Wildfires burning in western U.S. forests have increased in size and severity since the late 20th century, with a number of recent fires exhibiting characteristics that match the criteria for mass fires – or fires that burn with high intensity over large continuous areas for long durations of time.
Operational fire behavior models, commonly used by federal and state fire suppression agencies to predict how wildfires will behave, cannot predict mass fire behavior, largely because they do not include the important combustion and fire-atmosphere interactions. The Creek Fire, which exhibited mass fire behavior when it burned through the southern Sierra Nevada in 2020, was analyzed to better understand the mechanisms and forest conditions that contribute to devastating wildfires.
Scott Stephens, Alexis Bernal and Brandon Collins in the Department of Environmental Science, Policy and Management at University of California, Berkeley, along with other colleagues used both ground-based and remotely sensed data to analyze behavior patterns of the 2020 Creek Fire to determine which variables were important in predicting fire severity.
Findings indicated that dead biomass and live tree density were the two most important variables – more so than treatment history (i.e. timber harvesting, fire hazard reduction treatments, etc.), fire history or topography. Areas with the highest amounts of dead biomass and live tree densities were also positively related to high-severity fire patch size — indicating that large homogenous swaths of these types of conditions resulted in adverse, landscape-scale fire effects.
“Forest restoration must be increased greatly in California forests, the Creek Fire shows us what will happen if we don't move decisively ” said Stephens, lead author on the work, which is published in a new paper in the journal Forest Ecology and Management.
Additional analysis revealed that although the first two days of the Creek Fire were abnormally hot and dry, weather during the days of the greatest fire growth was largely within the normal range for the time of year (late summer). The spatial distribution of fire intensity during those days, however, revealed some notable patterns, with the concentration of heat from the fire being in the opposite location of where it would be expected. Specifically, on the day of the largest fire growth (Sept. 6), not only was the greatest heat concentrated away from the fire perimeter (or “flaming front,” which is the expected location for heat concentration), but a significant amount of heat was still being generated within the previous day's fire perimeter. This finding is critical to better understanding how traditional fire behavior models may or may not accurately predict fire behavior in forests that have large, contiguous areas of dead trees and high live tree density – an increasingly common forest fuel condition in Sierra Nevada forests.
The findings of this study have important implications for forest managers, as they indicate that in certain forest structures (i.e. those with large, homogenous swaths of dead biomass or high densities of live trees) conventional fire models may dramatically underpredict the spread rate and area burned because these models do not correctly capture the physics driving the fire.
The Creek Fire is one of a number of fires that shows how vulnerable forests are to current frequent-fire forest conditions, suffering high tree mortality and offering fuel conditions capable of generating mass fires from which future forest recovery is questionable because of type conversion and probable reoccurring high severity fire.
Scott Stephens, Department of Environmental Science, Policy and Management, University of California, Berkeley.
Alexis Bernal, Department of Environmental Science, Policy and Management, University of California, Berkeley.
Brandon Collins, Department of Environmental Science, Policy and Management and Center for Fire Research and Outreach, University of California, Berkeley.
Mark Finney, USDA Forest Service, Rocky Mountain Research Station.
“The first question I like to ask is, ‘Can you run through the forest?'” York says.
York, an assistant cooperative extension specialist and adjunct associate professor of forestry at UC Berkeley, poses the question while standing in a grove of pine trees during a tour of Blodgett Forest Research Station, a 4,000-acre experimental forest in the northern Sierra Nevada. While fire suppression has allowed many of California's forests to grow thick and dense, this patch of forest is one you could actually run through: The area is punctuated by large trees spaced a few meters apart, separated by a smooth carpet of dried pine needles.
“The idea is, if it doesn't have a lot of buildup of surface fuel on the ground — sticks and logs — you should be able to run through it,” York adds. “Looking through this forest, I might have to jump over that log, but, generally, I could take a jog through it.”
For more than 50 years, York and other Berkeley forestry researchers have used Blodgett as a living laboratory to study how different land management treatments — including prescribed burning, restoration thinning and timber harvesting — can reduce the risk of severe wildfire and improve a forest's resilience to the impacts of climate change. In addition to research, Blodgett regularly hosts workshops to demonstrate different land management techniques to landowners.
After another year of record-breaking wildfires in California, the work at Blodgett is more critical than ever, and state and federal agencies are motivated to enact more effective forest management practices. In 2020, the state and the U.S. National Forest Service jointly committed to managing 1 million acres of California forests a year, and last month the Biden administration pledged billions in new federal funding to reduce wildfire risk in the state.
“[Blodgett] was really designed to eventually demonstrate land management alternatives and offer a glimpse into how they might look at bigger scales,” York said.
