Wildfire activity has increased markedly and steadily in the Western United States since the 1970s, causing damage to ecosystems as well as human property, health and life (Bowman et al., 2020). This has led to proposals to combat the problem by intentionally reducing the density of vegetation in much of US forestland so that there is less material to burn when fires occur (often called “fuel treatment”). In particular, the Biden administration has introduced a plan to use mechanical thinning (vegetation removed with machinery) and prescribed fire (starting controlled fires intentionally to remove surface vegetation) to treat fuels on 50 million acres of US forests by 2030.
In a July 31 New York Times op-ed, Chad Hanson, from the John Muir Project, and Michael Dorsey, from Arizona State University, disagree with this plan and others like it. They argue that policies should focus on protecting US forests from efforts to reduce fuel loads, efforts they refer to as “commercial logging.” Much of their case draws on the negative aesthetic of loggers harming pristine forests and ecosystems. Their article is illustrated with a picture of felled trees in a national forest, and they colorfully describe chain saws, bulldozers, and “dinosaur-like logging machines.” Aesthetics aside, their primary substantive claim is that reducing vegetation density in forests actually increases wildfire danger rather than decreasing it. Their claim flies in the face of decades of well-tested, on-the-ground firefighting experience, empirical experiments, and advanced modeling. It is even ill-supported in their own op-ed.
It is widely accepted that the two main contributors to the increase in wildfire activity in the US West over the past several decades are 1) the increase in aridity associated with climate change and 2) ill-advised policies, starting in the early 1900s, to suppress all fires, including natural wildfires (Fourth US Climate Assessement). These natural wildfires (as well as intentional burning by indigenous populations) would have cleared surface vegetation periodically, but their suppression has caused vegetation in forests to build up to unnaturally high levels. Prior to western settlement, much of the yellow pine and mixed conifer forests of US West experienced fires about once every decade. Most of these forests have now not burned since the early 20th century (Van de Water and Safford, 2011). This is one of the reasons that it is wrong to think of the current state of the forests as pristine and natural; humans’ recent artificial suppression of fires in these fire-adapted ecosystems is as "unnatural" as logging itself.
The century of excess buildup of vegetation leaves these forests much more vulnerable to the most severe and harmful wildfires. For one, increased vegetation makes the ecosystem more sensitive to drought due to increased competition for water. Second, there is much more fuel to burn when fires occur. And third, so-called ladder fuels allow fires to more easily reach the canopies of forests where they can do more damage to mature trees and spread more rapidly. All this creates a situation where fires can be much more intense, unpredictable, and more likely to kill the entire forest (so-called stand-replacing fires). The excess fuels also increase the smoke that has such a detrimental impact on air quality and human health (Wang et al., 2021).
In response to this situation, the Biden administration’s plans to reduce tree density only to what it was prior to the 20th century. The graphic below illustrates how such “ecologically managed forests” allow for fires that are of lower intensity and spare the most mature trees.
Graphic by Erica Simek Sloniker via USDA.
The conceptual model above has been continuously validated in practice. A nice visual example is the photo below of the aftermath of Oregon’s 2021 Bootleg Wildfire. The fire destroyed much of the forest that had no recent removal of fuels (right), but flame lengths drastically decreased as they progressed through a region where mechanical thinning and prescribed fire had removed fuels (middle). On the left, the fire progressed into a region that had been mechanically thinned but had not yet experienced prescribed burns. There, the damage to the most mature trees was again larger.
© Steve Rondeau, Klamath Tribe, Nature.org
This reinforces a truth known to forest managers: a combination of mechanical thinning and prescribed burns is better for reducing wildfire severity than either tool alone (Vaillant et al, 2009) and far better than doing nothing. This is because mechanical thinning can reduce canopy base height, making it harder for flames to climb into the canopy and spread quickly, but it can leave too much fuel on the forest floor. Clearing out that excess fuel with prescribed burns makes it particularly difficult for wildfires to produce the most devastating impacts. Finally, fuel treatments also give firefighters the time and ability to contain wildfires threatening communities, as was exemplified during California’s 2021 Caldor Fire.
Together, theory, firefighting experience, and rigorous study have led to a strong mainstream consensus within the wildfire science community that reducing fuel loads reduces the likelihood of the types of fires that produce the most smoke, are the least controllable, and are the most likely to kill the forest’s mature trees (Agee et al., 2000, Pollet and Omi, 2002, Martinson and Omi, 2003, Finney et al., 2005, Agee and Skinner, 2005, Raymond and Peterson, 2005, Ritchie et al., 2007, Moghaddas and Craggs, 2007, Roccaforte et al., 2008, Schmidt et al., 2008, Vaillant et al., 2009, Graham et al., 2009, Earles et al., 2014, Krofcheck et al., 2017, Tubbesing et al., 2019, Sanchez et al., 2019)
Yet despite the overwhelming evidence, Hanson and Dorsey make the conspiratorial claim that Yosemite National Park’s plan to reduce tree density to its pre-twentieth century state is only under the “auspices” of thinning for wildfire control because the US forest service is “in the commercial logging business.” In support of this, they claim that reducing fuel loads actually increases wildfire severity rather than decreasing it.
One of their contortions to arrive at this conclusion is that they equate wildfire severity with overall tree mortality, and then they consider trees removed during the fuel treatment process as counting towards overall tree mortality. This reasoning would make sense if the goal was to maximize the total number of individual living trees in the forest. However, we do not wish to maximize the number of trees but rather protect the existence of the forest itself, and human settlements, from the most devastating wildfires.
In addition to claiming that fuel treatments do not maximize the number of living trees, Hanson and Dorsey claim that reducing fuels does not reduce the severity of fires because thinning the forest creates windier conditions and reduces “the cooling shade of the forest canopy.” As demonstrated by the studies and examples above, any wildfire-enhancing effect from reduced windbreaks and shade tends to be overwhelmed by the wildfire-mitigating effect of reductions in fuel. Thus, this is an extraordinary claim that requires extraordinary evidence.
Hanson and Dorsey do not provide such evidence. Instead, they offer 1) Chad Hanson’s own correlation-based study of a single fire and 2) a link to an advocacy letter “signed by 200 scientists and ecologists”. The advocacy letter does cite a smattering of studies supposedly supporting the notion that fuel treatments do not reduce wildfire severity. But when one looks at these studies in detail, they mostly show situations where the effects of fuel treatments were inconclusive, or they demonstrate that the effects of fuel treatments vary substantially depending on many other factors like weather conditions, other fuel characteristics, and topography. This is completely uncontroversial.
One point that comes out from the literature cited in the letter is that intensive plantation forestry is particularly prone to severe wildfires. This is because plantations can be very densely packed with relatively young and thus more fire-vulnerable trees. While noteworthy, this point is irrelevant to current proposals like the Biden administration’s plan because they intend to decrease fuel loads in US forests, not convert them all to plantations.
A final argument that Hanson and Dorsey make is that removing excess vegetation from US forests increases carbon emissions. It is true that US forests are currently acting as unnaturally large reservoirs of carbon because they currently have unnatural amounts of vegetation. Thinning does reduce this stock of carbon, and to the extent any of this biomass is burned, for example, for energy, then yes, it does increase atmospheric CO2 concentrations relative to the forest unsubjected to wildfire. However, the forest unsubjected to wildfire is a fantasy. Sooner or later, these forests are going to experience fires. We can forgo fuel management, and when fires inevitably return, they will burn intensely, with ultimately higher CO2 release, or we can undertake fuel management to assure that they will burn less intensely and have less negative impact on mature trees, wildlife, human property, and human health.