Fire and Fuels

So the current fire retardant issue is  interesting, not least because the current and past fire retardant lawsuits have been brought by one of our very own TSW contributors, Andy Stahl.  Who may or may not be able or want to talk about it much. It doesn’t seem like the agency “cone of silence” around litigation applies to plaintiffs.

Anyway, I’ve spoken to current wildland firefighters, retired fire folks, and Andy, and as usual tried to fit all the points of view into a whole zone of agreement.  I was listening to, of all things, a podcast called  “The Witch Trials of J.K Rowling” developed by The Free Press.  On one episode (Chapter 6), a teenager named Noah spoke about (on a completely different subject) encouraging the good things that can happen and discouraging the bad things that can happen.  And of course, isn’t that applicable to many policy issues?

What makes fire retardant more interesting than many of our TSW debates, I think, is that we agree on what good things and bad things are.. or do we?  Good things would be helping fire suppression folks save valuable human and ecosystem assets, and for human health and safety- of firefighters and residents.  Bad things would be dumping in streams or on firefighters. So first TSW discussion question… Do we agree on good things and bad things? Are there more good things or bad things I haven’t considered?

But the meta-dialogue is “how best to increase the good things and decrease the bad things?”  Who should be in the room to decide? Should there be carrots or sticks or some combination?  How are wildfire practitioners and the air resource industry involved?

If I had to guess about Andy’s motivations, it would be to put pressure on the USG to work harder to decrease the bad things.  But the danger with this approach is that involving another federal agency- in this case the EPA-may not be actually all that helpful in terms of adding value to the work- the ultimate work of trying things and doing lessons learned that the fire community is generally better at than most.

So let’s think about some other examples.  For some reason, I thought of healthcare-associated infections. The below is from the Florida state website.

The Agency for Healthcare Research and Quality has shown that the implementation of recommendations for HAI prevention from the CDC Healthcare Infection Control Practices Advisory Committee (HICPAC) can reduce HAI by 70% overall and virtually eliminate some specific types of infections.  Broad implementation of the CDC guidelines saves lives, reduces suffering, and decreases health care costs.  Through partnerships and the commitment of stakeholders, the Florida Department of Health (FDOH) HAI Prevention Program supports health care facilities implementing best practices for preventing the spread of health care-associated infections. Health care facilities are asked to conduct surveillance or track HAI infections, ensure health care workers perform hand hygiene before and after patient contact and when they come in contact with body fluids, use personal protective equipment such as gowns and gloves, minimize use of devices (i.e. catheters), and ensure the patient care environment has been cleaned including the proper cleaning of shared medical equipment such as blood pressure cuffs and glucometers.

So suppose a similar approach was taken for fire retardant. Maybe we would disagree on who would be the best people to put on the advisory committee (HICPAC)   Or would we?

It seems as if in this health case, it is assumed that everyone wants HAIs to be reduced and the academics and practitioners work together to figure out ways to do that. On the other hand, what might make the fire retardant issue is that big bucks are involved and specialized technologies in extreme and dangerous conditions.  Not exactly handwashing or antiseptics.

An Advisory Committee of knowledgeable and experienced people could help add to knowledge and dive into the weeds of specific practices, jointly look at accidents or errors, different formulations, new technologies and lessons learned.  And actually with the big bucks out there for new technologies (drones, AI, and so on) maybe an advisory committee should tackle all that? And no, I don’t think the current Wildland Fire Commission has the right folks to take that on.

With open meetings, public comment and all that.  Because it seems like trust is super-important.  As in the classic difference between the chicken and the pig at breakfast- the chicken is involved, but the pig is committed.  Similarly, communities in wildfire areas (which can be much of the US) may not want to be guinea pigs for new technologies- nor may wildland firefighters.  So it seems to me in this case, the more involvement and trust-building the better.

To me, this would seem like a more useful approach than asking the (in their words) overburdened and (in my words) currently non-expert EPA to do a rule and then apply for a permit.

