UC Irvine scientists reveal what fuels wildfires in Sierra Nevada Mountains

Text from a press release. The open-access paper is here.


UC Irvine scientists reveal what fuels wildfires in Sierra Nevada Mountains

Irvine, Calif., Sept. 25, 2023 — Wildfires in California, exacerbated by human-driven climate change, are getting more severe. To better manage them, there’s a growing need to know exactly what fuels the blazes after they ignite. In a study published in Environmental Research Letters, Earth system scientists at the University of California, Irvine report that one of the chief fuels of wildfires in California’s Sierra Nevada mountains is the decades-old remains of large trees.

“Our findings support the idea that large-diameter fuel build-up is a strong contributor to fire severity,” said Audrey Odwuor, a Ph.D. candidate in the UCI Department of Earth System Science and the lead author of the new study.

Researchers have known for decades that an increasing number of trees and an increasing abundance of dead plant matter on forest floors are the things making California wildfires more severe – but until now it was unclear what kinds of plant debris contribute most to a fire.

To tackle the question, Odwuor and two of the study’s co-authors – James Randerson, professor of Earth system science at UCI, and Alondra Moreno from the California Air Resources Board – drove a mobile lab owned and operated by the lab of study co-author and UCI alumna Francesca Hopkins at UC Riverside, to the southern Sierra Nevada mountains during 2021’s KNP Complex Fire.

The KNP Complex Fire burned almost 90,000 acres in California’s Sequoia and Kings Canyon National Parks. In the fire’s smoke, the team took samples of particulate matter-laden air and analyzed the samples for their radiocarbon content at UCI’s W.M. Keck Accelerator Mass Spectrometer facility with co-author and UCI Earth system science professor Claudia Czimczik.

Different fuel types, explained Czimczik, have different radiocarbon signatures, such that when they analyzed the smoke they discovered radiocarbon values associated with large fuel sources like fallen tree logs.

“What we did was pretty distinctive, as we were able to identify fuel sources by measuring the wildfire smoke,” said Czimczik. “Our approach provides what we think of as an integrated picture of the fire because we’re sampling smoke produced over the course of the fire that has been transported downwind.”

The team also saw elevated levels of particulate matter that is 2.5 microns in diameter or less, which includes particles that, if inhaled, are small enough to absorb into the bloodstream.

The preponderance of large-diameter fuels is new in western forests. “We’re really in a situation that’s a consequence of both management strategies and climate warming since European-American settlement began in California,” Odwuor said. “These fuels are building up on the forest floor over periods of decades, which is not typically how these forests were maintained.”

It’s information that, according to Odwuor, could help California better manage its wildfires.

“The knowledge that large-diameter fuels drive fires and fire emissions – at least in the KNP Complex Fire – can be useful for knowing which fuels to target with fuel treatments and what might end up in the smoke from both wildfires and prescribed fire,” said Odwuor. “The idea is that because we can’t control the climate, we can only do our best to manage the fuels, which will theoretically have an impact on fire severity and the composition of the smoke.”

But the solution isn’t as straightforward as removing trees from forest floors, because, among other things, they provide habitat for wildlife. That, and “once you get them out, where do you send them? There are only so many mills in California that can handle all the wood,” Odwuor said.

Where the new knowledge could be helpful is with prescribed burns, wherein teams burn tracks of forest in a planned fashion with the aim of reducing the amount of fuel available for future wildfires.

“We’re hoping to build some urgency for these management strategies,” said Odwuor.

10 thoughts on “UC Irvine scientists reveal what fuels wildfires in Sierra Nevada Mountains”

  1. Interesting idea. However, since most large trees are not fully consumed during these kinds of fires, wouldn’t the fires preferentially consume the outer portions of standing trees, which presumably would represent recent decades of growth?

    • The massive amounts of dead trees prior to those fires was the problem. Those dead trees burned into complete ash. It is also a bit of a moot point, anyway, when the next inevitable wildfire will burn through in less than 30 years, thanks to human-caused ignitions.

  2. I see there is no mention of the lack of salvage logging in those areas. Of course, a lack of salvage logging is a HUGE reason for the abundance of large woody debris, during the last 30 years. As far as I am concerned, those southern Sierra National Forests are a lost cause. The likely outcome is high-intensity wildfires. I do think those Forests make a great example for needing more active management, though.

    • The KNP Complex fire was in an area hit really hard by beetles from 2012-2016, a mortality event associated with the most intensive drought in 1,000 years (climate change isn’t an issue though, so don’t worry about it, it was a natural drought and part of the cycle of things, so drill for oil anywhere we can because if we don’t they’ll just do it someplace else and our economy will crash and we wont be able to afford a motorhome and several worldwide cruises a year when we retire and we all deserve anything we can afford, it’s our right under the ten commandments…but I digress).

      Beetle-killed trees do not have the salvage value that fire-killed trees do. Value declines rapidly over 6 months due to blue stain. Not sure how salvage could have done much here. Also, this area is really steep with limited access. There’s one mill that takes trees from this area…there was simply nowhere near enough mill capacity to handle 100 million dead trees over 5 years.

      The only available tool to deal with fuels in a lot of this landscape is Rx fire or fire managed for resource benefits, but that would be giving in to the spotted owls, the real culprits that caused the problem in the first place…cutting down all the large fire resilient trees and disrupting the natural disturbance processes…stupid enviros and stupid owls made the whole place burn. Don’t forget the fishers, they caused this too, stupid fishers.

      Instead of returning the natural disturbance process that maintained resilience, we should log more. Never met a fuels management problem we couldn’t log our way out of. That fire in Maui, should have logged more…grass. The Mendocino Complex, should have logged more…shrubs. LNU Lightning Complex, should have logged more…oak. We need 10% canopy cover in all our forests, because clearly trees are the issue everywhere there is fire (any fewer trees and it wouldn’t technically be a forest and we need to protect our forests), except where we planted millions of baby trees on 10×10 spacing, I really don’t like hurting babies, save the babies to kill later.

      • Are you finished with all the eco-themes and cliches? How is that crazy rhetoric working out for you? Are there any other groups of opinions you want to alienate?

        You’re trying to make it sound like I am all for logging ALL of it, without regards to anything else. Conspiracy theories aren’t working well for the Republicans. Why would you think they would work on the people of this forum? You are insulting our intelligence.

  3. This study seems to support the view that mature & old-growth forests are not the problem.

    “Large diameter” is relative.

    The paper actually finds through the carbon isotope analysis that “the mean age of fuels combusted in the KNP Complex Fire was 40 years, with a range of 29–57 years”

  4. The article reflects a level of climate illiteracy, and lack of knowledge of the severe climate impacts of messing up forest ecosystems with logging and “fuel reduction.”

    • 100% agree with you Jeff… Makes me wonder how many of the commenters actually read the papers they reference. And yes, I’m still scratching my head about what they call “large diameter fine fuels.” Sounds like a contradiction of terms?

      Mostly just embarrassed not just for the person posting, but the people making comments who don’t understand how much damage happens when you repeatedly remove carbon cycling from forest soils on a massive global scale that’s wiping out the world’s top soil at an unprecedented rate.

      Rather than understanding the advancement of scientific inquiry over time and integrating it into further refining forests practices that do less harm so forests are better off in the long run they instead always want to object to the latest and best available science and continue to turn back the clock and argue for the only important value of the forest to them is what you take out of it to make money rather than what you leave behind to make the ecosystem better off/more fecund in the long run.

  5. They measured a single fire from a single sampling location within a single 26 hour window. Then they fitted their sample to a model which had never been calibrated against known fuels.
    Nobody should draw any conclusions from this study.


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