Fescue (Festuca rubra), Oregon Gumweed (Grindellia stricta), Douglas Aster (Symphyotrichum subspicatum), Fat Hen (Atriplex patula), Pacific Silverweed (Potentilla pacifica), Saltgrass (Distichlis spicata), Seaside Arrowgrass (Triglochin maritimum), Lyngbye’s Sedge (Carex lyngbyei), Pickleweed (Salicornia virginica), Mountain Rush (Juncus arcticus ssp. Littoralis), Seashore Plantain (Plantago macrocarpa), and Goose Tongue (Plantago maritima).

Associated sites

Similar sites

Table 1. Dominant plant species

Legacy ID

R002XA002WA