Do you have other examples of mechanisms for how agencies work to increase the good, and reduce the bad, of practices?

And people have studied aerial firefighting.. I think if you read Bill Gabbert’s post here you’ll get some idea.  What I got from it was that you need to actually know the business to know what needs to be improved and how. Which most of us do not.

I’m one of many people who has had the opportunity to be in interagency conflict resolution meetings, in my case FS, ACE and EPA.  My own experience (based on a few incidents) is that some EPA employees have an attitude toward land management agencies that is not conducive to collaboration.  Their attitude seemed to be “we know more than you do”, which they didn’t, and “we are more righteous than you” which they weren’t.  At the same time, I have to say that one of the best people I’ve ever worked with was a political appointee at EPA (a lawyer and law professor) during the Clinton administration. So I’m not saying they’re all like that.. but I’ve run into a few.

The theme of the next few posts  is “right hand and left hand” or… are federal agencies aligned in the same direction? I think all of us agree that federal tax dollars should not be wasted or mismanaged by opposing government programs (if it can be avoided) and that for important topics like decarbonization or wildfires, agencies should be aligned.  It’s the administration’s job to make sure that happens and enforce it within the executive branch. We’ve discussed before this EPA PM 2.5 thing. Too bad the GAO didn’t explore how the proposed rule could have gotten all the way to OIRA before anyone noticed it was a problem. That could really shed some light on coordination processes and efficacy. Thanks to alert TSW readers for noticing this GAO report.

Here’s a link to the GAO Report, published on March 13, titled “Wildfire Smoke:Opportunities to Strengthen Federal Efforts to Manage Growing Risks.”

It could be that there are so many wildfire commissions and task forces nowadays that the idea of high-level officials actually engaging with each other and resolving disputes has fallen by the wayside. Which is of great concern. So while the EPA says they will respond to the need for “better coordination” with land management agencies, they haven’t actually pulled the plug on revising the 2.5 NAAQS. Maybe they have sent signals to high level folks at USDA and Interior, but it would be nice for us here in the cheap seats to also get the message.

Here is the National Association of Forest Service Retirees letter on the proposed rulemaking, which I also received last week. If they are standing down on this for prescribed fire, it would save work by many in our space. If they are not standing down- well actions speak louder than words about “coordinating.” Maybe the need is for less “coordination” and more “woodshed.”

I’m always a fan of coordinating research..does this seem like something the USG should institute as a matter of course for research topics?

Additionally, officials from the CDC said that creating a framework for coordinating research related to community preparedness for wildfire smoke could help federal agencies more intentionally plan such research and create a community of practice on the topic.

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Here’s the air curtain discussion…

Also, to remove barriers to certain wildfire risk mitigation strategies, EPA officials said that the agency could, for example, finalize its proposed rule related to permit requirements for air curtain incinerators. Air curtain incinerators are devices for burning debris collected through methods such as mechanical thinning.92 According to EPA officials, these devices offer an alternative to prescribed burns and have much fewer emissions than burning debris piles or prescribed burns

And ..According to Forest Service officials, the use of air curtain incinerators helps reduce woody fuel on a site but does not necessarily reduce the need for prescribed burns to reduce fine surface fuels.

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I thought it was interesting to compare the views of the FS and BLM (Interior), who conceivably have the same issues:

In its written comments, USDA stated that the role of the Forest Service in responding to wildfire smoke and protecting public health, as well as the importance of mitigation efforts to address smoke impacts associated with
the wildfire crisis, was captured well in our report, and that this role and mitigation are critical to long-term efforts for wildfire risk reduction. USDA also stated that, as the environmental impacts of catastrophic wildfire
extend far beyond air quality, effectively implementing our recommendations requires focused collaboration beyond smoke and its impacts to public health. USDA said that only focusing on the effect of wildfire smoke on public health minimizes the breadth of the current crisis impacting the natural and human environment and neutralizes the most effective mitigation tool that also mimics natural processes—prescribed fire, which, according to USDA, can be managed to minimize impacts on public health.

