Study Reveals North American Forests Burn Less but with More Intensity

Recent data challenges the common perception that wildfires in North American forests are becoming more frequent. A study published in Nature Communications indicates a decline in fire occurrences, yet paradoxically, the fires are becoming more severe.

“What we see in the record is that widespread wildfires were happening very frequently, about every 10 to 20 years in many areas,” stated Donald Falk, a fire ecologist and professor at the University of Arizona, who co-authored the study with alumni and colleagues from the USDA Forest Service’s Rocky Mountain Research Station. “We also know that, by and large, these fires were not the severe fires we’re seeing on television today. They were often mixed-severity and surface fires occurring over very large areas.”

Supported by the John Wesley Powell Center, a research initiative of the U.S. Geological Survey, the study aimed to uncover historical patterns of wildfire occurrences.

Falk, part of the School of Natural Resources and the Environment at the University of Arizona’s College of Agriculture, Life and Environmental Sciences, emphasizes fire’s historical role in maintaining ecosystem balance. Fires historically managed underbrush and tree density, but the absence of regular fires has led to denser forests and an accumulation of flammable materials, increasing the intensity of current wildfires.

“As a result, today’s extreme wildfires are more likely to harm people and communities, while exposing forests to damaging effects on soils and natural vegetation, from which they may not recover,” Falk explained.

A Time Machine for Fire

Fire scars left on trees provide a historical record of past fires. These scars, created when fire heat penetrates tree bark, are like a time capsule, offering insights into the frequency and scale of past fires.

The North American Tree-Ring Fire Scar Network, originating from the University of Arizona’s Laboratory of Tree-Ring Research, compiles these scars into a comprehensive dataset. This data allows researchers to compare modern wildfire events, such as the 2020 California August Complex Fire and Arizona Bighorn Fire, with historical fires.

“In recent history, between 1984 and 2022, wildfires in 2020 seemed like they were unprecedented in terms of the area they burned, but historically speaking, they were not,” said Sean Parks, study leader and research ecologist at the USDA Forest Service Rocky Mountain Research Station. “There were several years between 1600 and 1880 where much more fire burned than what we experienced in 2020. This said, recent wildfires are unprecedented in terms of their adverse impacts to people, communities and forests.”

The fire scar data spans over 1,800 sites across North America and has been pivotal in other research, such as a study by U of A alumnus Ellis Margolis, linking wildfire patterns with climate phenomena like El Niño.

“We have records from Alaska all the way down to southern Mexico going back centuries. This gives scientists an unprecedented ability to understand how fire was working historically, before we started excluding it from the landscape,” noted Falk. “Our forests are overgrown now due to 140 years of fire exclusion, but the more we can do to make our forests more resilient to that inevitable fire, the better off we’re going to be.”