Blodgett Forest is “pretty representative of millions of acres of Sierra mixed conifer forest,” said Ariel Roughton, a research stations manager at Berkeley Forests. After the majority of its trees were logged in the early 1900s, the forest was donated to Berkeley in the 1930s with the intent that it would be used to study sustainable timber production. Aside from a few old relics that survived early logging, the majority of the trees are regrowth and approximately 100 years old.
The forest is currently divided into a patchwork of tracts, each having received a different series of treatments since active management began in the 1950s and 1960s. And while fire suppression was once the policy at Blodgett — early fire ecologist Harold Biswell was even banned from using prescribed burns out of fear that they would interfere with the timber harvest — fire is now one of the primary tools that Blodgett researchers use to maintain biodiversity and reduce the risk of severe wildfire.
“Back then, people thought, ‘Why would you ever want to use fire for land management?' They wanted to grow trees, they want to grow timber. The idea of seeing black and char was literally off the scale,” said Scott Stephens, a professor of forest science and co-director of Berkeley Forests. “It's amazing that just a few decades ago, researchers didn't have the opportunity to do the work that Rob and Ariel and others are doing up here now.”
In the open, airy tract of forest that York could easily jog through, blackened scorch marks extend 10 to 15 feet up the trunk of each tree. Ecologists believe that before European colonization, these forests experienced fire once every 10 years or less, leading to open forest structures very similar to this one. Here, two years ago, Roughton, York and their colleagues conducted a prescribed burn to remove excess fuel from the ground and reduce the risk of wildfire.
“I think it's important to remember that nature hasn't taken its course without a lot of human intervention since the last glaciation, because there was strong Indigenous burning here,” said John Battles, a professor of forest ecology at Berkeley. “There has always been intense human stewardship of one sort or another.”
According to the researchers, it took 15 to 20 years of active management, followed by regular maintenance, to get the forest tract to this state. Over the years, they have worked to achieve the open forest structure by harvesting some of the bigger trees for timber, but leaving the largest behind. They have also used a machine called a masticator to chip up smaller trees and conducted regular prescribed burns.
While there are forest management strategies that can be effective on a shorter time scale, it usually takes at least a few separate treatments over the course of a few years to successfully restore a forest and reduce its wildfire risk, York explains.
“It can be a challenge to get to the forest structure that we want,” York says. “It takes a lot of time, and it takes a lot of investment.”
Climate change is also narrowing the annual windows of time when conditions are best for prescribed burning, limiting when and how often foresters can safely burn. Hot, dry conditions usually make prescribed burning too risky during the summer, while rain and snow in the winter can leave the forest too wet and damp for fire to burn. However, research at Blodgett is showing that, with the right management decisions, prescribed burning during the winter can be made more viable.
“Because of timber harvests that removed some of the canopy and subsequent treatments to remove the ladder fuel, we now have more light hitting the ground, and it dries out faster,” Roughton said. “We've gotten to the point out here where we're able to burn more easily because of our past management actions.”
While York likes to imagine running through the trees, Battles has a slightly different metric for evaluating the health of a forest.
“You need to be able to run through the woods,” Battles said. “But I also want to see all six of my friends as I do my run.”
Battles' friends are the six tree species that make up the Sierra mixed conifer forest: oak, ponderosa pine, sugar pine, white fir, Douglas fir and incense cedar. Fire suppression — and the dense, overgrown forest structures that can result — often favor the survival of some of these species over others, leading to forests that are dominated by just one or two species. This lack of biodiversity can make the forest, as a whole, less resilient to stressors like bark beetles or tree pathogens, which often target some of these species, but not others.
According to Battles, the open structure and frequent fire at this tract of Blodgett has allowed all six of his friends to flourish.
“I see my friend, ponderosa pine, which you don't see as frequently in the unburned forest because it's shade intolerant — it needs light. I see oak, and it also requires fire to get a lot of the oaks,” Battles said. “I see all six of my friends all here, and you only see them when you have management like this.”
Over the past 20 years, research has shown that prescribed burning and mechanical thinning with tools like the masticator can also benefit soil quality and water availability, while having no significantly negative impacts on forest ecosystems. While burning or otherwise removing plants and trees can release carbon dioxide into the atmosphere, which accelerates the impacts of climate change, reducing the risk of severe wildfire can help maintain the whole forest for long-term carbon storage.
However, applying these techniques across 33 million acres of California forestland remains a monumental task. Prescribed burning requires a great deal of expertise and is also limited by weather conditions and air quality regulations. Meanwhile, mechanical tree thinning can be costly, and unlike timber harvesting, it does not generate any revenue for landowners — though Berkeley researchers have suggested that creating a market for small trees and other woody biomass could help offset the cost while limiting carbon emissions.