In addition, USDA said that, as air quality standards become more stringent, expanded interagency discussions are needed to ensure the increased use of prescribed burning as the primary mitigation to catastrophic wildfire. USDA said that bolstering current authorities and approaches mentioned in our draft report should balance impacts on firefighter and public safety, water quality, and protection of municipal water supplies, among other environmental effects. As discussed in our report, EPA has raised other concerns related to the increased use of prescribed burning. We believe USDA would have important opportunities to raise these and related issues as it works with EPA and Interior to
implement our recommendation to better align air quality and land management goals.

Here’s Interior, which seems like the answer is “we’ll hire more people.”

In its written comments, Interior stated that to achieve our recommendation to work with EPA and USDA to better align air quality and land management goals, it plans to increase staffing to plan for and manage smoke emissions at the departmental and bureau levels and to work across agencies at the national and regional levels, as well as with tribal, state, and local governments and other external partners. Interior also stated that its management of air quality and wildfire risk mitigation goals will include an increasingly wide array of communications, data management, planning, budget development, wildfire operations, environmental justice efforts, and fuels management implementation, which will be supported by its additional staffing. Interior said these efforts will be initiated this year and will enable coordination of its existing efforts with EPA and USDA and the joint development of further efforts. Interior stated that this will support efforts to increase the pace and scale of fuels management treatments and address the overall wildfire risk reduction objectives included in the  Infrastructure Investment and Jobs Act. The actions Interior described, if implemented effectively, would address our recommendation.

For some reason, there is a tendency in some outlets to talk about the Jefferson County controversy about fuel treatment in some open spaces, and pivot from there to the national debate about federal lands. it’s interesting to  see how different reporters cover it.   Apologies for this being so long, but I got very interested in the different takes on the same subject,  as well as the omnipresence in these stories of a certain Californian.

Here’s the most recent by the Denver Post:

Tagline:

Logging opponent describes “complete shock and horror” at the sight of felled trees

Evergreen resident JoAnn Hackos, who also serves as a board member with the Evergreen Audubon chapter, said Jefferson County is targeting too many mature trees.

“I’ve seen truckloads of large, old-growth trees being driven away from our neighborhood parks,” Hackos said. “There is lots of money to be made in selling big trees, but it irreparably damages the forest.”

The county, she said, is using a technique called mastication, which essentially chops up treated areas into mulch. Hackos said the practice disrupts the soil and damages roots.

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Chad Hanson, forest and fire ecologist with the John Muir Project in Big Bear Lake, Calif., said Jefferson County “is doing everything wrong.”

“Removing mature trees increases wildfire spread and severity,” he said. “When they do these logging projects under the guise of thinning, that reduces the cooling shade of the canopy. Denser forests do not burn more intensely.”

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But Germaine said it’s a misnomer to call the ponderosas that are being cut old-growth trees, a term that evokes more of an emotional response.

“Ponderosa pine trees only begin to take on old-growth characteristics between 200 to 300 years of age, and they may live to 400 to 500 years,” he said. “Some of the trees we are cutting are large, and some are approaching 125 years in age, but none are old-growth.”

And the notion that the county is making a windfall from timber sales resulting from the felling is simply untrue, Germaine said.

“We hire local small business people to do most of our forest thinning. A lot of the material is ground up and spread around on-site because it has no market value,” he said. “We hold firewood sales to provide wood to local residents, and the county does not profit from any of this.”

Jefferson County pays contractors about $3,500 an acre for thinning, Germaine said.

Steve Germaine is the Natural Resources Supervisor for Jefferson County.

Here’s another  Denver Post article.

