When Tom Swetnam joined the United States Forest Service as a seasonal firefighter in the 1970s, his mandate was to "put everything out," he recalled. But when Swetnam enrolled in graduate school at the University of Arizona's Laboratory of Tree-Ring Research, he was surprised to find a record of repeated blazes dating back hundreds of years before Europeans arrived on the continent. Some of the trees he analyzed in New Mexico bore more than 20 fire scars spaced among their rings.
The fact that fires happened so often meant they couldn't have been severe enough to kill most trees — "If every time they're burning they're killing the forest, you're not going to have any forest left," said Swetnam, who is now a professor emeritus at the University of Arizona.
Instead, over the following decades a growing body of research showed that frequent, low-severity fires made many ecosystems healthier. They rid the forest of dead and sick trees, reducing competition and curbing the spread of disease. They helped release nutrients into the soil. And because flammable material couldn't build up on the landscape, blazes tended to move slowly and peter out when they reached the footprints of previous burns.
In 2022, Swetnam and more than five dozen fellow scientists teamed up to compile a comprehensive database of fire-scarred trees from across the continent. Their North American tree-ring fire-scar network (NAFSN) provided the basis for the fire deficit study from Abatzoglou and his colleagues.
The new analysis considered tens of thousands of fire-scarred trees from more than 1,800 sites across the continent, Abatzoglou said. Between 1600 and 1880, the researchers found, an average of 6 percent of these sites burned each year. Some started naturally from lightning strikes, but archaeological and tree-ring data suggest that many were also lit intentionally by Indigenous people who used fire to shape the landscape.
In some years, including 1748, fire was even more widespread. From the width of tree rings that formed around that time, the researchers could tell that 1748 was an extremely hot and dry year. Landscapes would have rapidly ignited, and there was nothing to stop the blazes except the arrival of rain.
When the scientists compared the historical fire cadence to the wildfires recorded over the past few decades, they uncovered a striking shortfall. The NAFSN sites experienced less than a quarter of the number of fires that would have been expected without fire suppression.
This deficit is a testament to the effectiveness of modern firefighting, said Kelly Martin, a past president of the International Association of Wildland Fire.
Yet the combined consequences of suppression and climate change have eroded humanity's ability to suppress fires, particularly those that ignite under the most dangerous weather conditions, Martin said.
To prevent entire ecosystems from going up in smoke, Martin said, people must bring healthy fire back to the places that need it.
At Yosemite National Park, Martin oversaw the use of what are known as prescribed burns to make the landscape more resilient. These fires were carefully planned and intentionally ignited during periods when weather kept the blazes gentle and easy to control, and helped eliminate some of fuel that had built up around important park facilities.
Research shows that these prescribed burns make subsequent wildfires less severe, even if the later fires happen under the most dangerous weather conditions.
Yet even as scientists and public officials increasingly agree on the need for more fires in our forests, climate change is making this tactic more challenging, experts said.
"It's a double-edged sword because wildfires are getting more severe and larger under climate change and we need this work even more, but then the work gets more challenging," said Susan Prichard, a fire ecologist at the University of Washington who helps agencies decide how to manage wildfires.