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(UC Irvine) Irvine, Calif… 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.