Anthropogenic Influence of Wildfire Regimes in the Willamette Valley (16 March 2024; ENVS)

 In 2020, the Willamette National Forest experienced three major wildfires that, combined, burned more than 170,000 acres of national forest land (“2020 Willamette”). Oregon is no stranger to wildfires, observing them nearly every summer, making headlines, and oftentimes with smoke lingering over homes and cities. While wildfires have always been a part of nature, it is difficult to argue that humans have played no role in their increasing prevalence over the last few decades. A history of fire suppression and human-accelerated climate change have worked together synergistically to the detriment of forest land (Abatzoglou et al. 2016). In this paper, I’ll explore the anthropogenic influence on wildfire regimes in the Willamette Valley. 

The Willamette Valley can be characterized as having high severity, infrequent burns, especially when juxtaposed with eastern/southern Oregon, areas which tend to have high frequency but low severity burns (Reilly et al. 2022). Burn severity, in this context, is defined as a wildfire's direct effect on the environment. An area with low burn severity has little or no soil/vegetation damage, coupled with potential long-term benefits to soil health; an area with high burn severity reduces biomass over time, increases soil damage, and makes it far more difficult for an ecosystem to recover. Because of this, severe fires might permanently alter the makeup landscape’s ecosystem (Grabinski et al. 2022). In nature, this is a normal disturbance, just as climate change is; however, in both cases, human contribution and acceleration has turned both natural phenomena into environmental hazards.

Given historic fire trends in the Willamette Valley, a clear recipe for its high-severity fires emerges. These often large fires all have an ignition source, occur during a late summer drought, and importantly, with a strong synoptic (wide-spread) east wind event. Early records of these high-severity fires exist, dating back to late colonial times, in which colonizers would set brush clearing fires which would then be spread by the east wind and morph into large, severe fires (Reilly et al. 2022source).

Because these large fires are dependent upon a late summer drought, one of the most obvious anthropogenic factors at play would be human-accelerated climate change, which may exacerbate or prolong those drought periods. A longer summer drought means there’s a larger window in which the high-severity fires that characterize the Willamette Valley can occur, and a more severe drought means that those fires might have more fuel: “The duration of the fire-weather season increased significantly across western US forests (+41%...) during 1979–2015” (Abatzoglou et al. 2016). Climate change’s most detrimental impact on fire regimes is its modulation of available fuel. According to a study on the impact of anthropogenic climate change (ACC) on western US fires, increased temperature, vapor pressure deficit, and fuel aridity (figure 1) have contributed to forests experiencing 75% higher fuel aridity during fire season between 2000-2015, and 55% from 1979-2015 (Abatzoglou et al. 2016). 

Figure 1: Correlation between fuel aridity and forest fire area in the western US (Abatzoglou et al. 2016).

Figure 2: Quantifying the anthropogenic climate change (ACC) impact. Charted is a model from 1984-2015; the red line is the hectares burned based on current ACC, the black line without ACC, and the orange line is the difference between the two (Abatzoglou et al. 2016).

Climate change is far from the only factor in the Willamette Valley: In fact, the article that discusses the impacts of ACC supports the claim that increases in fire activity due to ACC are supplemented “by the co-occurrence of changes in land management and human activity that influence fuels, ignition, and suppression” (Abatzoglou et al. 2016). Logging plays a significant role in increasing fire activity in the Willamette Valley. Westerners have been utilizing logging practices to extrapolate forest resources since the early colonial period. Contemporary logging practices often result in clearcut patches, easily distinguishable from the older stands of trees that surround them. Despite advances in logging techniques over the last few hundred years, the practice continues to produce unnaturally high fuel loads. 

