Fire in the Willamette Valley margin
Forested areas of the Willamette Valley margin are often drier than forests in the neighboring Coast Range and Cascade Range. Yet they are sometimes lumped in with moist westside forest types. The warm and dry forest type of the Willamette Valley margin was historically maintained by frequent low- to mixed-severity fire. Human-caused fire, both intentional and accidental, was (and is) the primary ignition source. Before the late 1800’s, cultural burning by Indigenous people in adjacent grasslands and woodlands likely affected much of the valley margin area and helped maintain relatively low fuel conditions.
With Euro-American settlement and forced cessation of burning practices by local Tribes, dense forest patches now grow on sites that were historically subject to more frequent fires that maintained open forest conditions. In these more heavily wooded areas, weather conditions, such as humidity, wind, temperature and increasing drought are now the major drivers of fire behavior.
The valley margin is subject to significantly warmer and drier air in summer compared to adjacent higher elevation areas of the western Cascades or eastern Coast Range. Rainfall from June–September is often less than 2 inches. This seasonal low humidity and lack of rain leads to increased plant moisture stress, drier fuels and higher fire risk. The effort to maintain more fire-resistant vegetation and infrastructure is worthwhile, especially given the high use and high values at risk, including homes and communities in the valley area.
Willamette Valley margin forests
The valley margin is a transitional area between historically open grassland and woodland ecosystems and heavily forested slopes of the eastern Coast Range or western Cascades. Elevation ranges from 300 to 1,200 feet with precipitation from 35–55 inches annually. Woodlands and forests in the valley margin have developed on a variety of soil conditions. Shallow soils are common (due to poor drainage, clay layers or rock) and soils often dry out in summer, which can stress Douglas-fir, western redcedar and grand fir, making them vulnerable to insects and disease. Deeper forest soils with high water-holding capacity are also common, mitigating summer drought stress and supporting productive forest growth. Dense vegetation accumulates on these more productive sites, creating relatively high fuel hazards.
Douglas-fir, either naturally regenerated or planted, is the predominant tree species across a range of soils and topographic positions within the valley margin. This results in forest types ranging from dry Douglas-fir forests with a salal or snowberry understory to moister Douglas-fir forests with sword fern associations in the understory. However, historic oak and oak/conifer woodlands were common in areas that are now occupied by dense Douglas-fir stands. Remnant patches of Oregon white oak with Douglas-fir in the valley margin indicate the historic prevalence of more frequent fire that maintained open woodland types. Pacific madrone persists on rocky, south-facing upper slopes and ridgetops that were also subject to relatively frequent fire.
Western redcedar and grand fir are common shade-tolerant associates with Douglas-fir. These species are more susceptible to fire damage, and they have increased in abundance in the absence of fire and after selective logging of large Douglas-fir in the early 1900’s. Bigleaf maple is a common shade-tolerant associate that sprouts vigorously after fire or cutting and colonizes the understory in older conifer forest stands. Willamette Valley ponderosa pine, Oregon white oak and Oregon ash are found in shallow soil that is wet in winter and spring, drying out in the tree rooting zone in summer. Black cottonwood, red alder, white alder and Oregon ash are common along moist lower slopes, floodplains and riparian areas.
Woodlands and forested patches are often interspersed with farm and rural-residential lands, in a fragmented landscape with dense road systems. Human ignitions increase with human presence, so chances of ignition are higher in proximity to developed areas and rural-residential areas compared to the larger forest zone.
Fire resilience strategies for small woodland owners in this forest type
Assess your situation and plan to address fire hazards, starting with the “Home Ignition Zone” or HIZ. The HIZ includes structures and everything around it out to 100 feet (or 200 feet on steeper slopes). If you have structures on your forest property that you want to protect, focus on improvements to the structure itself (by upgrading to fire-resistant building materials like a metal roof, for example) and modifications to vegetation in the first 100 feet around the structure. These treatments are the most likely to reduce the chance of structure ignitions due to a wildfire.
Become familiar with your soils, topography and forest vegetation types to understand forest productivity and management options in context of fuel hazard reduction and other objectives. The valley margin has a range of growing conditions across diverse soils, topography and climate. These conditions transition from warm and dry low elevations to cooler/moister conditions at higher elevations or north-facing slopes. Landowners can apply a combination of forest and fuels management approaches and techniques for both dry and moist forest types, depending on site-specific conditions.
Warm/dry sites have lower growth rates with opportunities to manage for widely spaced trees, open canopies and low ground fuels with frequent low-intensity maintenance treatments. More moist soil/site conditions favor higher growth rates, higher fuel hazards, and dense ground vegetation in open canopy conditions. The high rate of vegetation growth requires increased effort and costs to maintain low fuel hazards. Here it may be appropriate to maintain more closed canopy conditions to reduce understory growth and ground fuels. Across the range of site conditions, it is advisable to reduce ladder fuels (ground fuels that connect with higher canopy layers) and favor large-diameter, well-pruned trees that are fire resistant.
Modifying or reducing fuel to limit a fire’s ability to spread rapidly in strategic areas (called “fuel breaks”) is a key element of a fire-resilient or -resistant landscape. Identify locations for fuel breaks based on site-specific risks and existing features that create management opportunities. Consider the locations on your property that you most want to protect (structures, riparian areas, a special stand of trees or access roads, for example) and create fuel breaks to buffer these valued assets. Identify areas with the highest risk of wildfire ignitions and develop fuel breaks around these to reduce the chance of wildfire spreading into your woodlands. This may include along property lines or publicly accessible roads. Consider opportunistic areas to enhance natural fuel breaks, such as roads, rock outcroppings, ridgelines or riparian areas with a significant number of hardwoods.
Maintaining a diversity of forest types, with gaps and fuel breaks, results in a more fire-resilient landscape compared to large areas in one type of continuous cover, such as a Douglas-fir forest. Look for ways to enhance natural variation. Incorporating a hardwood component, (oak, maple, alder, ash or cottonwood, for example) can improve the fire resilience of conifer-dominated woodlands. Hardwoods in general maintain moisture and are less susceptible to ignition. However, heavy moss accumulation and dry leaves on some hardwoods can be a fire hazard under extremely dry conditions.
In dense, more productive forest types, it is usually necessary to use thinning, mastication, pruning and piling (with or without burning) to reduce fuels. Once fuel loads are reduced, there may be opportunities to conduct understory burns, which are often more cost-effective than other forms of understory maintenance. This may be most appropriate in areas with more oak components, open meadows or patches of low-density conifers.
We are still learning
As we face an era of increased wildfire occurrence in the region, we need to learn more about managing forests for wildfire resilience in the Willamette Valley margin. There are tradeoffs to consider when managing for wildfire hazards and risks. Fuels reduction treatments, maintaining more diverse patches, lower stand densities and mixtures of hardwoods may increase costs and reduce timber production values. Thinning and shaded fuel breaks can open the canopy and reduce the spreading of a crown fire. But opening the canopy may invite intensive growth of understory vegetation, including invasive species, and increase fuel loading on the forest floor. If woody debris (slash) from thinning is not adequately reduced, fire hazards are increased temporarily. Opening the canopy may have the effect of drying out the understory as more sunlight is able to penetrate. The appropriate density of thinning 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.
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.