The government faces opposition from forest lovers and environmental advocates who contend logging contractors operating with minimal oversight often mow down trees — rather than thinning — converting forests to grasslands, which the opponents argue could actually accelerate wind-driven fire. They accuse federal authorities of short-circuiting legally required environmental impact reviews. They favor “fire-wise” home safety as a smarter way to shave wildfire risks.

The argument comes down to ecological nuance and costs, which range from $500 to $7,000 an acre. It can be more feasible for loggers to cut broadly across an acre or more, rather than thinning that sometimes requires hiking on steep and hard-to-reach terrain, lugging chainsaws to remove trees selectively and optimizing spacing and species diversity.

There are two claims we usually don’t see.. that contractors don’t follow requirements and that feds short-circuit environmental reviews. It would be handy to have a quote from a person here, who could later be asked about specifics.

Fury over forest cutting intensified this summer and a grassroots Eco-Integrity Alliance deployed a billboard in central Denver urging President Biden and Colorado’s senators to “stop wasting $3 billion” for logging national forests.

Here’s the link to Eco-Integrity Alliance.  It appears to be an alliance of groups including:

Eco Advocates NW

Friends of the Clearwater

Friends of the Wild Swan

John Muir Project

Protect Our Woods

Swan View Coalition

And I suppose a billboard costs some bucks, so there must be some funding associated with these groups.

What’s interesting about the Finley piece to me is he brought up Hanson as an advocate, not as the voice of science.

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Here’s a Westword article. Westword is one of those independent papers you usually see in newstands in coffee shops in cities.. lots of culture and marijuana ads.

First the article talks about how Jeffco Open Space decided to do the project, including the mapping by Colorado Forest Restoration Institute of CSU and a verbal description of the rationale.

As JCOS implements forest-thinning projects, it tries to cut younger trees out of areas that have more trees per acre than what it estimates would have been there without historic fire suppression.

Sound familiar.. a la East Side screens?

Not all forests fit that mold, though, including those populated by ponderosa pines, points out Chad Hanson, practicing ecologist and director of the John Muir Project, which works to improve ecological management of public forests.

“The science strongly contradicts that narrative,” he says. “This is true for forests all across the West. This is true in the Colorado Front Range. … Everywhere scientists have looked at this, we’ve found the same thing: that historical forests were much denser overall than the U.S. Forest Service, or some state agencies that are involved in logging, have told the public they were.”

An expert person on the ground says “our forests are like this’ to Chad saying “the science says that many are not”. I think the question is simply “is this true of the forest we are working int?”

Schlossberg’s (he of the Steering Committee of the Eco-Integrity Alliance- sf) concerns come not only from the lack of scientific considerations in the forest-thinning plan, but also from the age of some of the downed trees. According to Germaine, the department doesn’t remove trees that are technically considered “old growth.” Those trees provide habitat for small mammals and birds and, if they are over 150 years old, were likely around before European intervention in American forests. It’s important to eliminate younger trees to keep the forest healthy, he contends.

“We don’t get a lot of water here on the east side of the Rocky Mountains, so when those trees are too crowded, they’re competing,” Germaine says.

According to Brett Wolk, assistant director of CSU’s Colorado Forest Restoration Institute, ponderosa pine tree systems take centuries to develop, and the trees can live for 500 years.

Schlossberg still takes issue with the idea that cutting down trees that have lived for a century is the answer, especially when it comes to the ponderosa pine.

“Why is it that ponderosa pine can grow that old?” he asks. “The answer is because they’re fire-resistant trees.”

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“The notion that removing trees from the forest will curb fire has been soundly discredited,” Hanson says. “Wildfires are driven mostly by weather and climate, and therefore also by climate change. In drought years, you get the ignition, and you get hot, dry, windy conditions. Those are conditions for fires. It’s not mainly about forest density.”

In forest fires, trees like the ponderosa pine are rarely consumed; removing them doesn’t change fire intensity because they don’t contribute combustible materials. A study of California forest fires showed that even large wildfires consumed only about 2 percent of tree biomass.

