AI method uncovers millions of dead trees hidden among living trees before California’s historic wildfires in 2020

Scientists at the University of Copenhagen may have found a new explanation for the 2020 California wildfires. By applying AI to detailed aerial photography, they created a unique dataset that breaks down mortality down to individual trees for the entire state of California. This showed that individual and grouped trees are dying on a large scale. The new AI model will improve understanding of tree mortality and give us a chance to prevent droughts, beetles and flames from destroying the world’s forests.

For better or worse, climate change has put forests in the global spotlight. Initiatives have emerged around the world to plant trees and increase forest cover because of trees’ ability to absorb and store atmospheric CO.₂At the same time, massive and more frequent wildfires raged, 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 the land mass went up in smoke. Now scientists from the University of Copenhagen are presenting a new picture of the state of California’s forests. They provide a new number of dead trees in the region and possibly a new explanation for the large-scale fires in a study published in. Nature communication.

Using an optimized AI model applied to sub-meter resolution aerial imagery, researchers were able to comprehensively map tree health across the entire state of California (over 90 million trees) and depict the distribution of dead trees with unprecedented precision. Importantly, this achievement uncovered an undercount of dead trees that all shared a unique characteristic.

“Our data show that a large proportion of these trees occur in isolation or in small groups of only a few trees. As a result, they can be seen scattered among healthy, living trees on coarse-resolution satellite images. This is new knowledge,” says Stéphanie Horion from the Institute of Geosciences and Natural Resource Management at the University of Copenhagen.

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

“This raises the possibility that such scattered clusters 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,” says Horion.

Fire is not the biggest tree killer

The aim of the study was not to understand fire. Rather than studying forest fires per se, the researchers wanted to understand the global phenomenon of massive tree dieback, in which large areas of forest suddenly die. The phenomenon is becoming more and more common and is being driven by climate change.

Due to the impressive and threatening nature of wildfires, they are often mistakenly viewed in the public perception as the leading cause of tree death. However, the case study from California shows that this is not the case. In fact, it is quite the opposite.

“The new data show that drought and the subsequent insect infestations are the biggest forest killers. Fires can be an indirect consequence. For a forest fire to break out, three basic elements are necessary: ​​hot, dry weather and climatic conditions that are becoming more common due to climate change; an ignition source – such as lightning or careless people – and finally an abundance of flammable materials. Drought weakens the immune system of trees, which increases the risk of tree death due to bark beetle infestation. And dead trees burn well,” explains Horion.

As an example of massive forest dieback, she cites the Harz Mountains in Germany, where drought and later bark beetles destroyed huge areas of forest long before forest fires occurred.

“Ironically, many locals were excited when bark beetles were first discovered there, as this was seen as a sign of a healthy forest and biodiversity. Since then, it has been shown that these beetles spread like epidemics in times of drought and that a third of the trees in the Harz forest have now died as a result. We must learn from this if tree planting is to play an important role as a climate solution,” says the researcher.

She emphasizes that the new AI model could be an important tool in the future, as effective mapping of tree mortality can provide researchers and authorities with an early warning system that enables timely intervention.