Fire Adapted Oak Habitats in the South Willamette Valley

The nexus of oak habitat restoration, hazardous fuels reduction and the potential for healthy, functioning oak habitats to reduce wildfire risk around communities.

In the southern Willamette Valley, native prairies, oak savannas and oak woodlands were once widespread. A history of agricultural and forestry practices, urbanization, altered wildfire patterns and the introduction of non-native invasive plant species have contributed to the decline of these habitat types. These plant communities support a variety of rare plants and animals absent from conifer-dominated forests. Without active management, the remaining oak habitats on the landscape are at risk of local extinction.

Well-managed oak habitats support hundreds of plants and animals while reducing local fire danger. When properly maintained, oak habitats can lower the risk of fire in an area from high to low intensity because the vegetation is more spread out and there are fewer opportunities for fires on the ground to move into the canopy.

In this publication, we will discuss the ecology of fire and oak habitats and how the two interact. We will then offer some options for restoring oak habitat while reducing the threat of severe wildfire around homes built in and around oak woodlands and forests.

Fire plays an important ecological role in the forests and woodlands of Oregon’s southern Willamette Valley. Fires help reinitiate young or early seral forests, create biodiversity gaps and reduce flammable material buildup. Fire helps to recycle nutrients, regenerate plants and stimulate biological diversity among plant species and habitats. Tribes historically lived and worked with fire. With a keen understanding of fire’s interactions with plants and animals on the landscape, tribes today use fire to meet many of their cultural and subsistence needs.

Oxygen, heat and fuel are the three components required for a fire to occur. If any are removed, then the fire goes out. In the forest, oxygen is present in the air. Heat is supplied through lightning or human activity. The final ingredient, fuel, is anything that burns, including trees, shrubs, grasses or woody debris.

Fire behavior is a measure of how quickly a fire spreads. Weather, topography and fuel are the three components that influence how quickly a fire moves through the landscape. Weather affects the rate and degree to which fuels dry out. Hot, dry days result in drier fuels that are more combustible. Hot, dry winds increase the rate of fire spread by maximizing the available oxygen and heating and drying fuels more quickly. Topographic features such as slopes affect how quickly a fire moves. For example, fires tend to move faster uphill because the rising hot gases preheat and dry the fuels upslope, making them more combustible. Lastly, the amount and arrangement of fuels will affect the fire behavior. The sparser and more spread out the fuels are, the slower the fire will spread.

The southern Willamette Valley ranges between 200 and 900 feet, with Eugene at 430 feet. In Eugene, the average annual high temperature is 63°F and the average annual low temperature is 42°F. The average annual precipitation is 46.1 inches and snowfall is 5 inches, most of which occurs between October and May, making mild summer drought a typical occurrence.

Prior to Euro-American settlement, fires occurred frequently in this region, with low intensity every three to 55 years. Rarely, lightning-ignited fires occur during late summer and early fall, when moisture is lowest and ground heating is greatest. Most other fires were ignited as part of Native American cultural burning practices by the local Kalapuya people.

These fires mutually benefit the Kalapuya people and the oak-dominated ecosystems of the southern Willamette Valley. Fires are set in the fall for easy control and proper timing to support healthy oak populations, over 50 species of prairie plants, and game animals that all provide food and other cultural resources. The burning also discourages insects that damage the acorns. It halts the encroachment of upland forest species such as Douglas-fir and incense-cedar. If these forest species are allowed to persist at high densities, they can overtop and eventually shade out the various oak ecosystem species. This would change the composition of the vegetation in the area, predominantly a mosaic of open prairies, oak savannas and oak woodlands. Forests dominated by Douglas-fir (Pseudotsuga menziesii) naturally occur in shadier, moister patches on north-facing slopes and mountainous foothills.

By the mid-1800s, the small population of local Kalapuya people that had survived several diseases brought on by European contact were moved to reservations. Willamette Valley oak habitats were converted to agricultural fields and homesteads in the desirable areas. In the other locations, the exclusion of frequent cultural burning practices allowed Douglas-fir and associated upland forest vegetation species to increase in density and outcompete the oak savanna, woodland, and forest species. People continued to use fire as a tool for land use and clearing activities through the 1930s, when widescale fire suppression was adopted to avoid any threats to their crops, homesteads and wood supplies.

