Fire in northwest Oregon oak ecosystems
Oak-dominated ecosystems of the Willamette Valley have adapted to relying on fire for establishment, maintenance and reproduction.
Before Euro-American colonization, fires on these landscapes were a common occurrence, burning every three to five years. Most of these fires were ignited as part of the cultural burning practices by the local Indigenous people. These frequent fires burn with smaller flames because woody vegetation and other ladder fuels were sparse, decreasing opportunities for the fire to move into the overstory trees. The few fires that were lightning-ignited occurred in the late summer and early fall when temperatures and dry conditions were highest.
By the mid-1800s, most oak habitats had been converted to agricultural fields and homesteads. Until 1930, settlers exclusively used fire for land-use and clearing activities. After the 1930s, widescale fire suppression was adopted to avoid any threats to crops, homesteads and timber supplies. Today, most fires in oak ecosystems are started accidentally by humans and, in most cases, are quickly suppressed due to their proximity to communities and homes. However, cultural burning practices are returning to the landscape through the collaborative efforts of the Tribes and their partners.
Northwest Oregon oak ecosystems
Historically, the foothills of the Coast and Cascade ranges and the Willamette Valley were predominantly covered in oak-dominated plant communities forming a complex mosaic of upland and wet prairie, oak savanna, oak woodland and oak riparian forest. Today, these plant communities are in decline, in part because of fire exclusion.
Two species of oak are native to these habitats: Oregon white oak and California black oak. These species are typically found in the lower elevations — white oak from 0 to 3,800-foot elevation and black oak from 0 to 1,000 feet south of Monroe, Oregon — on exposed, dry or rocky sites. White oak is also present on moist or riparian areas that dry in summer. 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.
More than 200 species of native wildlife, including at least eight considered imperiled, rely on these oak habitats.
Forests dominated by oak can be described on a gradient from open to closed canopy. Oak savannas are open, with fewer than 10 trees per acre, grassy and herbaceous vegetation understories and scattered Douglas-fir and ponderosa pine.
Oak woodlands are denser with five to 40 trees per acre and a mixed understory with open, grassy and herbaceous vegetation and woody shrub clumps. Oak forests with at least 75% tree cover have a more closed canopy.
In these forests, oak share the canopy with either conifers, such as Douglas-fir, pine, cedar and grand fir, or other hardwoods, including Pacific madrone, bigleaf maple, bitter cherry or Oregon ash. On wetter sites, the understory is comprised by a dense shrub layer and saplings, but in drier areas the understory can be a more open mixture of shrubs and grasses. Riparian oak habitats can represent any of the previously mentioned oak habitat types. The difference is that these habitats are situated next to a body of water. These wetter sites typically have a denser forest and include a greater diversity of deciduous trees and shrubs.
Fire resilience strategies for small woodland owners in this forest type
Fire suppression techniques of the last century encouraged denser vegetation that compromises oak ecosystem health 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. Oregon white oak resists fire with its protective corky bark structure, deep taproots, and heat-tolerant buds, which resprout quickly following fire damage. The reduced resins in oak wood and leaves and open stands with minimal shrub cover are less prone to sustaining crown fires than dense conifer forests. Restored and maintained Oregon oak ecosystems are a safer choice for vegetation around rural neighborhoods.
A management plan can help organize restoration activities into an effective maintenance strategy that reduces the fire risk. When planning, take a landscape management approach and think about how your property fits into the mosaic of the properties around you. Can you work with neighbors to expand the benefits of the oak habitat? Are there areas where oak savannah historically occurred on your property? A landscape approach can also help you maximize your efforts around landscape features that are naturally fire resistant and tend to slow the spread of fires. These features include riparian areas, ponds, rock outcroppings and major paved roads.
Oaks need lots of space to grow, and thinning is a useful tool for decreasing the canopy and avoiding overtopping from faster growing trees. When thinning, focus on decreasing canopy overlap so trees receive full sun on all sides by removing the less vigorous and unhealthy oaks. Additionally, remove trees that grow more quickly than oak, such as Douglas-fir, grand fir, ponderosa pine, incense-cedar and bigleaf maple.
The most efficient means of reducing fire risk both ecologically and economically in oak-dominant forests is prescribed fire. Prescribed fire helps reduce vegetation densities, thatch accumulation and surface fuels and encourages oak regeneration. Prescribed fire is currently not that common on private lands, but small private landowners can use pile burning, biochar and other methods to reduce fire risk. These methods include:
- Mechanical equipment that chops, chips, crushes or breaks apart plant material.
- Hand tools for cutting, pulling, digging and pruning.
- Chemical treatments, such as EPA-approved herbicides.
- Biological efforts, such as livestock grazing.
These methods can all be applied to overgrown shrubs and young trees, both native and non-native, that have accumulated within the understory over many years without disturbance. Removing overgrown shrubs and young trees reduces ladder fuels and creates space between the remaining woody shrubs and trees. Excess material can be piled and left for wildlife habitat, burned or used in biochar creation. If leaving the piles for wildlife, keep them low and spread out more than 100 feet from your structures. Following any of these techniques, seeding with native grass and herbaceous species can help to control the spread of invasive species and encourage the re-establishment of native oak ecosystems.
What are the tradeoffs of managing for fire in Northwest oak habitats?
Managing for fire and oak can provide major benefits, but there are some tradeoffs. Oak habitats require the regular maintenance of unwanted invasive species and woody vegetation. If this vegetation cannot be maintained regularly, the fire risk increases. This can be a large economic investment, and oak trees do not carry high commercial viability. In some cases, financial assistance may be available for maintenance. Talk to your Extension agent about the resources in your area. The conifer logs from thinning operations can sometimes be sold to offset the costs of the fuel reduction or oak restoration project. If the trees are not merchantable, they can often be used in other ways — as fence posts, firewood or mulch. Widely spaced large logs or low brush piles created during logging can be placed around the property to aid in creating wildlife habitat. Do not put these materials within 100 feet of a structure because they may increase fire risk.
We are still learning
Forest managers and scientists are still looking for the answers to many oak management questions. Many are interested in the most economic methods for understory maintenance to make oak restoration more approachable for landowners. Others are concerned about the potential fire risk increase in riparian oak forests when most ash trees succumb to the Emerald ash borer. Lastly, little is known about the interactions of climate change. Land managers are hopeful that the drought tolerance of oak will make it a candidate for long-term survival, but more information is needed to determine how increasing yearly temperatures, variable annual precipitation averages and the recent introduction of the invasive Mediterranean oak borer will influence the future of Oregon's Northwest oak ecosystems.
Related publications
For more information
Five local partnerships have emerged in Oregon with long-term goals for oak and prairie habitat conservation. These five partnerships work under the overarching framework of the Cascadia Prairie-Oak Partnership.
Bailey, J.D. 2024. A walk with wildland fire. Waveland Press, Inc. Long Grove, Illinois.
Bennett, M, S. Fitzgerald, A. Christiansen and K. Baylog. 2017. Keeping your home and property safe from wildfire: A defensible space and fuel reduction guide for homeowners and landowners (EM 9184). Oregon State University Extension Catalog.
Detweiler, A.J., S. Fitzgerald, A. Cowan, N. Bell and T. Stokely. 2023. Fire-resistant plants for home landscapes: Reduce wildfire risk with proper plant selection and placement. (PNW 590). Oregon State University Extension Catalog.
Forest facts: Emerald ash borer, 2022, published by the Oregon Department of Forestry
Prescribed fire basics, a collection of 11 modules that provide introductory-level information.