Following a clearcut, the harvested area must be replanted with new, smaller trees; these stands have very short crown times (ceteris paribus, the time it takes for fire to reach the upper levels of a tree), an indicator of fire spreadability. These small trees have thinner bark and are more likely to burn severely, reaching high temperatures much quicker than older and thicker trees during low-moderate fire weather (Reilly et al. 2022). On Vancouver Island between 1950 and 1992, logging was the number one culprit for the consistent increase in acreage burned year over year. Additionally, during the timeframe of a study done in the Sierra Nevadas in 2009, researchers discovered that the observed increases in fire severity were due to forest fragmentation, a result of patchiness caused by clear-cuts (Dickey 2018). When an area is harvested, companies often leave behind what’s called “slash” material (that which is left behind after a timber harvest), which then dies and dries, contributing to an increase in fuel. If these fuel-rich patches are exposed to an ignition source, the likelihood of a stark increase in intensity and subsequent spread to other (older) stands is dramatic. To reduce the risk of fire, alternative logging practices have been suggested, such as thinning very large patches rather than clear-cutting smaller patches (Dickey 2018). Doing so would make fire spread much more difficult, and would mitigate fire intensity by increasing average crowning time over current practices. These suggestions come with their own host of issues, which tend to be more impactful when the checks are written. Thinning forests is a more difficult operation, which ultimately translates to being more costly. Policymakers should weigh the cost difference between thinning forests and capital investments lost during wildfires. Additionally, harvests should be scheduled around fire season to reduce the likelihood of unintentionally starting a wildfire whilst harvesting trees (Dickey 2018). Yet, logging practices are only effective to a point; during high-fire weather, these changes make little difference.

Figure 3: Clearcuts in federal public lands documented by Oregon Wild using aerial photography and logging records. Clearcuts in private, state, and tribal lands were inventoried using Landsat imagery from 2001 to 2017 (“Logging”).

In the 2020 Labor Day fires, residents of the Willamette Valley (WV) witnessed the largest Oregonian burn in recent history. The valley received its signature “high severity” fire, driven by a synoptic east wind event, burning more than 150,000 acres in just a few days. On average, the fires were 75% more severe than all WV fires since 1984 (Reilly et al. 2022). While forest management practices, such as advances in logging technology are often beneficial in mitigating adverse wildfire behavior, such practices become obsolete in “high fire weather,” which is when conditions meet those listed in the recipe earlier in this paper. In low-moderate fire weather, forest management may make a significant difference, but in these conditions, all stand classes burn with equal severity. This is because the Labor Day fires, and similar monumental fires, are weather-driven (Reilly et al. 2022). Additionally, fuel treatment, which tends to work in many parts of the nation to mitigate large fires, is also ineffective during high fire weather conditions; evidence of this is that each stand class burns with equal severity. WV forests are highly productive, regardless of stand class, producing very large amounts of fuel to burn during extreme conditions (Reilly et al. 2022). 

Climate change and logging are crucial factors in increasing wildfire activity, especially when it comes to spreadability and severity. However, each fire must begin somewhere; the Labor Day fires began as a result of a downed power line. Anthropogenic wildfire ignitions are a continuing problem in the Willamette Valley (figure 4). Controlled burns to the benefit of biodiversity, such as those carried out by Native Americans to maintain the oak/prairie landscape of the Willamette Valley are exempt from this critique, as they are not “wildfires” as such (Hamman et al. 2011). Natural ignitions by way of lightning are much less common in the Willamette Valley than in other parts of Oregon where other fire regimes preside (high frequency/low intensity); generally, lightning ignitions occur at higher altitudes. Thus, the overwhelming threat is anthropogenic ignitions (Reilly et al. 2022). 

Based on mapping done in Sheehan 2011, human ignitions are strongly correlated with proximity to roads, and are correlated with road density. Sources conflict as to whether or not more ignitions occur near main roads, such as I5, or backroads, such as forest service roads that run through the foothills of the cascades. What is clear is that ignitions steadily taper off as the proximity to a road decreases. Backroad correlation is linked with recreational activities, such as camping or hiking. For future study, researchers should map the correlation between human ignitions, weather, and large fires to better understand and mitigate wildfire risk (Reilly et al. 2022).

Figure 4: Human and lightning ignitions in the Willamette Valley. The hot spot of green dots in the upper left quadrant of the map is Portland, with the map extending past the cascades to the east, where lighting ignitions are most prevalent. Ignitions from 2000-2009, mapped in Sheehan 2011.

