Fire in Douglas-fir – western hemlock forests
The Douglas-fir-western-hemlock forest type is prevalent in the Coast Range and western slopes of the Oregon Cascades, where the mild maritime climate and abundant rainfall support dense stands of large Douglas-fir and western hemlock trees. Historically, wildfire patterns in this forest type were variable. Through emerging science on fire scar data, we are learning that these areas experienced small, low- to mid-intensity fires on a relatively frequent basis that shaped the local ecology and forest development.
These low-intensity fires were likely ignited by Indigenous peoples to meet land stewardship objectives prior to Euro-American settlement. This forest type also experienced larger, more severe wildfires on an infrequent basis, often coinciding with periods of drought and extreme fire weather. These larger wildfires played a crucial role in maintaining a mosaic of vegetation communities and promoting biodiversity across Western Oregon landscapes.
Douglas-fir–western hemlock forests
The Douglas-fir-hemlock forests of Western Oregon occur in the Coast Range and Western Cascades. On the coast, they begin east of the Sitka spruce fog-belt forests and extend across the Coast Range to the western edge of the Willamette Valley ecoregion. In the Western Cascades, we find Douglas-fir-hemlock forest types at an elevation of 1,000–3,500 feet. Average annual precipitation ranges from 80 inches per year in the Cascade foothills to over 160 inches per year in areas of the Coast Range. These forests include western redcedar, grand fir, red alder, bigleaf maple, Pacific madrone, Oregon white oak and western white pine, across various site characteristics. Riparian areas may contain Oregon ash, black cottonwood and willow. The understory is comprised of vine maple, thimbleberry, salmonberry, vanilla leaf, boxwood, elderberry, huckleberry, rhododendron, serviceberry, ocean spray, Pacific dogwood, salal, Oregon grape and others. This forest type is highly productive and contains a lot of fuel that can burn with high intensity when weather conditions are right.
Fire resilience strategies for small woodland owners in this forest type
Recent fires such as the Beachie Creek, Riverside and Holiday Farm fires of 2020 remind us of the importance of managing Douglas-fir-hemlock forests with fire in mind. To improve the ability of your woodland to resist rapid wildfire spread, withstand most negative impacts associated with crown fire behavior and recover quickly from wildfire events, consider applying three principles in your management approach:
- Protect structures through home hardening and creating defensible space.
- Utilize strategic fuel breaks.
- Increase the patchiness and diversity of your woodlands.
Home hardening and defensible space within the Home Ignition Zone
If you have a home or other buildings on your forest property that you want to protect, focus on structure improvements, such as upgrading to fire-resistant building materials like a metal roof and installing ember-resistant vents, and vegetation modifications in the nearest 100 feet or 200 feet on steeper slopes around your home and other structures. These localized practices are the most likely to reduce the chance of structure ignitions due to wildfire.
Fuel breaks
The productive conditions of this ecoregion make Douglas-fir-hemlock forests incredibly fast growing. That growth rate results in dense, closed canopy stands with minimal understory vegetation. In this forest type, thinning to a very wide spacing creates space between tree canopies. This reduces the probability of canopy fire spread but promotes understory vegetation growth, which increases flammability of surface fuels. Managing understory growth after thinning requires significant effort to maintain, so consider strategic thinning instead. Strategic thinning breaks up the continuity of fuels in priority locations. In these “shaded fuel breaks,” trees in targeted areas are spaced far enough apart to keep their crowns separated and the understory fuels are regularly managed. The maintenance schedule depends on site conditions and may require hand cutting and mastication. The goal of shaded fuel breaks is not to extinguish fire. Rather, these fuel breaks support fire behavior that is less intense and easier to control than fires in untreated areas.
