Indigenous Americans broke the cycle of destructive wildfires. Here’s how they did it | Science

In the southwestern United States, wildfires typically follow a grim, but predictable cycle. When unusually wet years are followed by hot, dry ones, fuel builds up—then burns. But a new look at thousands of fire-scarred tree trunks from the region, combined with archaeology and oral histories, indicates that between 1500 C.E. and 1900 C.E., Indigenous fire management practices managed to break that cycle, buffering the landscape from climate-related conflagrations.

The study, published today in Science Advances, does “a really nice job of addressing questions paleoecologists have struggled with,” says Dave McWethy, a paleoecologist at Montana State University, who was not involved with the research. “They’re able to demonstrate that on a smaller scale, people are having a very strong influence that’s able to dampen the effects of climate.”

To tease out the impact of human activity on wildfires, researchers collected information from a database of 4824 fire-scarred trees in Arizona and New Mexico. The scars—known as “catfaces” for the pattern they make in the tree rings—are formed when fire damages the tree enough to halt growth in one spot on the trunk, but not enough to kill the plant. Over time, new growth covers the scar with tissue from the sides.

Together with annual tree rings, the scars form a permanent record of when fires took place—“down to the year or even what part of the growing season,” says co-author Christopher Roos, an environmental archaeologist at Southern Methodist University. “A tree might record fires up to 20 or 25 times over a 200- or 300-year period.”

The authors combined the fire data from these scars with other information that can be gleaned from tree rings, such as how wet or dry the years leading up to the blaze were. Working with Diné (Navajo), Hemish (Jemez Pueblo), and Ndée (Apache) archaeologists, ecologists, and fire experts, whose traditional lands were covered by the tree ring data, they then matched the fire and climate information with data on Indigenous land use over time.

Archaeologists have been studying pottery and radiocarbon dates from settlements throughout the region for more than a century, letting them know when and where people were living, and when they shifted from hunting and gathering to herding sheep. Oral tradition, meanwhile, offered complementary clues on traditional fire use and Indigenous presence in the area.

“There are only a few spots in the U.S. where we have access to that kind of high-quality data,” McWethy says. “The archaeology is the smoking gun—when you see changes in fire activity that depart from climate patterns, you can mount a strong argument that’s human influence.”

The team found that during periods of light land use, when Native American groups were pushed off their traditional land by conflict with European settlers or warfare with neighboring groups, wildfire hewed to its predictable cycle: Plant fuel built up during wet years and became more likely to burn during subsequent dry ones. But in time periods and geographic areas where Native people were actively managing the landscape in traditional ways—gathering wood for fuel or construction and setting small-scale fires to clear land for agriculture or grazing—they reduced or eliminated excess fuel from the landscape before it burned out of control. That short-circuited the connection between climate patterns and wildfire.

“At local scales, this clearly shows people were able to blunt the impacts of fire,” says University of British Columbia, Vancouver, forest ecologist Lori Daniels, who was not involved in the study. “This is what Indigenous people have been telling us for a long time, but I don’t think Western science has always listened.”

As more tribes in the United States and Canada seek to restore traditional fire management practices on their land, the authors say, historical data can serve as a powerful reference for decision-makers. “Knowing a little bit about that story may help the next generation craft better policy or have better conversations about fire management,” says Northern Arizona University archaeologist Kerry Thompson, a co-author and citizen of the Navajo Nation.

Though the swings between wet and dry years are likely to be more extreme in centuries to come, researchers say they’ll mirror the past in important ways. “What we’re going to see is the same whiplash between wet years that build fuel and dry years that burn, but even more extreme,” says University of Arizona dendrochronologist Kevin Anchukaitis, who was not involved in the work. “To have an effect that can overcome climate is going to take a lot of burning. It’s a big task to overwhelm the influence of climate on these scales.”

And although the changing climate means the centuries to come will be different from the period scrutinized in the study—which took place during the unusually cool Little Ice Age, a global stretch of bitterly cold winters and mild summers that lasted from the 16th to the 19th centuries—colleagues say the study’s findings remain relevant.

As the world warms, for example, conditions similar to the southwestern United States will become common elsewhere—such as in Canada’s British Columbia, which lost nearly 1 million hectares to wildfire in the past 2 years. Daniels says, “Lessons learned from the Southwest will become hugely important as climate shifts these conditions further north.”