“Fire used to be so common in this system, and that's no different than in most forests in California. But, when you take it out for that long, you begin this transformation,” Stephens said. “That's why we have to get both public and private entities together to come up with a philosophy to be able to move forward on this. Blodgett is 4,000 acres — that's interesting, but it doesn't really address the needs of the state. We always hope that our work shows people what's possible and then enables them to continue it.”
Berkeley News writer Kara Manke discusses a new UC Berkeley report that shows how allowing lightning fires to burn in Yosemite's Illilouette Creek Basin recreated a lost — and more resilient — forest ecosystem. She spoke with Scott Stephens, a professor of environmental science, policy and management at Berkeley and co-director of Berkeley Forests. “We can't just allow wildfire to manage our landscapes,” Stephens told Manke. “If we don't change it in 10 or 20 years, the forest ecosystems are going to change right in front of our eyes, and we're just going to be passengers.”
Yosemite National Park's Illilouette Creek Basin in September 2018. (UC Berkeley photo by Emily Gonthier)
Read a transcript of Berkeley Voices episode #83: “How wildfire can create healthier forests.”
Intro: You're listening to Berkeley Voices. I'm Anne Brice.
A lot of us might remember Smokey Bear. He was that serious cartoon bear wearing a pair of blue jeans with a belt buckle and a hat that read “Smokey.” He was in PSAs everywhere — on posters, on TV and the radio, saying his unforgettable slogan:
Created by the U.S. Forest Service and the Ad Council in the 1940s, Smokey Bear was the symbol of responsibility. I remember when I was 8, I met someone dressed as Smokey Bear on a class field trip. And I left this meeting having learned something that would stick with me for a long time: that fire was the enemy — something that should be suppressed, extinguished — and definitely not allowed to run rampant through our precious forests.
But Smokey Bear, we now know, wasn't giving us the full picture.
A new report from UC Berkeley has shown that allowing certain wildfires to burn can have a lot of benefit to the ecology of a forest, and for the humans and wildlife that depend on forest resources.
[Music fades]
My colleague at Berkeley News, Kara Manke, spoke with the senior author of the report, Scott Stephens. He's a professor of environmental science, policy and management at UC Berkeley and co-director of Berkeley Forests.
Anne Brice: Hi Kara, welcome to Berkeley Voices.
Kara Manke: Hi Anne, it's great to be here.
Anne Brice: So, Kara, can you start by talking about this report and what Scott Stephens and his team found?
Kara Manke: Yeah, certainly. So, for many years now, Scott Stephens has been leading research in a watershed in Yosemite National Park called the Illilouette Creek Basin. And what makes Illilouette unique is that since the early 1970s, forest managers have had a policy of trying to let lightning fires burn pretty much as they naturally would: trying to keep a close eye on them, but not putting them out unless they absolutely have to.
And, as a result, they have created this patch of forest — it's about 60 square miles — that has become almost like a natural laboratory for forest ecologists who want to understand better the interaction between wildfire and forest ecosystems.
Scott Stephens is a professor of environmental science, policy and management at UC Berkeley and co-director of Berkeley Forests. (UC Berkeley photo)
Here's how Stephens describes the Illilouette Creek Basin today:
Scott Stephens: I mean, it really is a place that shows tremendous change, variability, tree death, tree regeneration, meadows, areas also with big trees — there are a lot of big trees out there. So, I think it's really what the Sierra Nevada was 200 years ago.
Anne Brice: What were the forests like in the Sierra Nevada back then?
Kara Manke: We don't know exactly what the forests of the Sierra Nevada looked like 200 years ago, but we do know that there used to be a lot more fire. Lightning would regularly start fires that would burn parts of the forest, and also a number of Native American tribes would light fires to help with hunting and to encourage biodiversity.
However, with the arrival of European colonists in the late 1800s, fire really came to be seen as the enemy, and when the U.S. Forest Service was created in 1905, one of its main missions was fire suppression. And as a result, many of the forests in the Western U.S., including in the Sierra Nevada, are now very dense and have this almost wall-to-wall tree cover.
Scott Stephens: We look at forests with an eye that they all need to be green all the time, and they're all made with big trees. It turns out no forest can do that. You have to regenerate it. You have to get young trees and allow them to regenerate. So, Illilouette is doing that.
Anne Brice: We are in the middle of wildfire season. And we've already seen several catastrophic fires, like the month-old Dixie Fire in California, which is now the largest fire in the U.S. It has destroyed more than 1,000 homes and businesses and isn't close to being contained.
Many people are saying these fires are happening mostly because of climate change. But Stephens says it's more complicated than that — that it has more to do with the state of our forests. Can you explain what he means by that?
Kara Manke: Yeah, one of the most interesting things that Stephens said is that he thinks this hotter, drier weather that we're seeing as a result of climate change is only about 20 to 25% responsible for the recent increases in catastrophic wildfires in California.