Where should you develop fuel breaks? Consider property areas you most want to protect — such as structures, riparian areas, a special stand of trees, access roads and powerlines — and create fuel breaks to buffer these assets. Creating fuel breaks adjacent to roads (a non-burnable surface) can make fuel breaks even more effective. You can also assess where you have the highest risk of wildfire ignitions and develop fuel breaks around these ignition sources to reduce the chance of wildfire spreading into your woodlands. This may include areas along property lines or publicly accessible roads. Finally, consider opportunistic areas to enhance natural fuel breaks, such as rock outcroppings or riparian areas with a significant number of hardwoods, which are less susceptible to ignitions.
Patchiness and diversity
Another feature of fire-resilient woodlands is managing for diverse stand densities and tree species. Areas of low density can create gaps in the canopy cover and reduce the probability of crown fire spread. To develop this mosaic of low- to high-density patches, start by looking for ways to enhance natural variation. For example, in the foothills of the Coast and Cascades ranges, you may find areas where oak savannah historically occurred but is now dominated by Douglas-fir. This may be in drier areas or on south-facing slopes. Consider removing Douglas-fir in these areas to release existing Oregon white oak, which require significant sunlight and space between trees to thrive. This will also serve to maintain and improve biodiversity as oak habitats have significantly declined over the last 175 years.
Incorporating a hardwood component, whether it be oak, maple or others, can improve the fire resilience of your Douglas-fir-hemlock dominated woodland. Hardwood foliage has higher moisture and low-resin content and is less susceptible to ignition. Riparian areas may contain higher proportions of hardwoods. Encouraging the growth of hardwoods amongst stands of conifers can decrease the chance of a crown fire spreading by breaking up the coniferous canopy.
To further enhance the patchiness and species diversity in your Douglas-fir-hemlock-dominated woodland, consider where prescribed fire treatments may be used to manage the understory vegetation. This may be most appropriate in areas with more oak, in open meadows or in patches of low-density conifers. If you are interested in conducting prescribed fire as part of your management plan, consult with the Oregon Department of Forestry regarding burn permits.
In more dense, younger stands of Douglas-fir, the use of underburning would require substantial and long-term site preparation, including mechanical treatments to reduce stand density and treatment of ladder fuels and understory vegetation. Consider your land management objectives prior to integrating prescribed fire into your management plan. While small understory burns may enhance habitat for wildlife and help create a diversity of stand compositions in your woodlands, young Douglas-fir stands managed for timber may be damaged by the application of prescribed fire.
We are still learning
During the mid-20th century, Western Oregon experienced a period of minimal wildfire due to cooler climatic conditions and active wildfire suppression by land managers. As we enter an epoch of increased wildfire occurrence in the region due to a warming climate and many other complex factors, we are adjusting management guidance to address the increased need to manage westside forests for wildfire resilience, in addition to the other forest management objectives that landowners may have. Here, we consider a few tradeoffs, caveats and unknowns when it comes to managing Douglas-fir-hemlock forests in Western Oregon for wildfire.
A shaded fuel break – thinning to a wide-enough spacing that tree canopies are discontinuous — can reduce crown fire spread. The trade-off is that an open canopy invites increased growth of understory vegetation (fuel), including invasive species. The appropriate density of thinning to create a shaded fuel break that moderates fire behavior while also allowing for moisture retention and minimizing understory growth is likely site dependent. More research is needed to provide specific guidance. Because it would be difficult or impossible to treat your entire property at once to improve fire resilience, you can start by using fuel reduction around important features and values on your property, such as your home and outbuildings, powerlines and other areas of importance to you.
Extreme weather events, such as the east wind events that occurred in September 2020, can produce extreme fire behavior in dense, westside forests. Coupled with drought conditions, these weather events may diminish or eliminate the positive effects of management actions such as thinning or the development of fuel breaks that are designed to moderate fire behavior. In these extreme events, the best course of action is to focus on life and safety of first responders and residents by evacuating quickly and managing expectations about firefighters’ ability to protect your property.
For more information
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.
Prescribed fire basics, a collection of 11 modules that provide introductory-level information.