California’s Record Wildfires Spurred by Millions of Hidden Dead Trees

California’s record wildfires may have been spurred on by millions of hidden dead trees, according to a new study.

Scientists from the University of Copenhagen studied the 2020 wildfires, applying artificial intelligence (AI) to aerial photos to create a data set of trees across the state.

They discovered that individual and clustered tree death was widespread among living trees. This presents a brand new explanation for the severe wildfire season, published in Nature Communications.

California’s 2020 wildfire season was one of the most devastating in the state’s history, with unprecedented scope and intensity. Over 4.2 million acres were burned, marking a record-breaking year for wildfire damage. It is believed that the fires were fueled by a combination of factors including extreme heat, prolonged drought, and strong winds, all exacerbated by climate change.

However, research into wildfire causes is still ongoing, as knowing the contributing factors will help mitigate risk in the future.

Using an optimized AI model applied to high-resolution aerial photos, researchers mapped tree health across California, covering over 90 million trees with unprecedented precision. This detailed mapping revealed an undercount of dead trees, all sharing a specific characteristic.

“Our data show that a vast amount of these trees are isolated or located in small clusters of only a few trees, which has allowed them to hide scattered amidst healthy, living trees from coarse-resolution satellite images. This is new knowledge,” Stéphanie Horion from the University of Copenhagen’s Department of Geosciences and Natural Resource Management who worked on the study, said in a statement.

Horion added that the rapid spread of the 2020 wildfires was linked to “uneven distribution of fuel in both density and flammability.”

“This makes it reasonable to speculate that such scattered enclaves of dead dry trees could have acted as kindling between living trees, affecting the intensity and spreading of the wildfires. This new knowledge is interesting both as a possible part of the explanation for California’s violent wildfires, but also very much for our attempts at understanding the phenomenon of tree death more generally,” Horion said.

During the 2020 wildfires, thousands of structures were destroyed, and numerous communities were evacuated, resulting in widespread displacement and significant economic loss. The wildfires also had severe environmental impacts, including degraded air quality that affected millions and widespread habitat destruction. The 2020 season highlighted the urgent need for comprehensive wildfire management strategies and climate resilience planning in California.

The researchers actually did not intend to research wildfires in their study. Rather, they were seeking more information on mass tree death and why it happens. They discovered that it is becoming more common and more widespread as climate change worsens.

“The new data shows that drought and subsequent insect attacks are the biggest killers in forests. Fire can follow as an indirect consequence,” Horion said.

“For a wildfire to erupt, three basic elements are necessary: hot, dry weather and climatic conditions that climate change has increased the frequency of, an ignition source—such as a lightning strike or careless human—and finally, an abundance of combustible materials. Drought weakens ‘the immune systems’ of trees, which increases the risk of tree mortality in the wake of bark beetle attacks. And dead trees burn well.”

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