Record wildfires in California triggered by millions of hidden dead trees

Millions of hidden dead trees may have sparked California’s largest wildfires, according to a new study.

Scientists at the University of Copenhagen studied the 2020 wildfires and applied artificial intelligence (AI) to aerial imagery to create a dataset of trees across the state.

They discovered that individual and clustered tree deaths were widespread among living trees. This provides a brand new explanation for the severe wildfire season published in Nature communication.

The 2020 wildfire season in California was one of the most devastating in the state’s history, with unprecedented scale and intensity. Over 4.2 million acres of land burned, a record year for wildfire damage. The fires are believed to have been fueled by a combination of factors including extreme heat, prolonged drought and strong winds, all exacerbated by climate change.

However, research into the causes of forest fires is not yet complete, as understanding the contributing factors will help reduce the risk in the future.

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

“Our data show that a large number of these trees occur in isolation or in small groups of only a few trees, which has allowed them to appear scattered among healthy, living trees on coarse-resolution satellite images. This is new knowledge,” said Stéphanie Horion of the University of Copenhagen’s Institute of Geosciences and Natural Resource Management, who worked on the study, in a statement.

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

“This raises the possibility that such scattered enclaves of dead, dry trees may have acted as kindling between living trees, influencing the intensity and spread of wildfires. These new findings are interesting both as a possible part of the explanation for the severe wildfires in California and for our attempts to understand the phenomenon of tree mortality more generally,” Horion said.

The 2020 wildfires destroyed thousands of structures and forced numerous communities to evacuate, causing widespread displacement and significant economic loss. The wildfires also had severe environmental impacts, including degradation of air quality that affected millions and large-scale habitat destruction. The 2020 season highlighted the urgent need for comprehensive wildfire management strategies and climate resilience planning in California.

The researchers did not actually want to study forest fires with their study. Rather, they wanted to learn more about the mass death of trees and the causes behind it. They found that mass deaths are becoming more frequent and widespread as climate change worsens.

“The new data show that drought and the resulting insect infestation are the most destructive forces in forests. Fires can be an indirect consequence,” Horion said.

“For a wildfire to break out, three basic elements are required: hot, dry weather and climatic conditions that are becoming more common due to climate change; an ignition source – such as lightning or careless humans – and finally an abundance of combustible materials. Drought weakens the immune system of trees, which increases the risk of tree death after bark beetle infestation. And dead trees burn well.”

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