Fires still exist in today’s landscape, mostly from accidental human ignition. However, the rare lightning-started fires still occur in the upland areas surrounding the valley foothills when conditions are windy and dry. Additionally, modern descendants of the Kalapuyan people — members of the Confederated Tribes of the Siletz Indians and Confederated Tribes of Grand Ronde — and partnering agencies are working together to increase the intentional lighting of cultural fire on the landscape. Their goal is to restore access to cultural traditions, food resources and oak-associated habitats that are now greatly diminished.

Two oak species are native to the southern Willamette Valley: Oregon white oak (Quercus garryana) and California black oak (Quercus kelloggii). Oregon white oak is generally found in the lower elevations (up to 3,800 feet), most typically along the foothills of the Coast and Cascade ranges and within the Willamette, Rogue and Umpqua valleys of Oregon. This species tolerates a variety of sites but is often outcompeted on productive soils by trees that grow much faster, such as Douglas-fir. As a result, Oregon white oak is most often found on exposed, droughty or rocky sites and moist or riparian areas that dry in summer.

In Oregon, the California black oak ranges from north of Eugene south through the valleys west of the Cascade Mountains. California black oaks are common on low, dry slopes up to 1,000 feet, because they cannot tolerate persistently wet soils.

Both species can live up to 500 years, but due to their shade intolerance, they will succumb to being overtopped and outcompeted by faster-growing Douglas-fir, grand fir or bigleaf maple trees. Douglas-fir, grand fir and bigleaf maple struggle in the presence of frequent fires or extremely dry soils, allowing oaks to persist and provide vital resources in these otherwise low-productivity conditions.

These oak habitat refugia are especially important because they support communities of plants and wildlife different from other Willamette Valley ecosystems and more intensively managed agricultural and forestry lands. More than 200 native wildlife species rely on these oak habitats. Columbian white-tailed deer, white-breasted nuthatch and Fender’s blue butterfly are some of the imperiled species found here. The oak trees alone provide shade, cover, forage, perches and cavities that support these various species.

Within the Southern Willamette Valley, these oak habitats can be divided into five categories: oak savanna, oak woodland, oak forest, oak with pine and riparian oak.

• Oak savannas are characterized as open with grassy and herbaceous vegetation and few (one to 10) trees per acre. Tree species are predominantly white or black oak but can also include Douglas-fir and ponderosa pine mixtures. These habitat types are usually the intermediary between prairies and woodlands. The low density of trees allows full sunlight to reach each canopy, causing full mushroom-shaped crowns in oak and large-branched full-length crowns in conifers. With the reduction of fire on the landscape, current habitats characterized as savanna may include some shrub species and slightly elevated numbers of young deciduous and conifer trees.
• Oak woodlands are characterized by an oak overstory with an understory consisting of variable shrub densities mixed with more open, grassy and herbaceous vegetation. Some woodlands see a sprinkling of bigleaf maple and conifer species, including Douglas-fir, ponderosa pine, western hemlock, western redcedar or incense-cedar depending on the site conditions. Woodlands are sometimes further subdivided into open and closed subcategories. Open woodlands usually contain five to 20 trees per acre, while closed oak woodlands have 10–40 trees per acre. Trees in either subcategory tend to be a mix between open mushroom-shaped canopies and more columnar-shaped trees. Oak woodland habitats often occur between savannas and upland forests.
• Oak forests are characterized by a closed overstory of at least 75% tree cover. The density of these trees lends them to be more columnar in nature. Oak forests can be further subdivided to identify the composition of the overstory species. For example, we may use oak forests to refer to a predominantly oak overstory. In oak and fir forests, oaks share approximately 50% of the canopy with Douglas-fir, often with dying oak trees due to competition for sunlight. In oak and hardwood forests, oak shares the canopy with other hardwoods, including Pacific madrone, bigleaf maple, bitter cherry or Oregon ash. Lastly, oak and pine forests combine oak and ponderosa pine. On wetter sites, the understory is composed of a dense shrub layer and saplings, but in drier areas, the understory can be a more open mixture of shrubs and grasses. These habitat types are uncommon on the valley floor and usually occupy higher elevations and slopes that are more protected from fire.
• Riparian oak habitats represent any of the previously mentioned oak habitat types. The difference is that these habitats are situated next to bodies of water. These wetter sites typically have a denser forest and a greater diversity of deciduous trees and shrubs.

The exclusion of fire from the Oregon white oak ecosystems that once blanketed the southern Willamette Valley has resulted in drastic changes in the vegetation composition of the landscape and an overall decline in oak regeneration. Oregon white oak ecosystems are considered fire-dependent because they rely on fire for establishment, maintenance and reproduction.

