Willamette Valley forests
Willamette Valley forests are situated between the Oregon Coast Range and the Cascade Mountains in northwest Oregon, featuring a diverse landscape that includes low basalt hills and the meandering Willamette River. This valley, characterized by a mild climate with cool, wet winters and warm, dry summers, has historically been home to a rich tapestry of ecosystems, including oak woodlands, grasslands, and Douglas fir stands. Over the last 150 years, human activities have significantly altered these ecosystems, leading to the loss of much of the original forest cover due to agriculture and urban development.
The region's biodiversity is supported by a variety of flora and fauna, including many native tree species and wildlife such as birds, mammals, amphibians, and fish. Indigenous tribes historically managed the landscape through controlled burns to promote the growth of essential plants for food and medicine. Despite the ongoing pressures from habitat loss, invasive species, and climate change, current conservation efforts aim to restore and protect the remaining habitats of the Willamette Valley, highlighting the importance of these forests for ecological health and biodiversity. The area's complex history and ongoing environmental challenges make it a significant focus for both conservationists and researchers.
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Subject Terms
Willamette Valley forests
Category: Forest Biomes.
Geographic Location: North America.
Summary: The Willamette Valley has changed dramatically from its historical landscape, once dominated by oak woodlands and prairies with occasional coniferous stands, because of human disturbance.
The Willamette Valley is bounded on the west by the Oregon Coast Range and on the east by the Cascade Mountains. It spans a level alluvial plain with scattered groups of low basalt hills. The valley extends through northwest Oregon, and through a very small area in southern Washington state. The Willamette River, a major tributary of the Columbia River, meanders through this valley, flowing mainly south to north with historically great fluctuations in its flow, ranging from low summer flows to massive winter and spring floods. The river’s bottomlands extend across low-lying floodplains, depositing rich sediment. The entire valley ecoregion stretches across approximately 5,800 square miles (14,900 square kilometers).
The climate of this region is relatively mild year-round, characterized by cool, wet winters and warm, dry summers. Temperatures seldom rise above 90 degrees F (32 degrees C), and even less frequently do temperatures drop below 0 degrees F (minus 18 degrees C). Most rainfall occurs in the late autumn, winter, and early spring, when temperatures are the coldest. The valley gets relatively little snow—5 to 10 inches (13 to 25 centimeters) per year.
This ecoregion was once dominated by a vast network of oak woodlands and grasslands interspersed with Douglas fir stands and wetlands, but human disturbance has dramatically changed the landscape over the past 150 years. Fire and seasonal flooding shaped the original characteristics of this landscape. The fire regimes, historically developed by seasonal wildfires and those set by Native Americans, were crucial for maintaining the grassland and savanna communities that dominated the region.
Oak prairies are able to endure on fire-prone landscapes where other forest types are unable to become established. The indigenous tribes of the Willamette Valley set fire to a large amount of land every year to regenerate the prairie plants on which they depended for food and medicine, and to generally clear the brush for hunting and other purposes. Some of the most important plants that were sought after were camas, tarweed, Oregon white oak acorns, and wapato. Other woodlands were purposely left unburned to provide areas where game would concentrate, so that animals could be hunted more successfully.
Biodiversity
Oregon white oak (Quercus garryana) occurs as scattered trees in savanna communities, and in pure or mixed-species closed-canopy woodlands. Pacific madrone (Arbutus menziesii), western serviceberry (Amelanchier alnifolia), ponderosa pine (Pinus ponderosa), and Douglas fir (Pseudotsuga menziesii) are among some of the most common tree species that co-occur with Oregon white oak. Poison oak (Rhus diversiloba) and common snowberry (Symphoricarpos albus) are some characteristic shrubs of Oregon white oak habitats.