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Hanson warned of emerging “sterilized landscapes” where forests once stood around cities.

The cited paper is by Harmon, Hanson and DellaSala (he who reviewed the Proforestation paper we discussed).  It’s perfectly OK to be against commercial logging.  It’s when you claim that “the science” says something and it turns out that we can find plenty of scientists who don’t agree.  To reporters: anytime someone says “the science says this,  ask them “are there any scientists who disagree with you?” and interview them.

They cannot go toe-to-toe with us on the scientific evidence,” he says. “Every time they try, they lose, so now, in desperation, they’re hitting below the belt, and they’re going personal and engaging in character assassination.”

Hanson believes that the logging industry relies on people buying into a positive narrative around cutting down trees, so it lobbies Congress to support forest-thinning management to keep that narrative alive. That lobbying trickles down to local forest management, he adds.

Or maybe other scientists  can go toe-to-toe, and have in the 10 Common Questions paper.  This is where some skepticism by the reporter might have paid off.

Here’s a link to a Colorado Sun story from last September. The Colorado Sun is a journalist-owned independent news outlet.

Again, Schlossberg

“What’s happening is we’re using the public lands as the sacrifice zones. We don’t think any of this tree cutting should be happening on public lands,” Schlossberg said. “It’s not justified scientifically or ecologically.”

A quote from Denver Mountain Parks.

The portions of Flying J that have raised the most public ire are actually controlled by Denver’s mountain park system, which cooperates with Jeffco when they have adjacent land.

“We got the short end of the stick at Flying J,” said Andy Perri, a Denver Mountain Parks program manager for forestry and natural resources. “We were dealt a very unhealthy forest on our two sections.”

Thick lodgepole stands on Denver’s portion of the property were plagued by beetles and dwarf mistletoe, which grows in the canopy and effectively places a tinderbox high in the forest. So Denver simply had to remove more, creating the shorn-land look that riled many visitors.

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“We try our best to contact neighbors, but it’s just me and two others in our program,” Perri said. Reaction ranges from extremely upset to extremely grateful, he added. “I try my best to explain the science — and we’re not here to sell timber, our wood is basically worth nothing.”

If a loud voice falls in the forest, does anyone hear? 

Hannah Brenkert-Smith watched some of the same thinning vs. perception dynamic play out during a 2,460-acre cutting program on Arapaho-Roosevelt National Forest lands around Nederland. The plan included a public conflict-resolution process, which Brenkert-Smith later studied from her position in the Environment & Society Program at the University of Colorado’s Institute of Behavioral Science.

“What we found is that the people who are in opposition were actually in the minority,” Brenkert-Smith said. “They’re just really vocal and well organized, and they get a lot of attention.”

Forest managers the researchers worked with were often braced for much more overwhelming opposition that never really materialized, Brenkert-Smith said.

“So one of the things that I think is really important is not to assume what the public is going to think. And also not to let loud voices shape the narrative,” she said. “You actually have to go out and find out what people think, and not just the people who have the time and the motivation to show up at everything, and to write all the letters and to harness their social capital, but the silent majority who tends to be supportive.”

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This story gets points for including local scientists, and a social scientist.

Les Joslin’s story last Saturday about Smokey Bear signage reminded me of Nagy et al. ‘s paper, which I wrote about on TSW here on Monday.  There must be something in the atmosphere, because today Nick Smith had a link to an article on this Higuera et al. paper in his newsletter today.

Here’s the story in the Bozeman Chronicle. A few interesting points..