Oregon white oak itself is rarely killed in understory burns. It is considered a fire-resilient tree species that uses strategies to both resist and endure the presence of fire. It resists fire with its protective corky bark structure, deep taproots and heat-tolerant buds that resprout quickly following fire damage. As a younger tree, it endures fire by experiencing topkill. This makes it appear dead above the ground, but the roots remain alive. In the next growing season, the tree will sprout new branches at bud sites on the stem and at the tree’s base. Larger and older trees do not typically experience full topkill, but instead some branches in the crown die back. While this process can be aesthetically unpleasant to people, fire-induced topkill and crown dieback can help to thin dense stands and provide snags and downed wood for wildlife habitat.

While fire does kill seedlings, the overall decline in regeneration of Oregon white oak is linked to reductions in fire on the landscape. This is because the absence of fire causes oak habitats to have too many trees and invasive species, creating enough competition to block light critical to new oak seedlings in unburned sites. Additionally, fire stimulates the burned oaks to sprout, which may be more effective than seed regeneration. So, reductions in fire limit new oaks’ ability to develop from seeds and resprouts.

Without fire, upland oak ecosystems increase in density and transition into conifer-dominated forests or shrublands. Species that are far less fire-resistant than oak but faster growing, such as Douglas-fir and other woody shrubs, can survive and then shade out and limit the reproduction of the oak and associated grass and forb species. The introduction of invasive species, thatch accumulations and moss cover limit the germination and survival of native plants. This has caused widespread changes in habitat structure and function. This transition in vegetation species has contributed to the decline of several associated plant and animal species. The reintroduction of cultural or prescribed fire can help to conserve and restore imperiled oak ecosystems by limiting the introduction of non-native, fast-growing, shade-inducing species and encouraging oak regeneration.

Fire suppression techniques of the last century have encouraged an explosion of dense vegetation that threatens oak ecosystems and increases the threat of high-severity fires. This is of special concern in and around the increasing number of homes built in forests and wildlands. With proper restoration techniques and subsequent maintenance, the same features that make Oregon white oak trees and ecosystems fire resilient make them safer choices for vegetation around these more rural neighborhoods with intermixed oak woodland and forest vegetation. The reduced resins in oak wood and leaves and the open stands with minimal shrub cover are less prone to crown fires than dense conifer forests.

For these reasons, managing Oregon white oak ecosystems is inherently beneficial for meeting simultaneous goals for fire risk reduction. Many techniques that support oak also reduce the risks of adverse impacts from wildfire. Landowners have several opportunities to both benefit oak ecosystems and reduce fuels on their properties. Many of these management practices are also eligible for various financial assistance programs. Contact your local OSU Extension Agent to learn more about these programs.

When the understory becomes overgrown with woody vegetation, oak-associated grasses and forbs are reduced and wildfire risk increases. To maintain low tree and shrub densities, oak savannas and woodlands require maintenance every three to 10 years. Fuel-reduction and oak-restoration maintenance techniques include burning; manual and mechanical vegetation removal; targeted seasonal grazing; chemical applications; or a combination of these methods. Reducing ladder fuels is especially important to keep fires moving slowly and close to the ground. Ladder fuels are fuels situated midcanopy, such as low branches on trees, large shrubs and small- to medium-sized trees that help to carry a fire from the ground into the canopy of the oak woodland or forest. Following any of these techniques, seeding or planting with native species can help to control the spread of invasive species and encourage the re-establishment of native oak ecosystems.

Thinning treatments

Thinning is a land management tool that can help land managers remove overstory trees to meet various goals and objectives. Due to oaks’ shade intolerance and slow-growing nature, thinning is useful for decreasing the density in the canopy and avoiding encroachment or overtopping by conifer trees. Oaks recover well from thinning by increasing their branch and diameter growth and producing more acorns. They also experience fewer impacts than conifers after thinning, such as windthrow, logging damage or thinning shock.

Oaks need lots of space to grow. Open-grown, healthy oaks have mushroom-shaped crowns and a few dead branches. Oaks needing more sunlight tend to lack height growth and have narrow crowns with fungi and dead branches present. When thinning, remove enough trees so that the oaks selected to remain receive full sunlight on all sides to allow for recovery and future growth. If you are working in a younger stand, thin trees before the canopies begin to overlap. Periodic thinning may be necessary throughout the life of the oak woodland. Trees to consider thinning grow more quickly than oak. These include conifers, pacific madrone, bitter cherry and bigleaf maples. You may also remove less vigorous and unhealthy oaks to make more room for the healthier ones.

