Willamette Valley (ecoregion)

This article is about the ecoregion designated by the EPA. For the river valley it contains, see Willamette Valley.

Willamette Valley
Wetlands near Eugene, Oregon
Willamette Valley ecoregion (3)
Ecology
Ecozone	Nearctic
Biome	Temperate broadleaf and mixed forests
Borders	Cascades, Coast Range and Klamath Mountains
Bird species	177^[1]
Mammal species	77^[1]
Geography
Area	14,900 km² (5,800 sq mi)
Country	United States
States	Washington and Oregon
Coordinates	44°48′N 123°12′W / 44.8°N 123.2°W / 44.8; -123.2Coordinates: 44°48′N 123°12′W / 44.8°N 123.2°W / 44.8; -123.2
Conservation
Habitat loss	65.679%^[1]
Protected	4.07%^[1]

The Willamette Valley ecoregion is a Level III ecoregion designated by the United States Environmental Protection Agency in the U.S. states of Oregon and Washington. Slightly larger than the Willamette Valley for which it is named, the ecoregion contains fluvial terraces and floodplains of the Willamette River system, scattered hills, buttes, and adjacent foothills. It is distinguished from the neighboring Coast Range, Cascades, and Klamath Mountains ecoregions by lower precipitation, lower elevation, less relief, and a different mosaic of vegetation. Mean annual rainfall is 37 to 60 inches (96 to 152 cm), and summers are generally dry. Historically, the region was covered by rolling prairies, oak savanna, coniferous forests, extensive wetlands, and deciduous riparian forests. Today, it contains the bulk of Oregon’s population, industry, commerce, and agriculture. Productive soils and a temperate climate make it one of the most important agricultural areas in Oregon.^[2]^[3]

Setting

The ecoregion covers an area of 14,900 square kilometers (5,800 sq mi),^[4] lying mostly in Oregon, with a small portion lying across the Columbia River in southern Washington. The ecoregion lies in the Willamette Valley, which runs from south to north between the Oregon Coast Range to the west and the Cascades Range to the east. The ecoregion is drained mostly by the Willamette River and its tributaries, which flows into the Columbia River straddled by Portland, Oregon.

Landscape and threats

Grove of Garry Oaks near Hillsboro

Fall colors in Wilsonville

Historically, the Willamette Valley forests were mostly an oak savanna, tall grasslands with scattered Garry oaks (Quercus garryana), and groves of coast Douglas-fir (Pseudotsuga menziesii subsp. menziesii). The river floodplains contained extensive wetlands, stands of willow, alder, and cottonwood, and gallery forests.

This landscape was maintained by the Native American inhabitants of the valley who set frequent fires which encouraged the open grasslands and killed young trees. The American settlers of the region, since the 19th century, suppressed fires and converted much of the valley to agriculture, which has caused much of the former grassland and savanna to revert to closed-canopy forest.

Less than one-tenth of one percent of the original savanna vegetation remains. The remaining enclaves include a section of Garry oak savanna preserved at Mount Pisgah Arboretum in Eugene.

Level IV ecoregions

Level IV ecoregions in the Willamette Valley. (Full map).

The Willamette Valley ecoregion is made up of four Level IV ecoregions:

Portland/Vancouver Basin (3a)

The Portland/Vancouver Basin ecoregion (named for the cities of Portland and Vancouver) is a geological depression at the base of the Portland Hills fault-block. It contains the confluence of the Willamette and Columbia Rivers and is composed of deltaic sands and gravels deposited by Pleistocene floods, notably the Missoula Floods. Elevation varies from 0 to 300 feet (0 to 90 m), with buttes as high as 650 feet (200 m). Historically, the basin was characterized by Garry oak groves and Douglas-fir forests on the uplands; black cottonwood groves on riverbanks and islands; Oregon ash, red alder, and western redcedar in riparian areas; and prairie openings maintained by Native American burning, with camas, sedges, tufted hairgrass, fescue, and California oatgrass. Numerous wetlands, oxbow lakes, and ponds can still be found, but today the region is dominated by urban and suburban development, pastures, cropland, and tree farms. The climate is usually marine-influenced, but easterly winds from the Columbia River Gorge periodically bring continental temperature extremes. The region covers 305 square miles (790 km²) in Washington and 269 square miles (697 km²) in Oregon, including the northern and eastern suburbs of the Portland metropolitan area. It contains several National Wildlife Refuges within the Ridgefield National Wildlife Refuge Complex.^[2]^[3]

