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Artificial intelligence method reveals millions of dead trees hidden among living ones ahead of historic 2020 California wildfires

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California tree mortality status due to individual dead trees detected in NAIP aerial imagery in 2020. Source: Nature communication (2024). DOI: 10.1038/s41467-024-44991-z

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California tree mortality status due to individual dead trees detected in NAIP aerial imagery in 2020. Source: Nature communication (2024). DOI: 10.1038/s41467-024-44991-z

Scientists at the University of Copenhagen may have found a new explanation for the 2020 California wildfires. Using artificial intelligence for detailed aerial imagery, they have created a unique dataset detailing mortality rates down to individual trees across the state of California. This revealed that single and concentrated tree death was spreading among the living on a large scale. A new artificial intelligence model will increase knowledge about tree mortality and give us the chance to prevent droughts, beetles and fires from destroying the world’s forests.

For better or worse, climate change has put forests in the spotlight around the world. Initiatives have sprung up around the world to plant trees and expand forests because trees can extract and store CO2 from the atmosphere.2. At the same time, massive and more frequent fires were raging, forcing hundreds of thousands of people to flee their homes.

California is one of the places most affected by droughts and wildfires. In 2020, 4% of its area went up in smoke. Scientists from the University of Copenhagen provide a new picture of the state of California’s forests, revealing a new insight into the region’s dead trees and perhaps a new explanation for widespread wildfires in a study published in Nature communication.

Using an optimized artificial intelligence model applied to sub-meter-resolution aerial imagery, researchers were able to map in detail the health of trees across the entire state of California (over 90 million trees), detailing the distribution of dead trees with unprecedented precision. Importantly, this feat revealed a small number of dead trees, all of which had a particular feature.

“Our data shows that a huge number of these trees are isolated or located in small clusters of just a few trees, which allowed them to hide among healthy, living trees from coarse-resolution satellite imagery. This is new knowledge,” he says. Stéphanie Horion from the Faculty of Earth Sciences and Natural Resources Management, University of Copenhagen.

According to the researcher, fire spread during forest fires is strongly related to the uneven distribution of fuel, both in terms of density and flammability.

“This makes it reasonable to speculate that such scattered enclaves of dead, dry trees may have acted as kindling between living trees, influencing the intensity and spread of fires. “This new knowledge is interesting both as a possible part of the explanation for the wild fires in California, but also largely for our attempts to understand tree dieback more broadly,” Horion says.

Fire is not the biggest killer of trees

The purpose of the study was not to understand fire. Instead of studying forest fires themselves, scientists have sought to understand the global phenomenon of mass tree dieback, in which large areas of forests suddenly die off. This phenomenon is becoming more and more frequent and is caused by climate change.


Single and clustered dead trees were scattered among living trees before the 2020 California wildfires. Photo: Yan Cheng. Source: Yan Cheng

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Single and clustered dead trees were scattered among living trees before the 2020 California wildfires. Photo: Yan Cheng. Source: Yan Cheng

Due to the impressive and dangerous nature of fires, the public often wrongly perceives them as the most common cause of tree death. A case study from California shows that this is not the case. In fact, it turns out to be the opposite.

“New data shows that drought and subsequent insect attacks are the biggest killers of forests. An indirect consequence may be a fire. For a fire to break out, three basic elements are needed: hot and dry weather and climatic conditions that exacerbate climate change, the frequency of the ignition source – such as a lightning strike or human carelessness – and finally, an abundance of combustible materials. Drought weakens the “immune system” of trees, which increases the risk of tree mortality from bark beetle attacks. And dead trees burn well,” Horion explains.

As an example of mass forest death, he points to the Harz forest in Germany, where drought and then bark beetles killed huge swathes long before any fire occurred.

“Ironically, many residents were delighted when bark beetles were first discovered there, as they were considered a sign of forest health and biodiversity. Since then, it has been shown that during periods of drought these beetles spread like an epidemic, and as a result, one third of the trees in the Harz forest died. We need to draw conclusions from this if tree planting is to play an important role in solving the climate problem,” says the researcher.

He highlights that the new AI model could become an important tool in the future, as effective tree death mapping can provide scientists and public agencies with an early warning system that will enable timely intervention.

More information:
Yan Cheng et al., Scattered Tree Death Contributes to Substantial Forest Loss in California, Nature communication (2024). DOI: 10.1038/s41467-024-44991-z

Information about the magazine:
Nature communication