Thinning also decreases fire risk by reducing ladder fuels and the continuity of the overstory canopy that could carry a crown fire. If mostly conifer species are removed, then fire risk is further reduced because conifers have higher amounts of flammable resins that more effectively perpetuate fire than deciduous trees such as oak do. Douglas-fir, grand fir, ponderosa pine, western redcedar and incense cedar are all conifers that grow alongside oak and may be candidates for removal. Not all conifers need to be removed, depending on the area and your objectives. Conifers — especially large, mature trees — were historically present in these ecosystems, provide vital resources and can have a place in oak savannas, woodlands, and forests. Still, it’s best to aim for fewer trees that are spread out.

Thinning can be accomplished by hand-felling trees with chain saws or using larger machines, such as a feller-buncher, harvester-forwarder, skid steer or another forestry machine. Site conditions and management objectives will dictate the best equipment for the job. Stumps left behind might limit some mechanical operations such as mowing and hand falling, which can be labor-intensive. Also, when planning your operation, it is important to consider the limited contractors available to small landowners doing restoration work.

Sometimes, the logs from thinning operations can be sold to offset the fuel reduction or oak restoration project costs. If the trees are not merchantable, you can often use them as fence posts, firewood or mulch, rather than rather than piling and burning them. Widely spaced large logs or low brush piles created during logging can be placed around the property to aid in creating wildlife habitat. These structures can provide important shelter, food, and humidity or temperature resources for small mammals, amphibians and reptiles.

These thinning operations will greatly open the canopy. With the addition of sunlight, these savannas and woodlands will continue to experience encroachment from young trees and woody shrubs. Understory restoration and fuel treatments — including prescribed fire, mechanical, manual, chemical and biological approaches — will also typically be required.

Prescribed fire treatments

Prescribed fire or cultural burning is the intentional lighting and carrying of fire across the landscape to achieve a specific goal, such as maintaining oak savannas, woodlands and forests and reducing wildfire risk. Prescribed fire reduces wildfire risk by reducing the available fuels, such as dead wood on the ground and ladder fuels that connect the ground to the canopy. In oak restoration management projects, prescribed fire helps reduce vegetation densities, thatch accumulation and surface fuels. It also encourages oak regeneration.

Prescribed fire can be accomplished through pile burning, biochar creation, broadcast and understory burning. Understory burning aims to retain most of the canopy while consuming ground and surface fuels. It is increasingly used in addition to broadcast burning, which aims to consume most, if not all, of the available fuels. Both types of burns are conducted in the Willamette Valley as the local Tribes and land management agencies burn to restore and maintain existing oak ecosystems. Low-intensity fires are typically lit in the late summer and early fall, ideally on a three-to five-year rotation to limit unwanted and invasive species and benefit the native species. Varying the size and configuration of burn units and how often these fires are set can increase the local biodiversity of plants and wildlife associated with shorter or longer fire return intervals.

Understory and broadcast burning are still uncommon on private lands. Small, private landowners should consider using broadcast burning with input from a state-certified burn manager or other experienced burn boss and in coordination with local fire response agencies to ensure safe fire operations are planned and executed. Most small, private landowners use pile burning and biochar coupled with one of the other methods to reduce fire risk. However, the most efficient means of reducing fire risk ecologically and economically is the application of prescribed fire.

Both pile burning and biochar use mechanical, manual, chemical or a combination of techniques to make the fuels more manageable by removing them and reducing their size. Then, the material is piled or collected for burning. Piles can be made by hand or machine and are burned in place. They help to recycle nutrients and disperse the fire’s effects on the landscape.

Biochar is made by combusting the slash material with limited oxygen. Biochar can be made by burning a slash or rick pile (wood stacked like a log cabin with kindling on top), or in a kiln. When done effectively, biochar reduces smoke emissions and captures carbon for long-term storage, as it requires lighting the material from the top and quenching the fire to maximize coal formation.
To minimize the risk of pile or biochar burns escaping, have fire tools and water on site and ready. Create a firebreak — an area void of flammable material (usually bare soil) — around your piles to stop the fire from transitioning to other parts of the property. Do not leave piles unattended.

For more information on prescribed fire, see Prescribed fire: Why we burn (EM9339).