He thinks the rest is actually due to the structure of our forests, basically the way that more than a century of fire suppression and past practices of timber harvesting has allowed so much excess fuel to build up. Illilouette provides evidence of this, he says, because even though the region has also been affected by climate change, just like everywhere else in the Sierras, the severity of the fires that they are seeing in Illilouette have stayed the same.
Anne Brice: Do they know why wildfires haven't been getting any more severe in Illilouette?
Kara Manke: Yeah, so one reason for this fire resilience might be that patches of grassland and wet meadow that are created by recent burns can actually form natural barriers that prevent the spread of future fires.
Another really interesting discovery that they have made is that, perhaps counterintuitively, frequent fire actually increased the amount of water that is available in the forest and may have also helped trees survive during recent droughts.
Scott Stephens: The thing that just sparked my interest was the Hoover Fire, a 2001 burn, and that was a pretty big one. So, we had actually had some plots in an area previous to the Hoover Fire. So, we went out afterwards to find them and just do some measurements. And I remember walking in that place. I had walked out there before. It's a very typical lodgepole pine forest, you know, kind of big trees, very little understory, dry ground.
But when I walked out there after the Hoover Fire, probably around 2003 or something like that, all of a sudden I'm walking in six inches of water, and I'm going, “What the heck happened here? This is really remarkable.”
Anne Brice: So, what the heck did happen? How could wildfire create water?
Kara Manke: Stephens actually enlisted the help of a number of hydrologists, including many at UC Berkeley, and what they found from a combination of measurements on the ground in Illilouette and also hydrological simulations, is there are likely two major factors at play.
The first is that, with fewer trees, there is simply less demand for water. And the second is that these small gaps in the forest canopy that are created by wildfires actually allow more rain and snow to reach the ground and be absorbed by the soil.
Anne Brice: So, Kara, it sounds like there are a lot of benefits to letting wildfires burn in forests. Are we trying to do this outside of Illilouette?
Kara Manke: Yeah, I think there is a growing recognition that reintroducing fire to these forest ecosystems is more important than ever. But, in a lot of ways, we've also now created a situation in which it's riskier than ever, as well.
With the increase in heat waves and droughts that we've seen, paired with these dense forests that are full of fuel, it's now even more difficult to control these fires once they start, which is what we're now seeing with these massive fires like the Dixie Fire.
And in early August, because of some of these concerns, the U.S. Forest Service actually changed its guidelines, now telling firefighters that, for now, they must put out every fire as quickly as possible. Stephens said that they will evaluate this new policy next year.
Anne Brice: Are there other less risky ways to help our forests be more resilient to fire without letting fires burn? Is there anything we can do beforehand to kind of prep them in some way?
Kara Manke: Yeah. Luckily, letting these natural lightning fires burn isn't the only tool we have for shaping our forest ecosystems.
Stephens advocates for more aggressive prescribed burning, which is basically going in and deliberately lighting fires during low-risk times. So, this is often in the late fall, winter or spring, and letting those fires clear out some of this underbrush and built-up fuel.
A similar option is restoration thinning, and that's where you go in with machines instead of fire to take out some of the underbrush and some of these extra trees, focusing on the vegetation that you want to keep. These types of modifications on their own can really help the ecology of the forest, and they can also serve as tools to help make the forest safer to allow more natural lightning fires to burn, as well.
Scott Stephens: You know, it's been 50 years. What we've learned there I think helps us understand what is possible. We can't just allow wildfire to manage our landscapes. That's crazy. I think we have 10 to 20 years to actually change the trajectory of forest ecosystems in the state. If we don't change it in 10 or 20 years, the forest ecosystems are going to change right in front of our eyes, and we're just going to be passengers. So, that's why it's so important in earnest to start to do work like this.
[Music fades]
Anne Brice: You also talked with Stephens about a project he and his team started this summer working with the Amah Mutsun Tribal Band in the Santa Cruz Mountains. They're working with the tribe to restore their cultural fire regimes. Can you talk about what this partnership looks like and what they're hoping to accomplish with it?
Kara Manke: Yeah. There is a growing recognition that the cultural burning practices of the Indigenous people of California really had a lot of benefit to forest ecosystems. So, when it comes to land management, it is important not just to implement the prescribed burning and restoration thinning that researchers are exploring, but also to work with Indigenous people so that they can resume some of these important cultural practices that involve fire.
Anne Brice: Incredible. Kara, thanks so much for joining me on Berkeley Voices.
Kara Manke: Of course, thanks for having me.
Anne Brice: Kara Manke is a science and health writer for Berkeley News in the Office of Communications and Public Affairs at UC Berkeley.