Oak savannas and woodlands are among the most endangered habitat types in the Pacific Northwest. They provide a unique habitat that is utilized by 200 species of wildlife including birds, mammals, amphibians, and reptiles, some of which rely on oak habitat to survive. There were an estimated 500,000 acres of oak dominated habitat in pre-settlement time at the turn of the 18th century. It was found from southern California through Oregon and Washington, and extended to British Columbia. Current estimates indicate that between one and five percent of that still exists today.
Protected areas are found mostly in parks and reserves. However, a significant decline of oak habitat continues. Greater than 80% of oak habitat in the Pacific Northwest is found on private lands, which offer some of the best opportunities for oak conservation in the region. The key to maintaining and restoring this critical habitat lies with private landowners.
History with Native Americans and Settlement
Oregon White Oak has a lifespan of up to 500 years, so many of the trees that remain today were here for Lewis and Clark to see in 1804. Native Americans repeatedly burned prairies and oak savannas for ten thousand years to maintain the habitat for deer and other game and to provide acorn production, an important staple in their diet. Repeated burning resulted in low intensity fires that would control other tree species and shrubs that competed with the more fire resistant oaks.
Given little credit for its economic value, acre upon acre of oak has been converted to other uses. Better quality sites have naturally converted to conifer in the Smokey Bear era of fire suppression, starting in the 1940’s. Faster growing Douglas fir and grand fir, as well as bigleaf maple, grow in the shade of oak where soil moisture and nutrients are more plentiful. They survive and eventually overtop and shade out the oak trees that “nurse” them through their early establishment. Landowners have opted to convert their land to conifer, or to vineyards and other agricultural crops. Urbanization and development have also contributed to the decline of oak habitat in a significant way.
Also known as Garry Oak, Quercus garryana grows on quality sites of deep, well drained soils where it can attain a height of eighty feet in one hundred years. It can also grow where other trees cannot survive, displaying an ability to tolerate extreme soil conditions that range from shallow, dry and rocky soils to saturated, heavy clay soils. Oak may only reach a height of twenty five feet in one hundred years on poorer sites.
Oaks are a very slow growing tree in comparison to other natives in their natural range. Good sites can support mature trees capable of producing up to twenty pounds of acorns per year, but they are not produced until trees are approximately 20 years old. Seed dispersal is primarily dependent upon animals.
The nuts drop between late August and November. Over half of the acorns can be removed and eaten or cached by stellars jays, squirrels, and other wildlife. Most of the remaining acorns desiccate on the ground surface. The majority of acorns buried by animals germinate in two to five weeks without a dormancy stage. This allows oaks to more quickly develop a deep taproot to survive on dry sites. The tree may die back for a number of years as it puts more energy into the root system. Dormant buds close to the surface at the root collar and from the roots themselves can sprout a new shoot, resulting in a root that can be ten years older than the stem.
Periodic disturbance by fire can be an important contributor to the persistence of oak dominated habitat. Frequent low intensity fires tend to kill competing vegetation, such as faster growing tree species, allowing the oak to perpetuate.
Oaks are the climax species on poor sites. Large dead and dying oaks can be seen on better sites where conifer have established and competed for light. Common trees associated with oaks are Douglas fir, grand fir, bigleaf maple, pacific madrone, Oregon ash, and some valley Ponderosa pine. Native shrubs include poison oak, snowberry, oceanspray, hawthorn, hazel, and serviceberry. Native oak savannas have bunchgrasses like Roemer’s fescue, red fescue, and California oatgrass.
Value to Wildlife
The benefits of oak habitat to wildlife are numerous. It provides shade and delivers large woody debris, nutrients and insects to streams directly benefitting fish. Big game, small mammals and birds eat the high calorie acorns (mast) that are provided in fall when other sources of nutrition decline. They are able to find hiding cover in what are otherwise very open areas. The leaf litter provides excellent microhabitats that offer cover and moisture for amphibians and reptiles. Birds of prey perch in oaks to rest or hunt. Insects forage on decaying wood in dead limbs and snags. They are in turn foraged by birds and other wildlife. Cavities are produced in the process that are utilized for nesting and cover by birds, small mammals, amphibians and reptiles. Downed wood is valuable in much the same way, maintaining fungi and micro-organisms while harboring insects and contributing hiding cover for amphibians, reptiles and small mammals. Ninety three species have been identified that utilize snags, logs and stumps. Several wasp species are dependent on Oregon White Oak for food or reproduction and up to 35 species of moths and butterflies can be found in a single oak tree. Even domesticated livestock benefit from the shade oak provides.
Preserving Existing Oak Dominated Habitat
A common threat to existing oak savannas and woodlands comes from overtopping tree competition by Douglas fir, grand fir, cherry and bigleaf maple. Thinning the competitive tree species, commonly referred to as a “release”, results in providing more water, nutrients and light to the oak. Oak has only a moderate tolerance to shade. Once it has been overtopped it is only a matter of a few years to a few decades before it succumbs and dies under the canopy of its competitors. Release can be done in a single treatment if done in the fall when there are no leaves on the oak. There is no evidence that oak respond negatively, or are “shocked”, by a sudden increase in sunlight at that time of year. It has been shown that the sooner they are fully released the better their performance will be.
Research indicates that removing competing trees for a full tree height distance from the oaks improves their growth and vigor within five years. They have better stem growth due to expanding crowns that grow more limbs and produce more acorns. Damage to the oaks from removal of the overtopping trees may be a concern in situations where they grew up through the oak canopy. Girdling is an alternative that kills the competing trees, but leaves them standing to provide greater wildlife benefits.
More detailed and helpful advice on release treatments is provided in the manual “A Practical Guide to Oak Release”. It explains how to assess your oak stand, how to design the release for maximum benefit, and what response you can expect.
Prescribed fire is a valuable tool to control competing vegetation. Incidence of periodic fire is considered necessary for the persistence of Oregon white oak woodlands and savannas.
Planting New Oak Dominated Habitat
It is important to assess the site you have in mind for planting to oak prior to proceeding. Factors that can determine its suitability include the soil type, elevation, aspect and existing vegetation. Depending on what you are starting with it may be necessary to do significant site preparation prior to planting. You may want to consider planting associated understory species at the same time to develop the most beneficial plant community that best mimics a natural oak stand or savanna.
There are three keys to successful oak planting. Be sure to start with quality seedlings that have fibrous root systems, not just a few woody roots. Fibrous root systems enable seedlings to better absorb water and nutrients, greatly improving their chances of survival and establishment. Control competing vegetation in the immediate vicinity of seedlings, including herbaceous plants and woody shrubs, to make more water and nutrients available. Protect seedlings from animal damage that can come from big game browse or girdling by small mammals. Expect slow growth for the first few years as oak puts energy into establishing a well-developed root system during that time.
Quercus garryana, U.S. Forest Service, U.S. Department of Agriculture,
“Fire in Restoration of Oregon White Oak Woodlands”, James K. Agee, in General Technical Report INT-GTR-341, (The Use of Fire in Forest Restoration), dated 1996
The Mighty Oak Faces Challenges in the Pacific West, SCIENCE UPDATE, Issue #20 / Fall 2010
“Planting Native Oak in the Pacific Northwest”, by Devine and Harrington. “General Technical Report, PNW-GTR-804”, dated February 2010. www.fs.fed.us/pnw/pubs/pnw_gtr804.pdf
“A Practical Guide to Oak Release”, by Harrington and Devine. “General Technical Report PNW-GTR-666”, dated February 2006. www.fs.fed.us/pnw/pubs/pnw_gtr666.pdf
“Oregon White Oak”, Oregon Forest Resources Institute.