Mechanical treatments

Mechanical treatment methods include reducing overgrown vegetation or removing invasive or unwanted plants using mechanical equipment that chops, chips, crushes or breaks apart plant material. Common equipment used for mechanical treatments include masticators, chippers, grinders, mowers and string trimmers. This process reduces ladder fuels by eliminating the vegetation that connects the understory plants to the overstory tree canopy. Fire risk is further reduced by increasing the space between woody shrubs and trees. The material left on the ground can create a dense fuel bed several inches deep, inhibiting herbaceous plant germination. Therefore, these techniques are often combined with building slash or wildlife piles. Piles left unburned can return nutrients to the soil, serve as wildlife habitat and suppress unwanted vegetation from returning. Still, the number of piles left for habitat should be limited and their size should be kept small. Piles can also be burned or used in biochar creation.

For oak restoration purposes, this treatment can be applied to overgrown shrubs and young trees, both native and non-native, which have accumulated within the understory over many years with no disturbance. Mechanical treatments may also be selectively applied to prevent the spread of unwanted species, such as blackberries, poison oak, Scotch broom, English ivy, clematis, Japanese knotweed, Russian olive, English hawthorn, tree of heaven, black locust, shiny geranium and false brome.

When performing a mechanical treatment, the vegetation type and site conditions will dictate the type of machine you use. Some things to consider about the kind of machine include the ground disturbance impacts, material size limitations, cost, public use restrictions, industrial fire precautions and hillside slope limitations.

To learn more about mechanical treatments, see Reducing hazardous fuels on woodland property: Mechanical treatments (EC1575).

Manual treatments

Manual treatments involve removing unwanted and non-native herbaceous and woody plants by hand. Methods for manual treatments include pruning, pulling, digging, lop and scatter, which is cutting up and leaving the dispersed material on the ground. These processes mostly remove surface fuels but can also remove ladder fuels if pulling or digging large woody shrubs or pruning low-to-the-ground branches. Pruning oak trees can also stimulate acorn production for cultural use, oak regeneration or wildlife forage. Lop and scatter are generally not appropriate for oak ecosystems unless coupled with a prescribed fire plan, because the material that is left behind suppresses native understory germination and growth. It also hinders mowing and grazing for future maintenance.

For pruning guidelines, see Reducing hazardous fuels on woodland property: Pruning (EC1576).

Chemical treatments

Chemical treatments are the removal or management of unwanted or invasive vegetation using EPA- approved herbicides. Chemical applications can be broadcast, spot or individual treatment.
Broadcast herbicide applications systematically treat the entire area you are working on. Spot treatments usually focus on smaller areas that will benefit specific trees or a location within the unit. For example, you would use a spot treatment around a new seedling to reduce competition for water and sunlight.

Lastly, individual treatments are selective. They usually focus on an individual plant through basal spraying, stump painting, hack and squirt, or injection. Application formulas for any of these techniques should be matched to the type of vegetation you are trying to control and the area in which it is being controlled.

Chemical treatments work quickly to achieve ecological goals. They are the most cost-effective when used to treat large-scale infestations of difficult-to-control species such as false broom, blackberries and scotch broom. Herbicides should be used cautiously to minimize human, wildlife and environmental health risks. Consult the PNW Weed Management Handbook and your local OSU Extension agent for more information.

Biological treatments

Biological treatments use livestock grazing to reduce persistent, unwanted and invasive vegetation such as blackberry, resprouting hardwood stumps, tree or shrub seedlings, and re-emerging shrubs. Managing the frequency, timing, duration, distribution and number of livestock, is necessary to reach the desired objectives. Overgrazing is a major concern. It can cause soil damage, accidentally encourage invasive species and result in loss of wildlife habitat. Consult with your local OSU Extension Small Farms agent to determine a grazing plan for your property and conditions.

Oregon white oak ecosystems are increasingly rare. But with rising concerns of wildfire risk in the southern Willamette Valley, restoration projects with multiple objectives that include wildfire risk reduction are increasing in interest. Removing overly dense trees and woody vegetation supports the health and establishment of vital oak habitats while reducing the potential fire intensity. Additionally, as climate patterns shift, the potential for hotter, drier summers and wetter winters makes Oregon white oak more adapted to the valley floor and foothills than most conifers. Investment in oak habitat restoration by private landowners could provide multiple benefits, including enhancing wildlife habitat, reducing wildfire risk and maintaining ecological diversity in a changing climate.

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