Willamette River and Tributaries Gallery Forest (3b)

The Willamette River and Tributaries Gallery Forest ecoregion includes low-gradient, meandering river channels, oxbow lakes, and meander scars incised into the broad floodplains of the Willamette River and its tributaries. Elevation varies from 40 to 500 feet (12 to 150 m). The region includes the historic floodplains of the Willamette River system, which rarely function today due to flood control dams in the upper basin that have reduced the frequency and volume of floods and contributed to the decline of the endemic, endangered Oregon chub. A small section, designated as the Willamette Floodplain, has been protected within the William L. Finley National Wildlife Refuge. Historically, riparian gallery forests containing ash, black cottonwood, alder, and bigleaf maple grew on fertile, alluvial soils. Today, most of the forests have been replaced by agriculture and residential development. The region covers 675 square miles (1,748 km²) in Oregon in a narrow band rarely more than 5 miles (8 km) wide that extends along nearly the entire length of the Willamette River and the lower reaches of the McKenzie, Long Tom, Santiam, Yamhill, Molalla, Clackamas, and Tualatin rivers, including the Tualatin River National Wildlife Refuge.^[2]^[3]

Ankeny National Wildlife Refuge in the Prairie Terraces ecoregion

Prairie Terraces (3c)

The Prairie Terraces ecoregion includes all terraces of the Willamette River upstream of the Portland/Vancouver Basin. It is dissected by low-gradient, meandering streams and rivers. Elevation varies from 160 to 500 feet (50 to 150 m). The region's broad fluvial terraces once supported oak savanna and prairies, featuring Garry oak, camas, sedges, tufted hairgrass, fescue, and California oatgrass. Wetter areas supported Oregon ash, Douglas-fir, bigleaf maple, black cottonwood, and an understory of poison-oak, hazel, and Indian plum, with some Ponderosa pine to the south. Today, only relict native prairie remains. The poorly drained soils derived from glacial lake deposits are extensively farmed for grass seed and small grains, as grasses tolerate poor drainage and poor rooting conditions better than other crops. Historically, seasonal wetlands and ponds were common, but many streams are now channelized, and the wetlands have been reclaimed for grain crops. In addition to agriculture, the Prairie Terraces have experienced the brunt of urban and suburban development along the Interstate 5 corridor. The region covers 1,971 square miles (5,105 km²) in Oregon along the length of the valley and includes the Baskett Slough and Ankeny national wildlife refuges.^[2]^[3]

Valley Foothills (3d)

The Valley Foothills ecoregion is a transitional zone between the agricultural Willamette Valley and the more heavily forested Cascade and Coast ranges. It contains rolling foothills with medium gradient, sinuous streams, and a few buttes and low mountains, rising to an elevation of approximately 1,500 feet (457 m). The region receives less rainfall than its more mountainous neighbors, and consequently the potential natural vegetation is distinct. The eastern foothills are wetter than those that lie on the western side of the valley in the lee of the Coast Range. Historically, the drier areas supported Garry oak and madrone woodlands and prairies, with California oatgrass, fescue, blue wildrye, brodiaea, and other prairie forbs; while the moister areas supported Douglas-fir forests, with sword fern, oceanspray, hazel, baldhip rose, poison oak, and alien Himalayan blackberry. Today, the valley foothills are characterized by rural residential development, pastures, timberland, vineyards, Christmas tree farms, and orchards. The largest of the Willamette Valley subregions, it covers 2,415 square miles (6,255 km²) in Oregon and 112 square miles (290 km²) in Washington.^[2]^[3]

References

1 2 3 4 Hoekstra, J. M.; Molnar, J. L.; Jennings, M.; Revenga, C.; Spalding, M. D.; Boucher, T. M.; Robertson, J. C.; Heibel, T. J.; Ellison, K. (2010). Molnar, J. L., ed. The Atlas of Global Conservation: Changes, Challenges, and Opportunities to Make a Difference. University of California Press. ISBN 978-0-520-26256-0.
1 2 3 4 5 This article incorporates public domain material from the United States Geological Survey document: Thorson, T.D., Bryce, S.A., Lammers, D.A.; et al. "Ecoregions of Oregon" (PDF). (color poster with map, descriptive text, summary tables, and photographs; with a Reverse side).
1 2 3 4 5 This article incorporates public domain material from the United States Geological Survey document: Pater, D., Bryce, S.A., Kagan, J.; et al. "Ecoregions of Western Washington and Oregon" (PDF). (color poster with map, descriptive text, summary tables, and photographs; with a Reverse side).
↑ "Willamette Valley forests". Terrestrial Ecoregions. World Wildlife Fund.

