Chapter 24
Ecological Subregions of the United States

Back

Contents
Forward

Pacific Lowland Mixed Forest

One Section has been delineated in this Province:

The area of this Section, which is located in Washington and Oregon, is about 14,900 mi2 (38,600 km2).

Section 242A--Willamette Valley and Puget Trough

Geomorphology. To the south, there are primarily cyclic flood deposits. These were laid down during the Spokane flood events, forming the smooth floor of the Willamette basin. Below the flood veneer, fluvial channel deposits and overbank clays interfinger with fans from the foothills. Studies of geomorphic surfaces with age and relationships clearly defined are described throughout the Willamette Valley. To the north, Pleistocene glaciers have deposited and eroded morainal debris. Generally, the glaciation produced predominately ground moraine to the north and outwash plains to the south of Tacoma. South of the glacial limit, a zone of branching drainages and low divides is sculpted in soft rocks. Throughout the Section, isolated basalt-capped mesas and islands of bedrock occur. Elevation ranges from sea level to 2,000 ft (700 m). Lithology and Stratigraphy. The southern trough's basement is Eocene basalts and pyroclastics. To the north, sedimentary rocks are also found. These are overlain by up to 2,000 ft (610 m) of river-deposited sands, silt, and clay with basalt interbeds. Of these river deposits, the upper two to 100 ft (31 m) may be flood-deposited silts. To the north, the chaotic nature of glacial deposition and re-working is evident. Below the glacial features and exposed to the south of deposition, Miocene and pre-Miocene volcanics are overlain by Tertiary continental and marine deposits. In the far north, the San Juan Islands expose a Cenozoic mountain root complex.

Soil Taxa. Soils in this Section are characterized by a mesic temperature regime and xeric moisture regime. On the floor of the Willamette Valley, soils formed in the Willamette silts, which were deposited by the great Pleistocene Missoula floods, and in alluvium from Coast Range and Cascade Mountain drainages. Soil development, texture, and drainage are specific to geomorphic surfaces expressed in the valley. The youngest soils, on flood plains represented by the Ingram and Horseshoe surfaces, are well drained to excessively well drained, coarse textured, and have dark, base-rich surface horizons (Fluventic Haploxerolls and Cummulic Ultic Haploxerolls). On the Winkle surface are well drained and moderately well drained soils with clay-enriched subsoils and thick, dark, base-rich surface horizons (Pachic Ultic Argixerolls). The Champoeg and Senacal surfaces are represented by silty, somewhat poorly drained soils with medium base saturation (Aquultic Argixerolls). Poorly drained, fine textured soils with light-colored surface horizons are on the Calapooyia surface (Typic Albaqualfs and Typic Endoaqualfs).

On forested foothills and uplands, south of the terminus of continental glaciation, well weathered soils have developed on old erosional surfaces (Eola and Dolph surfaces in the Willamette Valley). These well developed soils have clay-enriched subsoils and dark, organic matter-rich topsoils that are low in base cations (Xeric Palehumults and Haplohumults, and Ultic Palexeralfs and Haploxeralfs). Soils on upland terraces, formed in early to mid-Pleistocene alluvium, have mostly base-rich dark topsoil and are well drained to poorly drained (Ultic Argixerolls, Mollic Haploxeralfs and Fragixeralfs, Aquic Haploxerolls, and Typic Fragiumbrepts).

In the Puget lowlands and foothills bounding the lowlands, soils have formed in and on drift deposited by continental glaciation. Soils are related to a complex mosaic of deposits resulting from glacial processes. Soils with a silica-cemented hardpan that impedes drainage and root penetration (Dystric Entic Durochrepts and Entic Durixerolls) occur on gravelly till deposits. Excessively drained, coarse textured soils with low water-holding capacity (Dystric Xerochrepts, Typic Haplorthods, and Dystric Xeropsamments) occur in sandy and gravelly outwash deposits. Fine textured, poorly drained soils (Typic Endoaquolls, Aquic Palexeralfs, and Aquic Xerochrepts) occur in silty and clayey lake and marine deposits. On the floor of depressions in the glacial drift, soils are poorly drained and have accumulations of organic matter (Typic Medihemists and Humaquepts).