In the southwestern United States, wildfires typically follow a grim, but predictable cycle. When unusually wet years are followed by hot, dry ones, fuel builds up—then burns. But a new look at thousands of fire-scarred tree trunks from the region, combined with archaeology and oral histories, indicates that between 1500 C.E. and 1900 C.E., Indigenous fire management practices managed to break that cycle, buffering the landscape from climate-related conflagrations.

The study, published today in Science Advances, does “a really nice job of addressing questions paleoecologists have struggled with,” says Dave McWethy, a paleoecologist at Montana State University, who was not involved with the research. “They’re able to demonstrate that on a smaller scale, people are having a very strong influence that’s able to dampen the effects of climate.”

To tease out the impact of human activity on wildfires, researchers collected information from a database of 4824 fire-scarred trees in Arizona and New Mexico. The scars—known as “catfaces” for the pattern they make in the tree rings—are formed when fire damages the tree enough to halt growth in one spot on the trunk, but not enough to kill the plant. Over time, new growth covers the scar with tissue from the sides.

Together with annual tree rings, the scars form a permanent record of when fires took place—“down to the year or even what part of the growing season,” says co-author Christopher Roos, an environmental archaeologist at Southern Methodist University. “A tree might record fires up to 20 or 25 times over a 200- or 300-year period.”

The authors combined the fire data from these scars with other information that can be gleaned from tree rings, such as how wet or dry the years leading up to the blaze were. Working with Diné (Navajo), Hemish (Jemez Pueblo), and Ndée (Apache) archaeologists, ecologists, and fire experts, whose traditional lands were covered by the tree ring data, they then matched the fire and climate information with data on Indigenous land use over time.

Archaeologists have been studying pottery and radiocarbon dates from settlements throughout the region for more than a century, letting them know when and where people were living, and when they shifted from hunting and gathering to herding sheep. Oral tradition, meanwhile, offered complementary clues on traditional fire use and Indigenous presence in the area.

“There are only a few spots in the U.S. where we have access to that kind of high-quality data,” McWethy says. “The archaeology is the smoking gun—when you see changes in fire activity that depart from climate patterns, you can mount a strong argument that’s human influence.”

The team found that during periods of light land use, when Native American groups were pushed off their traditional land by conflict with European settlers or warfare with neighboring groups, wildfire hewed to its predictable cycle: Plant fuel built up during wet years and became more likely to burn during subsequent dry ones. But in time periods and geographic areas where Native people were actively managing the landscape in traditional ways—gathering wood for fuel or construction and setting small-scale fires to clear land for agriculture or grazing—they reduced or eliminated excess fuel from the landscape before it burned out of control. That short-circuited the connection between climate patterns and wildfire.

“At local scales, this clearly shows people were able to blunt the impacts of fire,” says University of British Columbia, Vancouver, forest ecologist Lori Daniels, who was not involved in the study. “This is what Indigenous people have been telling us for a long time, but I don’t think Western science has always listened.”

As more tribes in the United States and Canada seek to restore traditional fire management practices on their land, the authors say, historical data can serve as a powerful reference for decision-makers. “Knowing a little bit about that story may help the next generation craft better policy or have better conversations about fire management,” says Northern Arizona University archaeologist Kerry Thompson, a co-author and citizen of the Navajo Nation.

Though the swings between wet and dry years are likely to be more extreme in centuries to come, researchers say they’ll mirror the past in important ways. “What we’re going to see is the same whiplash between wet years that build fuel and dry years that burn, but even more extreme,” says University of Arizona dendrochronologist Kevin Anchukaitis, who was not involved in the work. “To have an effect that can overcome climate is going to take a lot of burning. It’s a big task to overwhelm the influence of climate on these scales.”

And although the changing climate means the centuries to come will be different from the period scrutinized in the study—which took place during the unusually cool Little Ice Age, a global stretch of bitterly cold winters and mild summers that lasted from the 16th to the 19th centuries—colleagues say the study’s findings remain relevant.

As the world warms, for example, conditions similar to the southwestern United States will become common elsewhere—such as in Canada’s British Columbia, which lost nearly 1 million hectares to wildfire in the past 2 years. Daniels says, “Lessons learned from the Southwest will become hugely important as climate shifts these conditions further north.”