Oral accounts from Native Americans have helped scientists reconstruct the 3,000-year history of a large fire-prone forest in California. The results suggest parts of the forest are denser than ever and at risk of severe wildfires1. The research is part of a growing effort to combine indigenous knowledge with other scientific data to improve understanding of ecosystem histories.
Wildfires are a significant threat to California’s forests. Clarke Knight, a paleo-ecosystems scientist at the US Geological Survey in Menlo Park, California, and her colleagues wanted to understand how Indigenous communities helped shape the forest by managing this risk in the state’s lush western Klamath Mountains . In particular, they studied the use by indigenous peoples of cultural burning — small, controlled fires that keep biomass low and reduce the risk of more widespread burning. The results will be published in Proceedings of the National Academy of Science.
“When I was a little kid, my grandmother used to burn around the house,” says Rod Mendes, fire chief for the Yurok Tribe Fire Department, whose family is part of the Karuk Tribe of Northern California. The Karuk and Yurok tribes have called the Klamath Mountains home for thousands of years. “She just kept the apartment clean. Native Americans probably performed some of the first prescribed fire operations in history,” says Mendes.
Understanding how indigenous tribes used fire is critical to managing forests to reduce the risk of wildfires, Knight says. “We need to listen and learn from the indigenous people and understand why they managed the landscape the way they did,” adds Mendes.
cooperation to confirm
To map the region’s forest history, the team drew on historical accounts and oral histories from Karuk, Yurok, and Hoopa Valley tribesmen, those of Frank Lake, a US Forest Service research ecologist in Arcata, California, and a Karuk descendant , were collected. as part of his PhD thesis in 2007. These accounts described the tribes’ fire and land use. For example, members lit small fires to keep paths clear; This also reduced the amount of vegetation and prevented the spread of wildfires from lightning strikes. Larger fires, known as broadcast burning, were used to improve visibility, hunting, and nut harvesting conditions in the forest. The fire’s effects on vegetation lasted for decades.
Knight says it was important to work with the tribes since they know about the region. The Karuk Resources Advisory Board approved a proposal for the study before it began. “In a way, it decolonizes the existing academic model, which doesn’t take indigenous history very much into account,” says Lake.
Researchers also analyzed sediment cores collected near two low-lying lakes in the Klamath Mountains that are culturally important to the tribes. Pollen layers in the cores were used to infer approximate tree density in the area at different times, and the modeling helped date the cores so they could estimate how that density was changing.
The team also measured charcoal in the cores’ layers, which helped map variations in the amount of fire in the region. Fire scars on tree stumps indicate specific fire events between 1700-1900. Since the rings of the tree stumps serve as an ecological calendar, the researchers were able to compare fire periods with corresponding tree density data. They then pieced together how this density varied with the occurrence of fires. Although these empirical methods could not specifically confirm that the fires were lit by the tribes, patterns suggested when this was more likely, Knight says. For example, increased burning during cool, wet periods when flashfires were likely less common suggested human influence.
Combining multiple lines of evidence, Knight and her team show that tree density in this region of the Klamath Mountains began to increase as the area was colonized, in part because European settlers prevented Indigenous peoples from practicing cultural burning. In the 20th century, total fire suppression became standard management practice, and fires of all kinds have been extinguished or prevented – although controlled burns are currently used in forest management. The team reports that tree density in some areas is higher than it has been in thousands of years, partly due to firefighting.
A dense forest isn’t necessarily healthy, Knight says. The Douglas fir, which dominates the lowland Klamath forests, is less fire resistant and more susceptible to catastrophic wildfires. “This idea that we should just let nature take its course is just not supported by this work,” she says. She adds that one of the strengths of the study is the abundance of evidence showing past indigenous burning helped manage tree density.
Fire ecologist Jeffrey Kane of California State Polytechnic University Humboldt in Arcata says the study’s findings on increased tree density are not surprising. He has made similar observations in the Klamath region. “There are a lot more trees than there were 120 years ago,” he says.
Dominick DellaSala, senior scientist at forest conservation organization Wild Heritage in Talent, Oregon, cautions that the results, which point to record tree densities, cannot be extrapolated to the entire Klamath region due to the limited range of the study’s lakeshore data.
However, Knight says the results can be extrapolated to other similar low-elevation lakes with similar vegetation types.
More indigenous voices
Paleoecology studies are increasingly incorporating indigenous knowledge — but there’s still a long way to go, says physical geographer Michela Mariani of the University of Nottingham, UK. In Australia, Mariani has also noted that tree density began to increase after British colonization hampered cultural burning. “It is very important that we now include the indigenous people in the discussion about fire safety,” says Mariani. “They have a deeper knowledge of the landscape that we just don’t have.”
Involving indigenous voices in research is also critical to decolonizing traditional scientific methods, Lake points out. It “is becoming a form of justice for those indigenous people who have long been excluded, marginalized and unrecognized,” he says.