A new study shows that structure loss from wildfires rose nearly 250% across the Western United States over the last decade and 76% of wildfires that burned homes and other structures were human caused.
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Just under 80% of the structures that burned in wildfires between 2010 and 2020 were located in California. While that state dominated the trend, the number of structures lost to wildfires increased overall in every Western state, apart from New Mexico.
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When it comes to policy solutions, Higuera sees several. First, people can re-emphasize the “Smokey the Bear” message, particularly later in the fire season. Second, they can consider large-scale changes in how, where and whether they build. (my bold)
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While solutions may differ from region to region, they could involve rethinking “how power gets transmitted across the landscape” and the affordable housing crisis, which is part of what’s leading to development in flammable vegetation, according to Higuera.

Here are the conclusions from the paper (open access, hurray!). I tried to remove all the page numbers but may not have been entirely successful.

Our study reveals key drivers of the wildfire crisis in the West, and therefore when, where, and which “levers” can be pulled to reduce the chances of future fire disasters. We focused on  structure loss as an acute negative impact of wildfires, highlighting a significant increase in structures destroyed by wildfires, by over 3x between 1999-2009 and 2010-2020 (Fig. 2B).  Critically, we also show that wildfires have become more destructive, with the number of 19 structures destroyed per 1000 ha burned increasing, by 160% between the last two decades (Fig. 20 2C). This West-wide pattern was dominated by California and from events in just three recent  years (2017, 2018, 2020), but nearly every Western state exhibited higher structure loss rates over the second decade of the 21st century (Fig. 4I, S4).

Increased loss rates indicate that wildfire-related structure loss did not increase simply due to  more area burning. Likewise, although our analysis did not resolve structure exposure to  individual wildfires, we estimate that the proportion of structures destroyed by wildfire outpaced  higher structure abundance in flammable vegetation alone (Table S1). The causes of increased  structure loss thus reflect complex human-environment interactions, not “either” more area burning “or” increased development, but both, and more. When and where human-related ignitions occur, how many structures are built among flammable vegetation, and how climate 30 and land use affect fuel abundance and fuel aridity over timescales of days to decades are all interacting to drive trends in structure loss.

Recognizing the complex drivers of fire disasters has important implications for mitigation and adaptation. For example, efforts to reduce structure loss must look beyond simply limiting area burned generically. In fact, across the West 88% of wildfires, accounting for 45% of the total area burned, had no associated structure loss (Fig. 1A). While not explicitly assessed here, much of this burning is not a crisis per se and can provide resource benefits (41–44). The wildfire crisis generally and structure loss specifically are largely driven by extreme events, highlighted clearly in recent years (Fig. 1B). The overwhelming majority of wildfires that result in structure loss are started by human-related ignitions, and they are occurring in regions with increasingly high structure density within flammable vegetation (21; Fig. 4J). Consequently, the number of  structures destroyed by wildfire in the West in any single year was well explained by the total  area burned from human-related ignitions; and, total structure loss in a state over the past two decades was well explained by the number of structures in flammable vegetation (Fig. 5). Both  elements have increased between the past two decades – area burned from human-related ignitions by 51%, and structure density by 39% (Fig. 4) – and are leading contributors to increased structure loss rates.

While numerous studies highlight increased fuel aridity from anthropogenic climate change as a  key enabler of rising wildfire activity in the West (6, 14–17), in parallel to these changes we have shown that human factors operating over shorter time scales are increasingly contributing to  wildfire disasters. The consequences of human-related ignitions, specifically when and where  fuel aridity is high and lightning ignitions are rare, are becoming magnified in the context of  climate change. Therefore, efforts to reduce human-related ignitions will be increasingly  important beyond historical lightning-caused fire seasons, for example in spring, fall, and even  winter months (Fig. 3F, S5).

(my bold, and I would add based on experience in my part of the country “the ability of fire departments to jump on them quickly when they do occur.”)

