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 1 percent of the valley’s native grasslands and oak savannas remains (Noss and Peters 1995).
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.
The Willamette Valley provides the only habitat for Bradshaw’s lomatium (Lomatium bradshawii), a yellow-flowered member of the parsley family (Noss and Peters 1995). The valley is also the sole wintering area of the dusky Canada goose (Branta canadensis occidentalis) (Bailey 1995).
Less than 1 percent of the Willamette Valley remains as intact habitat due to conversion to agriculture, urbanization, and fire suppression. Practically no prairie remains and the savanna is converting to forest. Most of the riparian areas have been lost, though some remain intact because their propensity for flooding makes them unsuitable for agriculture or development.
Remaining Blocks of Intact Habitat
The largest blocks of remaining habitat are no greater than 35 km2 and management of these areas focuses on waterfowl. The existing private reserves are all smaller than 0.15 km2.
Degree of Fragmentation
Remaining patches of habitat in the Willamette Valley are tiny, with effectively no 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
Management of the protected areas in the Willamette Valley seeks to provide waterfowl for hunters, not to maintain natural habitats or enhance natural 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.
Types and Severity of 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 a problem, and there are still moderate levels of wildlife exploitation.
Suite of Priority Activities to Enhance Biodiversity Conservation
•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.
•Association of Forest Service Employees for Environmental Ethics
•Northwest Ecosystem Alliance
•National Wildlife Federation - Western Division
•Pacific Rivers Council
Relationship to other classification schemes
Bailey combines the Willamette Valley with the Puget Sound Valley to form the Pacific Lowland Mixed Forest Province. We follow 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.
Prepared by: R. Noss, J. Strittholt, G. Orians, J. Adams