Willamette Valley forests

Content Cover Image

Willamette Valley scene, Oregon. Source: Maureen Raad

The Willamette Valley forests ecoregion is an element of the Temperate Broadleaf  and Mixed forests biome in the Pacific Northwest region of the USA. Once a prairie supporting oak stands and groves of Douglas-fir and other trees, cultivation and development have destroyed nearly all of the natural habitat in the Willamette Valley. Just one-tenth of one percent of the valley’s native grasslands and oak savannas remains.

caption Source: World Wildlife Fund Fire shaped the Willamette Valley, as it did most of the northwest grassland and savanna communities. Possibly dating back to the Pleistocene era, periodic burning by Native Americans created ideal conditions for native perennial grasses. More recent fire suppression activities–with the concurrent spread of agriculture and development–have contributed enormously to the destruction of the natural habitats of the Willamette Valley. Without regular fires, forest is gradually replacing most of the savanna in the valley.

The Willamette Valley has nearly level to gently sloping floodplains bordered by dissected high terraces and hills. The climate is generally mild throughout the year, with moderate rainfall reaching its maximum in winter. Prior to cultivation, the valley had abundant swamp or bog communities in addition to the grasslands and oak savannas.

Biological distinctiveness

caption Willamette Valley, Oregon, USA. Source: Maureen Raad

The Willamette Valley provides the only habitat for Bradshaw’s desert parsley (Lomatium bradshawii), a yellow-flowered member of the parsley family. The valley is also the sole wintering area of the Dusky Canada goose (Branta canadensis occidentalis). There are 268 vertebrate taxa identified in the ecoregion.


The following salamander family taxa are the only ones present in the Willamette Valley forests: the Near Threatened Clouded salamander (Aneides ferreus); Northwestern salamander (Ambystoma gracile); Rough-skinned newt (Taricha granulosa); and Pacific giant salamander (Dicamptodon tenebrosus). There is only one  anuranAn amphibian that has limbs but no tail (includes all frogs and toads) species occurring in the ecoregion: Pacific chorus frog (Pseudacris regilla).


There are only a few reptilian taxa within the ecoregion, namely: Western fence lizard (Sceloporus occidentalis); Western pond turtle (Emys marmorata); Western Rattlesnake (Crotalus viridis); Common garter snake (Thanophis sirtalis); Northwestern garter snake (Thamnophis ordinoides); Rubber boa (Charina bottae); and Sharp-tailed snake (Contia tenuis).


Many avifauna species are found within the Willamette Valley forests, representative taxa being: the Near Threatened Olive-sided flycatcher (Contopus cooperi); Prairie falcon (Falco mexicanus); Macgillivray's Warbler (Oporornis tolmiei); and Pine siskin (Carduelis pinus).


There are a considerable number of mammalian species present in this ecoregion, a sample of which follows: The ecoregion endemic Gray-tailed vole (Microtus canicaudus); Creeping vole (Microtus oregoni); Long-legged myotis (Myotis volans); Mountain lion (Puma concolor); and the Near Threatened Red tree vole (Arborimus longicaudus).

Conservation status

Habitat loss

Less than one percent of the Willamette Valley remains as intactThe condition of an ecological habitat being an undisturbed or natural environment habitat due to historic    conversion to agriculture, urbanization and overly aggressive modern fire suppression. Practically no prairie remains, and the savanna is converting to forest. Most of the riparian areas have been lost to agriculture or urbanization; however, some remain intact, because their propensity for flooding renders them less desirable for agriculture or development.

Remaining blocks of intact habitat

The largest blocks of remaining intactThe condition of an ecological habitat being an undisturbed or natural environment habitat are no greater than 35 square kilometers (km2) and management of these areas generally focuses on waterfowl protection. The existing private reserves are all smaller than 0.15 km2.

Degree of fragmentation

caption Bradshaw's lomatium (Lomatium bradshawii), Willamette Valley, Oregon, USA. Source: Ed Alverson

Remaining patches of habitat in the Willamette Valley are quite small, with effectively no biological connectivity in most areas and little core habitat due to edge effects. The individual fragments and clusters that remain are highly isolated, and the intervening suburban and agricultural landscape precludes dispersal for most taxa.

Degree of protection

Present management of the protected areas in the Willamette Valley chiefly seeks to provide waterfowl for hunters, not to maintain natural habitats or enhance natural environment values. Area managers do not allow natural disturbance events to proceed nor do they seek to mimic those events through interventions, and much of the use of these areas may degrade the quality of remaining natural communities.

Ecological threats

So little natural habitat remains in the Willamette Valley that there are few conversion threats to the ecoregion. Degradation of the remaining fragments continues to be an issue, and there continue to be moderate levels of wildlife exploitation.

Suite of priority activities to enhance biodiversity

  • Restoration activities should focus on riparian areas to regenerate gallery forests and to connect corridors to the Cascade Range and the Coast Range foothills.
  • Conservation efforts should promote small prairie/savanna areas, but these will likely remain fragmented from the overall linked conservation strategy cited above.

Conservation partners

  • Association of Forest Service Employees for Environmental Ethics
  • Northwest Ecosystem Alliance
  • National Wildlife Federation - Western Division
  • Pacific Rivers Council
  • Audubon Society

Relationship to other classification schemes

Bailey combines the Willamette Valley with the Puget Sound Valley to form the Pacific Lowland Mixed Forest Province. The present treatment of the Willamette Valley ecoregion follows Omernik in dividing this region into two separate regions and reclassifying the Willamette as a grassland/savanna. This more accurately reflects the communities present in the valley, and creates smaller, more manageable ecoregions. Conservation strategies will differ for the Willamette and Puget Sound Valleys, and would be unwieldy if the two were managed as a single large unit. The Willamette Valley forests ecoregion is designated by the ecocode NA0417 by the World Wildlife Fund, and the ecoregion is classified within the Nearctic Realm.


  • Eugene S. Hunn. 1990. Nch'i-Wana, The Big River. University of Washington Press. ISBN 0-295-97119-3.
  • Barbara Perry Lawton. 2007. Parsleys, Fennels, and Queen Anne's Lace. Portland: Timber Press
  • Herbert O. Lang & George Henry Himes. 2013. History of the Willamette Valley, being a description of the valley and its resources, with an account of its discovery and settlement by white men, and its subsequent history
  • Harry Nehls, Tom Aversa. 2004. Birds of the Willamette Valley Region.


Disclaimer: This article contains some information that was originally published by the World Wildlife Fund. Topic editors and authors for the Encyclopedia of Earth have edited its content and added new information. The use of information from the World Wildlife Fund should not be construed as support for or endorsement by that organization for any new information added by EoE personnel, or for any editing of the original content.






Fund, W. (2015). Willamette Valley forests. Retrieved from


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