The patterns highlighted here varied widely across the West, emphasizing the importance of understanding spatial and temporal characteristics of wildfires – fire regimes – as integrated social-ecological phenomena (29, 30, 45). Mitigating anthropogenic climate change, given its impacts on fuel aridity (14, 6, 19), is a clear overarching necessity for addressing the wildfire  crisis. At smaller scales of individual states, policymakers and managers may benefit from emphasizing other aspects driving increased structure loss, including structure expansion into  flammable vegetation, increased ignition from human-related sources, or in states with below- average structure loss, reducing the chances of lighting-ignited fires spreading into developed  areas (32, 43). Mitigation and adaptation approaches in predominantly rural states, like  Wyoming, Montana, and Idaho, may look different than in more densely populated states, like  California, Colorado, and Washington. Additionally, states with low structure loss rates may  look to those with high structures loss as harbingers of future change, and ideally glean ways to avoid similar outcomes as structure expansion and densification trends continue. Two clear  implications emerge from this and other recent work (20, 21, 32, 33) to help prevent wildfires from becoming disasters: reduce unintentional human-related ignitions, particularly near homes  and during periods of extreme fire danger; and carefully consider if and how structures are built, including building with fire-resistant materials, minimizing flammable vegetation near structures, and providing mechanisms to do so equitably across socioeconomic conditions.

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I think most of us would agree , as the authors say, that the entire wildfire problem or issue is partially social (as is pretty much everything)  in nature.  So my questions are:

1. Where is the social science around these human factors and what interventions work to change them?

Unplanned ignitions from human sources – ereafter “human-related ignitions” – include backyard burning, downed power lines, escaped campfires, etc., and are a well-recognized component of contemporary fire activity across the
United States (22, 23)

After all, there is a great deal of good literature on attitudes toward prescribed burning and fuel treatments.  But maybe I’m not aware of the literature on strategies to reduce human-caused ignitions.

2. If there is none, why not?

Is it because it’s unstudiable for various reasons (under the litigation cone of silence?), or because there aren’t enough social scientists around, or because it’s not popular with granting agencies, or ???

Les’s article about Smokey Bear signage reminded me of this (2018) Nagy et al. paper. I read lots of climate media and papers and it is a given in some that “climate causes increased wildfires.”  As we know, climate (or changes in weather over time) can certainly influence fire behavior, but so can fuel conditions, suppression strategies and tactics, and …. ignitions. This seems obvious, and more easily dealt with than decarbonizing  global energy production, so it seems like something scientists would want to look at.  We have seen fires start from power lines, people driving in dry grass, and people doing what appear to be really stupid things to celebrate events, and a variety of other causes.  Many of these ignition sources have been the targets of  behavioral change efforts via media, lawsuits or criminal charges.  It seems like a likely topic for social scientists to look at.. what are all the human caused sources? What has worked in practice to reduce them?

Does anyone know of such studies?

From the paper:

Large Fire Seasonality

The seasonal expansion of large wildfires in the spring in the eastern U.S. (Figure 3a) was similar to the pattern of fires of all sizes [5], with many large human-caused fires occurring earlier in the year than large lightning-caused fires (Figure S6) and outside of the lightning fire season (Figure S7). Given the strong correlations previously identified between large fires and climate [3,18,34,49,56], we expected large fires across the U.S. to be more likely during dry, hot summer months in the western U.S. In the eastern U.S., the seasonality of large human-caused fires was highly correlated with the seasonality of human-caused fires of all sizes across most eastern ecoregions (Figure 4). These results suggest that seasonal climate does not play an important role in facilitating large fires in the East. Instead, ignition pressure is the primary driver of large fires in these ecoregions. This finding is consistent with [57], who showed that human ignitions are a more important driver of fire than climate in the eastern U.S. In the northwestern U.S., the seasonal correlation between large fires and all fires was lower (Figure 4), suggesting that climate conditions may play a more important role in facilitating large fires here. Nonetheless, even in ecoregions of the intermountain west where climate is known to influence large fires [2,12,56], human ignitions explained a substantial portion of the seasonal pattern of large fires (r > 0.66; Figure 4). This finding suggests that ignition pressure, and human expansion of ignitions [5], plays a much larger role in influencing large fires than previously thought.