Through oral traditions from indigenous people, researchers have succeeded in reconstructing the around 3000 years of history of a large, particularly fire-prone forest in California. The results indicate that parts of the forest are denser today than ever - and the risk of becoming the scene of heavy forest fires. The research work is part of the increasing efforts to combine indigenous knowledge with other scientific data in order to better understand the history of ecosystems.
Wildfires pose a significant threat to California's forests. Clarke Knight, paleoecosystem scientist at the US Geological Survey in Menlo Park, California, and her colleagues have therefore investigated how indigenous communities have dealt with this threat scenario in the past. They were particularly interested in cultural fires – the custom of keeping biomass low through small, controlled fires and thus minimizing the risk of large-scale fires. The team published the results in the Proceedings of the National Academy of Science.
One who can still remember this fire is Rod Mendes, head of the Yurok Tribe Fire Department, whose family belongs to the Karuk in Northern California. The tribes of the Karuk and Yurok have been located in the Klamath Mountains for thousands of years, a mountains in the northwest of the state. "When I was a little child, my grandmother always lit fire around the house," says Mendes. »She just kept the place clean. Indigenous people have probably carried out some of the first fire brigade operations in history. «
Understanding how indigenous peoples used the fires is essential for reducing the risk of forest fires, Knight says. "We need to listen to them and learn and understand why they managed the landscape the way they did," Mendes adds.
Oral traditions and drilling nuclei reveal the history of the forest
In order to map the forest history of the region, Knight's team relied on historical accounts and oral narratives from members of the Karuk, Yurok, and Hoopa Valley tribes. Study author Frank Lake, who is also a descendant of the Karuk, had compiled the traditions as part of his doctoral thesis in 2007. They describe the fire and land use of the tribes. For example, the members lit small fires to keep the paths clear; this also reduced vegetation and prevented the spread of forest fires caused by lightning strikes. Larger fires, so-called area fires, were used to improve visibility, hunting and nut harvesting in the forest. The effects of the fire on the vegetation lasted for decades.
For Knight it was important to work with the trunks because they know the region well. The Karuk Resources Advisory Board approved a proposal for the study before it started.
The researchers also analyzed sediment nuclei, which had been collected near two low lakes in the Klamath Mountains, which are culturally important for the tribes. Based on the layers of pollen in the drilling nuclei, the approximate tree density in the area could be determined at different times, and the modeling helped to dating the drilling nuclei, so that the researchers could estimate how the tree density changed over time.
The team also studied the coal deposits in the layers of the drill cores to determine how often and when it burned in the region. Burn scars on tree stumps indicated certain fire sources in the period between 1700 and 1900. Since the rings of the tree stumps serve as an ecological calendar, the researchers were able to compare the fire periods with the corresponding data on tree density. Thus, they determined how this density fluctuated with the occurrence of fires.
These empirical methods could not be clearly confirmed that the fire had been lit by the tribes. However, certain patterns provided clues when this was more likely, says Knight. For example, the increased burning sensation in cool, damp periods, in which fires caused by lightning, rarely occurred to a human influence.
The trees are as dense today as never before
As Knight and her team discovered, the tree density in this region of the Klamath Mountains increased with the settlement of the area. This was partly related to the fact that the European settlers prevented the indigenous peoples from laying fire. In the 20th century, all efforts were finally concentrated to extinguish all the kind of fires and prevent it from the outset - even though controlled fires are still used in forest management. In some areas, the tree density is therefore higher than it has been for thousands of years.
But a dense forest isn't necessarily a healthy forest, Knight explains. Douglas firs (Pseudotsuga menziesii), which dominate the lowland forests of Klamath, are less fire-resistant and more susceptible to devastating forest fires. »The idea that we should just let nature take its course is not supported by our work,« she says.
The results of the study do not surprise the fire ecologist Jeffrey Kane from California State Polytechnic University Humboldt in Arcata. He made similar observations in the Klamath region. "There are much more trees there than 120 years ago," he says.
However, Dominick DellaSala, senior scientist at the forest conservation organization Wild Heritage in Talent, Oregon, points out that the results, which indicate a record tree density, cannot easily be transferred to the entire Klamath region. For this, the data of the study at the lake would have too limited a range. But at least for other, similarly located lakes with a comparable vegetation, the data could be meaningful, says Knight.
Paleo -ecological studies are now increasingly involved in the knowledge of indigenous peoples. But it is still a long way to go, explains geographer Michela Mariani from the University of Nottingham in Great Britain. In Australia, Mariani was able to uncover a similar trend as in the Klamath Mountains: Here, too, the tree density increased after British colonialists prevent the indigenous people from targeting fire. "It is very important that we now involve the indigenous people in the discussion about further fire management," says Mariani. "You have a deeper knowledge of the landscape than us."
Including indigenous voices in research is also crucial for the decolonization of conventional scientific methods, Lake emphasizes. In this way, the peoples who have been excluded, marginalized and misunderstood for a long time would at least receive a little justice.
© Springer Nature 10.1038/D41586-022-01232-X, 2022