Potential Natural Vegetation. According to K\"uchler, the Willamette Valley portion is a mosaic of two types, the cedar--hemlock--Douglas-fir and the Oregon oak woods forests. The Puget Trough portion is the cedar--hemlock--Douglas-fir forest. There is an area of the alder--ash forest along the Columbia River. Long-term human settlement has had pronounced influence on this area. More recent investigations suggest somewhat different interpretations of potential vegetation. The Willamette Valley portion is a mixture of Douglas-fir and White Oak series with local areas of Western Hemlock and Western Red Cedar series on more moist sites. A similar pattern occurs in the Puget Trough portion, but with a greater abundance of Western Hemlock and Western Red Cedar series and less amount of White Oak and Douglas-fir series. Riparian areas, with aquic conditions, include Cottonwood, Willow, Ash, and Alder series. Bigleaf maple occupies mixed sites. Prairies of Idaho fescue are on droughty, gravelly soils in the Puget Trough; in the Willamette Valley, grasslands of danthonia, bentgrass, orchard grass, needle grass, fescue, and prairie June grass are on the drier sites. Tufted hairgrass and shrub thickets are dominant in wetlands.

Fauna. Blacktail deer commonly inhabit the area from the urban-farmland interface to the conifer forested uplands. Elk are frequent along the forest-bottomland interface but not common. Red fox, tree fox, opossum, nutria, beaver, and striped skunk are common valley inhabitants. Red-tailed hawk, Cooper's hawk, sharp-shinned hawk, and kestrel are common avian predators during summer; the major valley bottoms serve as winter grounds for rough-legged, red-tailed and Swainson's hawks. Great blue heron, osprey, and kingfisher are aquatic-associated species, as are the mallard, wood duck, green winged teal, hooded merganser, and common merganser. Canada geese are common year-long residents. The camas pocket gopher, largest gopher in the Pacific Northwest, and the California ground squirrel, a recent invader, are common valley inhabitants. Band-tailed pigeon, once common to the valley and adjacent forested foothills, are now limited in their distribution and abundance. Lakes, reservoirs and streams support populations of rainbow trout and introduced crappy and largemouth bass. Anadromous fish runs of chinook and silver (Coho) salmon represent diminished runs and significantly reduced distribution. Shad and smelt are unique species that also occur in major river systems, but numbers are greatly reduced. More than 7,000 species of arthropods, a variety of amphibians and reptiles, and slugs occur.

Climate. Precipitation ranges from 25 to 60 in (630 to 1,620 mm), occurring mostly as rain during October through June. Mean annual temperature is 47 to 57 oF (9 to 13 oC). The growing season lasts 140 to 240 days.

Surface Water Characteristics. There are large riverine systems with merging streams. The Puget Trough portion is joined by estuaries and ocean sound. Wetlands are abundant. Both natural and man-made lakes are scattered throughout the Section.

Disturbance Regimes. Fires were commonly set by Native Americans and trappers, hunters, and settlers. Floods from intense winter storms occurred at frequent intervals prior to construction of dams.

Land Use. The Willamette Valley is a dominantly intensive agricultural area. More than 100 kinds of crops are grown--from grass seed and row crops to berries and nuts. Grazing by sheep and cattle is common. Private forestry and intensive industrial forestry occur on the footslopes of the valley perimeter. The Puget Trough is managed largely for intensive industrial and private forestry. Grazing in pasture, and crop land is dispersed throughout and is in small parcels. Several large metropolitan areas and industrial complexes occur. Small rural communities also occur throughout the Section.

Cultural Ecology. The earliest human occupation of the Willamette Valley and Puget Trough Section dates to about 9,000 years ago. Early peoples depended heavily on the hunting of deer and elk, and led a highly mobile existence. By 6,000 years ago, a more diverse economic strategy had emerged. In the south, this included an emphasis on the collection and processing of camas, acorns, and hazelnuts, in addition to hunting. In the north, there was a greater dependence on riverine salmon fishing and the gathering of intertidal shellfish. By 4,000 years ago an increase in sedentism was linked to greater reliance on stored food resources, and resulted in the establishment of villages on high river terraces, valley margins, and the shorelines of Puget Sound. Prairies in the Willamette Valley and around Puget Sound were fire-maintained to enhance productivity of various plant resources. As complex geomorphic changes in the Puget Sound basin affected the distribution of food resources, subsistence activities shifted to take advantage of more microenvironments. In the Willamette Valley, the localized abundance of a wide variety of food resources on the valley floor meant the opposite. greater sedentism and smaller home ranges. As the adaptive characteristics of classic northwest coast culture developed about 2,000 years ago on Puget Sound, social and religious systems involving the ownership of salmon and other resources arose, persisting into the historic period. British and American settlement began following the establishment of fur trade posts in the 1820's and 1830's. Open prairies were appropriated for agricultural use. Extensive clearing and the cultivation of non-native crops was central to the economy of 19th century communities. In the Puget Sound area, intensive timber harvest had a dramatic effect on ecological structure and hydrology. The historic development of towns and cities, river diking and damming, harbor construction, and roads and railroads all caused significant local changes to the environment.

Compiled by Pacific Northwest Region.

Top

Back

Contents
Forward