In the absence of fire, the remaining prairie, oak woodland, and savanna have quickly filled with the fast-growing conifer Douglas fir, Oregon’s state tree. Douglas fir trees are large, sun-loving conifers capable of living hundreds of years and reaching more than 200 feet (61 meters) tall and 10 feet (3 meters) in diameter. Douglas fir forests are the most extensive in Oregon and the most crucial for timber production. Douglas fir often forms immense, nearly pure stands. Western hemlock is the climax species for this conifer-dominated community in much of this region, while other associates can also be found, including western red cedar, noble fir, bigleaf maple, and red alder.
Douglas fir is used for a wide variety of building products, and millions of Douglas fir Christmas trees are exported from Oregon annually. Older Douglas firs provide an important habitat for an array of wildlife, including nesting birds and small mammals. Dying Douglas fir trees and snags are key sources of cavities for woodpeckers and other birds.
Despite all the changes to, and pressures on, the Willamette Valley’s forest lands, they continue to play a vital role in wildlife health in the region. The now more common mixed stands of deciduous and conifer forest, and the watershed that sustains them, provide habitat for an array of fauna, including 18 species of amphibians, such as the Pacific giant salamander, and at least eight reptile species. Fish in the Willamette basin include 31 native species, among them cutthroat, bull, and rainbow trout; several species of salmon; and sturgeon.
Beaver and river otter are among 69 mammal species living in the watershed. Other mammal species include: elk, coyote, bobcat, mule deer, weasel, mink, muskrat, red fox, black bear; many small mammals such as voles, shrews, squirrels; and many species of bats. There are also 154 bird species that have been identified here, including the great horned owl, northern saw-whet owl, northern pygmy owl, various hawks, woodpeckers, grosbeaks, terns, quail, ducks, and hummingbirds. Many conservation efforts now focus on restoring, protecting, and maintaining the region’s almost-lost oak woodland and grassland habitats.
Human Impact
The valley’s fertile soils, mild climate, and gently sloping floodplains nestled within volcanic peaks and wet temperate forests made this area highly desirable to European settlers. The rich soils and plentiful rainfall made the Willamette region the most crucial agricultural area in the state. This is also the fastest-growing region to be developed, now housing the majority of Oregon’s major cities and population.
Over the past 170 years, a significant change for the Willamette River Valley has been the loss of its floodplain forests, which in 1850 covered an estimated 89 percent of a 400-foot (120-meter) band along each river bank. By 1990, 63 percent of this forested area had been converted to farmland or to urban and suburban development.
By 2021, 96 percent of land within the Willamette Valley was in private ownership, with most used for agriculture. The landscape is a mosaic of farms, working forest lands, suburbs, and cities, with only small fragmented habitat of the types that were once present in continuous abundance. Less than 1 percent of the historically dominant oak savannas and prairies remain. Remnant pockets of historic oak woodlands and savannas are often invaded by conifers, Oregon ash, and an array of nonnative species.
The Willamette River itself has been disconnected from its floodplain. Dams now control the river’s flow, and vast stretches of the riverside have been channelized. Invasive plant species are currently considered to be one of the major causes of species becoming threatened and endangered, second only to habitat loss. Population growth, land-use conversion, invasive-species infestations, pollution, and effects of global warming are all likely to continue to increase stress on all the habitats throughout the valley.
Bibliography
Campbell, Bruce H. Restoring Rare Native Habitats in the Willamette Valley. West Linn, OR: Defenders of Wildlife, 2004.
McNab, W. Henry and Peter E. Avers. “Willamette Valley and Puget Trough.” In U.S. Forest Service, eds., Ecological Subregions of the United States. Washington, DC: U.S. Department of Agriculture, 1996.
Robillard, Tracy. "Public/Private Partnerships Restore Willamette Valley Wildlife Habitat." Natural Resources Conservation Service Oregon, March 2021, nrcs.usda.gov/wps/portal/nrcs/detail/or/newsroom/stories/?cid=nrcseprd1555463. Accessed 17 Aug. 2022.
Vesely, David G. and Daniel K. Rosenberg. Wildlife Conservation in the Willamette Valley’s Remnant Prairies and Oak Habitats: A Research Synthesis. Corvallis: Oregon Wildlife Institute, 2010.