To propose a precise forecast of when and where a characteristically severe wildfire will occur in the Willamette Valley would be to propose an oxymoron. Contemporary forecasting tools can provide only a vague estimate at best. Rather, planning should occur with reference to other “doomsday-esque” events, similar to what we see with planning for the Cascadia earthquake. East wind fires can only be stopped when the fire that's driving them weakens or ceases, and should be treated as immune to fuel treatment. At best, fire suppression would only slow down the fire’s westward expansion. 

Figure 5: Part of a fire-weather warning posted by the USDA in September 2022, showing a similar east wind to that seen immediately preceding the 2020 Labor Day fires

Human influence has undoubtedly altered wildfire regimes in the WV. To at least mitigate anthropogenically enhanced wildfires in the WV, there are a few things we can control. First and foremost, controlling and monitoring ignitions is the most direct way to prevent these fires from occurring. This begins with education and public transparency, teaching responsibility, and explaining the mechanisms of how wildfires spread with ease under certain conditions. These social strategies may prove to be more cost-effective than fuel treatments, which are ultimately a short-term and relatively ineffective solution to a long-term larger problem. That said, fuel treatments still have their place in protecting high-value assets (such as neighborhoods) during non-weather-driven wildfires (Reilly et al. 2022). The development of more advanced fire monitoring tools can aid in quelling an ignition before it turns into a large-scale wildfire during high fire season. Additionally, through the use of more responsible forest management, such as stand thinning rather than clearcutting, we can reduce fire spread attributable to fragmentation and patchwork. Finally, the most difficult to attain and bigger-picture threat would be to slow or reverse the acceleration of anthropogenic climate change. Until that larger goal can be obtained, planners should focus on the mentioned localized small-scale changes to better combat wildfires in the Willamette Valley.

Works Cited

Abatzoglou, John T., and A. Park Williams. “Impact of Anthropogenic Climate Change on Wildfire across Western US Forests.” Proceedings of the National Academy of Sciences, vol. 113, no. 42, 10 Oct. 2016, pp. 11770–11775, https://doi.org/10.1073/pnas.1607171113.

Dickey, W. (2018). Spatial Analysis of Human Activities and Wildfires in the Willamette National Forest (Order No. 27811948). Available from ProQuest Dissertations & Theses Global. (2408542782). Retrieved from https://uoregon.idm.oclc.org/login?url=https://www.proquest.com/dissertations-theses/spatial-analysis-human-activities-wildfires/docview/2408542782/se-2.

Grabinski, Zav, and Chris Smith. “Burn Severity Explained.” ArcGIS StoryMaps, 14 Mar. 2022, https://storymaps.arcgis.com/stories/ed9018a155af47ab904e5e0db5da3e14.

Hamman, S., Dunwiddie, P., Nuckols, J., & McKinley, M. (2011). Fire as a Restoration Tool in Pacific Northwest Prairies and Oak Woodlands: Challenges, Successes, and Future Directions [Review of Fire as a Restoration Tool in Pacific Northwest Prairies and Oak Woodlands: Challenges, Successes, and Future Directions]. Northwest Science, 85(2), 317–328. https://bioone.org/journals/northwest-science/volume-85/issue-2/046.085.0218/Fire-as-a-Restoration-Tool-in-Pacific-Northwest-Prairies-and/10.3955/046.085.0218.full.

“Logging in Oregon.” Logging.oregonhowl.org. Retrieved March 17, 2024, from https://logging.oregonhowl.org/.

Reilly, Matthew J., et al. “Cascadia Burning: The Historic, but Not Historically Unprecedented, 2020 Wildfires in the Pacific Northwest.” Ecosphere, vol. 13, no. 6, June 2022, https://doi.org/10.1002/ecs2.4070.

Sheehan, T. (2011, December). Modeling wildfire and ignitions for climate change and alternative land management scenarios in the Willamette Valley, Oregon [Review of Modeling wildfire and ignitions for climate change and alternative land management scenarios in the Willamette Valley, Oregon]. ProQuest; Department of Biology, University of Oregon. https://www.proquest.com/docview/917736866?pq-origsite=gscholar&fromopenview=true&sourcetype=Dissertations%20&%20Theses.

“2020 Willamette Wildfires Information” Usda.gov, 2020, www.fs.usda.gov/detailfull/willamette/fire/?cid=fseprd835361&width=.