Native American communities actively managed North American prairies for centuries before Christopher Columbus' arrival in the New World, according to a new study by researchers from Southern Methodist University and the University of Arizona.

Fire was an important indigenous tool for shaping North American ecosystems, but the relative importance of indigenous burning versus climate on fire patterns remains controversial in scientific communities.

The new study, published in Proceedings of the National Academy of Sciences, documents the use of fire to manipulate bison herds in the northern Great Plains. Researchers found that, contrary to popular thinking, burning by indigenous hunters combined with climate variability to amplify the effects of climate on prairie fire patterns.

"The important contribution of this research to paleoenvironmental science is a demonstration of the impact that relatively small groups of mobile hunter-gatherers could have on amplifying the broader climatic effect on wildfires," said study co-author María Nieves Zedeño, a professor in the UA School of Anthropology. "We have added a new human dimension to the discussion of interactions between people and climate by actually going back in time and showing how mobile hunter-gatherers manipulated the environment by improving the grassland through fire."

The relative importance of climate and human activities in shaping fire patterns is often debated and has implications for how we approach fire management today.

"While there is little doubt that climate plays an important top-down role in shaping fire patterns, it is far less clear whether human activities — including active burning — can override those climate influences," said lead study author Christopher Roos, a UA alumnus who is now an associate professor of anthropology at Southern Methodist University. "Too often, if scientists see strong correlations between fire activity and climate, the role of humans is discounted."

The impact of hunting bison with fire

Anthropologists and historians have documented a wide variety of fire uses by Native peoples in the Americas, but fire scientists have also documented strong fire-climate relationships spanning more than 10,000 years.

"People often think that hunter-gatherers lived lightly on the land," said study co-author and UA alumna Kacy L. Hollenback, who is now an assistant professor at Southern Methodist University. "Too often, we assume that hunter-gatherers were passive in their interaction with their environment. On the Great Plains and elsewhere, foragers were active managers shaping the composition, structure and productivity of their environments. This history of management has important implications for contemporary relationships between Native American and First Nations peoples and their home landscapes — of which they were ecosystem engineers."

Working in partnership with the Blackfeet Tribe in northern Montana, the researchers combined landscape archaeology and geoarchaeology to document changes in prairie fire activity in close spatial relationship to hunting features known as drivelines.

Drivelines consisted of a series of rock piles, spaced a few steps apart and arranged in a funnel-like shape up to 5 miles long, which were used to drive herds of bison off cliffs to be harvested en masse.

"We surveyed the uplands for stone features that delineate drivelines within which bison herds would be funneled toward a jump," Zedeño said.

"By radiocarbon dating prairie fire charcoal deposits from the landscape near the drivelines, we were able to reconstruct periods of unusually high fire activity that are spatially associated with the drivelines," Roos said.

The overlap between peak periods of driveline use, between about A.D. 900 and 1650, and prairie fire activity, between A.D. 1100 and 1650, suggests that fire was an important tool in the hunting strategy involving the drivelines.

The researchers suggest that fire was used to freshen up the prairie near the mouth of the drivelines to attract herds of bison, who prefer to graze recently burned areas. Episodes of high fire activity also correspond to wet climate episodes, when climate would have produced abundant grass fuel for prairie fires.

The absence of deposits indicating high prairie fire activity before or after the period of driveline use, even though comparable wet climate episodes occurred, suggests that burning by Native hunters amplified the climate signal in prairie fire patterns during the period of intensive bison hunting.

"We need to consider that humans and climate have more complicated and interacting influences on historical fire patterns," Roos said. "Moreover, we need to acknowledge that hunter-gatherers can be active influences in their environments, particularly through their use of fire as a landscape tool. We expect that future studies of human/climate/fire interactions will further document the complexity of these relationships. Understanding that complexity may prove important as we try to navigate the complex wildfire problems we face today."

UA partnership with the Blackfeet Tribe

The researchers' work was made possible by a longstanding research agreement between Zedeño and the Blackfeet Tribe. Zedeño was approached by the Blackfeet Tribe 14 years ago and has since been studying the tribe's pre-European contact history through the archaeological record.

Her work with the tribe has included two National Science Foundation projects dedicated to mapping and recording all of the bison hunting complexes along a 45-mile stretch of the Two Medicine River on the Blackfeet Reservation. She and Roos also co-authored a book chapter on the social and cultural aspects of the Blackfeet's fire use from an environmental history perspective.

"To me," Zedeño said, "the most interesting part of the work is to integrate science and traditional culture — to integrate ancient Blackfoot concepts and practices as they relate to their landscape and their homeland."