External links

Willamette Valley forests (Vanderbilt University)

Level III Ecoregions in Oregon

Marine West Coast Forest	1 Coast Range 3 Willamette Valley

Northwestern Forested Mountains	4 Cascades 9 Eastern Cascades Slopes and Foothills 11 Blue Mountains 78 Klamath Mountains

North American Deserts	10 Columbia Plateau 12 Snake River Plain 80 Northern Basin and Range

as defined by the United States Environmental Protection Agency

Level III Ecoregions in the United States

Marine West Coast Forest	1 Coast Range 2 Puget Lowland 3 Willamette Valley 111 Ahklun and Kilbuck Mountains 113 Alaska Peninsula Mountains 115 Cook Inlet 119 Pacific Coastal Mountains 120 Coastal Western Hemlock-Sitka Spruce Forests

Northwestern Forested Mountains	4 Cascades 5 Sierra Nevada 9 Eastern Cascades Slopes and Foothills 11 Blue Mountains 15 Northern Rockies 16 Idaho Batholith 17 Middle Rockies 19 Wasatch and Uinta Mountains 21 Southern Rockies 41 Canadian Rockies 77 North Cascades 78 Klamath Mountains 105 Interior Highlands 116 Alaska Range 117 Copper Plateau 118 Wrangell Mountains

Mediterranean California	6 Southern and Central California Chaparral and Oak Woodlands 7 Central California Valley 8 Southern California Mountains

North American Deserts	10 Columbia Plateau 12 Snake River Plain 13 Central Basin and Range 14 Mojave Basin and Range 18 Wyoming Basin 20 Colorado Plateaus 22 Arizona/New Mexico Plateau 24 Chihuahuan Deserts 80 Northern Basin and Range 81 Sonoran Basin and Range

Temperate Sierras	23 Arizona/New Mexico Mountains

Great Plains	25 Western High Plains 26 Southwestern Tablelands 27 Central Great Plains 28 Flint Hills 29 Central Oklahoma/Texas Plains 30 Edwards Plateau 31 Southern Texas Plains 34 Western Gulf Coastal Plain 40 Central Irregular Plains 42 Northwestern Glaciated Plains 43 Northwestern Great Plains 44 Nebraska Sand Hills 45 Piedmont 46 Northern Glaciated Plains 47 Western Corn Belt Plains 48 Lake Agassiz Plain

Eastern Temperate Forest	32 Texas Blackland Prairies 33 East Central Texas Plains 35 South Central Plains 36 Ouachita Mountains 37 Arkansas Valley 38 Boston Mountains 39 Ozark Highlands 51 North Central Hardwood Forests 52 Driftless Area 53 Southeastern Wisconsin Till Plains 54 Central Corn Belt Plains 55 Eastern Corn Belt Plains 56 Southern Michigan/Northern Indiana Drift Plains 57 Huron/Erie Lake Plains 59 Northeastern Coastal Zone 60 Northern Appalachian Plateau and Uplands 61 Erie Drift Plain 63 Middle Atlantic Coastal Plain 64 Northern Piedmont 65 Southeastern Plains 66 Blue Ridge 67 Ridge and Valley 68 Southwestern Appalachians 69 Central Appalachians 70 Western Allegheny Plateau 71 Interior Plateau 72 Interior River Valleys and Hills 73 Mississippi Alluvial Plain 74 Mississippi Valley Loess Plains 75 Southern Coastal Plain 82 Laurentian Plains and Hills 83 Eastern Great Lakes and Hudson Lowlands 84 Atlantic Coastal Pine Barrens

Northern Forests	49 Northern Minnesota Wetlands 50 Northern Lakes and Forests 58 Northeastern Highlands 62 North Central Appalachians

Tropical Wet Forests	76 Southern Florida Coastal Plain

Southern Semi-Arid Highlands	79 Madrean Archipelago

Taiga	101 Arctic Coastal Plain 102 Arctic Foothills 103 Brooks Range 104 Interior Forested Lowlands and Uplands 106 Interior Bottomlands 107 Yukon Flats 108 Ogilvie Mountains

Tundra	109 Subarctic Coastal Plains 110 Seward Peninsula 112 Bristol Bay-Nushagak Lowlands 114 Aleutian Islands

as defined by the United States Environmental Protection Agency

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