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Past Distribution in the Pacific Northwest

Dave Vesely, Pacific Wildlife Research, Corvallis, Oregon.
The distribution of Oregon white oak has shifted across the Pacific Northwest for thousands of years as climatic conditions have evolved. As glaciers retreated from the region (18,000 to 10,000 years ago), spruce, fir, and tundra plants became established as soon as the ice melted. During the next 5000 years, the climate became warmer and drier, and fires became more frequent across the landscape. These conditions favored xeric (adapted to warm, dry conditions) plant communities. Pollen analysis indicates that prairies, oak savanna, and dry forests dominated by Douglas-fir and pine replaced cold-climate plant communities. Oregon white oak savannas probably reached their maximum extent at this time. Fires ignited by lighting maintained wide expanses of savanna across the Willamette Valley and southern Puget Lowlands. Pollen records suggest that oak habitats and other xeric vegetation eventually began to decline as the region became cooler and wetter. Forests of western hemlock, western redcedar, and other mesic (adapted to moderate conditions) plants became widespread. By about 6,000 years ago, the present climate and natural patterns of vegetation were established in the Pacific Northwest.

In spite of climatic conditions that have favored the expansion of conifer forests, early European explorers and settlers found vast areas of prairie and oak savanna across the Puget Lowlands and Willamette Valley. Traveling through the Willamette Valley in 1841, explorer Charles Wilkes described the landscape as being "destitute of trees, except oaks". The means by which forests were prevented from encroaching upon the prairies and savannas was fire. Although the present climate has made lightening-ignited fire a rare event in the lowlands west of the Cascades, American Indians used fire as a tool for managing natural resources. Oregon white oaks are able to endure on fire-prone landscapes where other trees are unable to become established.

The Cowlitz and Upper Chehalis Indians of the Puget Lowlands and the Kalapuya tribes of the Willamette Valley set fire to thousands of acres every year to regenerate the prairie plants on which they depended for food and medicine. Some of the most important plants were camas (Cammassia quamash), tarweed (Madia spp.), and wapato (Sagittaria spp.). Fire also cleared the brush from underneath oaks and made collecting acorns easier. A mush made from acorns was very important in the diet of these Indians. Some woodlands were deliberately left unburned to provide areas where deer, elk, grouse, and other game would concentrate, so that these animals could be hunted more successfully.

Pioneers arriving in western Washington and Oregon during the mid-1800's were able to suppress the practice of annual burning by the American Indians. Since then, the wide expanses of savannas and prairie have largely been converted to agricultural fields, pastureland, suburbs, and cities. The few patches of oak savanna and open woodlands that have remained undeveloped, are now being encroached upon by Douglas-fir, grand fir, bigleaf maple, and other trees that can out-compete Oregon white oak in the absence of fire. The loss of oak-dominated habitats is recognized as a serious threat to native biodiversity of the Pacific Northwest.

Threats to Oak Habitats

Oregon white oak savannas and woodlands are among the most endangered ecological communities in the Pacific Northwest. Oak habitats face several serious threats:

  • More than 99% of pre-settlement prairies and savannas in western Washington and Oregon have been converted to farms, urban areas, and other human developments.
  • Valley woodlands once dominated by widely-spaced oaks are slowly transforming into forests crowded with conifers and shade-tolerant trees. Oregon white oaks are unable to survive for more than a few decades in such conditions.
  • On rural landscapes, legacy oaks that persisted on pastures and woodlots for centuries are being cut down as agricultural practices intensify.
  • Foresters viewed Oregon white oak as an undesirable species because no strong market has developed for the wood. Therefore, there has been no economic motivation to maintain oak woodlands.
  • Invasive, non-native plants such as Scot's broom, Himalayan blackberry, and false-brome reduce the survival and growth rate of oak seedlings. Invasive plants also compete against wildflowers and grasses that are associated with oak habitats, thus reducing native biodiversity of the site.
  • Park managers and homeowners do not often plant Oregon white oak for landscaping because of its reputation for slow growth.

Conservationists and public land managers in the Pacific Northwest recognize the critical role oak savannas and woodlands play as wildlife habitat and maintaining ecosystem functions. However, most federal and state lands are concentrated in the Cascades, Coast Range, and Olympic Peninsula, regions with few suitable sites for growing oaks. Therefore, the future of oak savannas and woodlands depends upon the active participation of private landowners.
The pastoral scenes that are common across today's farmlands appear somewhat similar to pre-settlement savannas and prairies. However, the plant communities and wildlife habitats on these landscapes have changed dramatically in the last 200 years. Dave Vesely, Pacific Wildlife Research.
The seeds of tarweed (Madia elegans) were an important food to American Indians living in the lowlands of western Washington and Oregon. Lynda Boyer, Heritage Seedlings.
Oregon white oak woodlands will eventually become dominated by faster growing conifers and bigleaf maples without active management designed to maintain oaks. Lynda Boyer, Heritage Seedlings.
 

Threats to Oak Habitats

This Oregon white oak woodland in Washington is threatened by an invasion of Scot's broom and over-topping conifers. Warren Devine, US Forest Service.
Perhaps no other wildlife species is as closely associated with oaks as the western gray squirrel (Sciurus griseus). The species is listed as State Threatened in Washington and listed as a Sensitive Species in Oregon due to habitat loss and fragmentation.

Sudden Oak Death

  • California Oak Mortality Taskforce website is designed to be a resource for all persons concerned about Sudden Oak Death, from foresters and private landowners, to legislators, educators, and those in the research community.


  • Oregon Department of Agriculture SOD Alert Oregon's oaks, tanoaks, huckleberries, and rhododendrons can best be protected by preventing further introductions of the disease to Oregon.


  • USDA/APHIS/PPQ SOD Fact Sheet The goal of APHIS-PPQ is to define the extent of SOD distribution in the US and limit its artificial spread beyond the infected area through quarantine and a public education program.

Historic Oak Distribution

  • Oregon Natural Heritage Information Center,Oregon State University.
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