In the past several decades, prescribed burns have been integrated into land management policies in the United States. This active approach to conservation is driven, in part, by the belief that people have been using fire to shape American landscapes for thousands of years.
In New England, fires are used to maintain diverse habitats like heathlands, scrublands, and grasslands that support rare and endangered species. According to new research analyzing nearly 15,000 years of paleoecological and archeological data, however, those open habitats are not a reflection of the “natural” landscape. They are the product of European agricultural practices introduced just a few hundred years ago.
A team of researchers led by Wyatt Oswald, an environmental scientist at Emerson College in Boston, looked at charcoal and pollen grains that had accumulated in the sediment of lake beds across New England, comparing them to archeological records in the region. “This is the first time that we bring together these two points of view on how ecosystems have changed, thinking about it in terms of environment-human interactions,” Oswald said.
The team’s findings indicate that climate, rather than human activity, has been the primary driver of fires in the region. But prior to European colonization, fire hadn’t played an active role in the landscape for 8,000 years. Mature forests dominated the region until Europeans began clearing them for grazing and agriculture.
Digging into Lakes to Look Back in Time
Oswald’s team collected sediment cores from 21 bodies of water across the region, favoring coastal sites with a history of high-density human settlement and a diverse array of vegetation types.
To extract charcoal and pollen data at the right scale, the team had to be careful about the size of lake they selected. A large lake accumulates debris from too wide an area, and small ponds provide information about only the immediate vicinity. “The ideal pond is about the size of a soccer field,” Oswald said.
Once the sites were selected, it was time to dig into the lakes. The coring setup involved two canoes with a plywood board strapped onto them “catamaran style.” Oswald’s team anchored the setup to the shore at three points, then lowered the coring device through a hole in the plywood to extract sediment from the lake bed 1 meter at a time.
Back at the lab, researchers used radiocarbon dating to determine the age and rate of accumulation of the sediment in each core, finding that 13 of the cores stretched as far back as 9,600 to 14,000 years and eight extended back roughly 2,000 years. The cores were sampled at 50-year intervals. At each interval, the researchers identified the relative percentage of pollen from each species of plant present. They also counted pieces of charcoal and calculated the charcoal accumulation rate to get a sense of the relative frequency of fire over time for each site.
To understand whether humans might be the primary drivers of fire frequency, the team compared their findings to human population levels in the area over time using carbon dating of archeological materials. They also compared their findings to preexisting reconstructions of moisture and temperature data.
No Evidence of Native Americans Using Large-Scale Controlled Burns
The team’s findings indicate that human populations in New England peaked twice over the past several thousand years: once between 3,000 and 5,000 years ago and again between 500 and 1,500 years ago. Human societies became larger and more densely populated, numbering in the tens of thousands. But their increased numbers do not correspond to increased charcoal accumulation or changes in vegetation.
“These were complex societies that undoubtedly had local-scale impacts on the landscape,” Oswald said. “But the story that comes away from this analysis is there simply weren’t big regional-scale impacts on ecosystems.”
The findings demonstrate that “Native Americans in what is now called New England were, among other things, conservators of forest environments,” said Joyce Chaplin, an early American historian at Harvard University who was not involved in the study.
What did affect fire frequency in the paleoclimate record was climate change—but the team found little evidence of fire’s influence in the past 8,000 years. Around 8,000 to 10,000 years ago temperatures began to increase, and boreal forests gave way to grasslands and oak savannas.
Dry grasses provide perfect kindling for fire, so charcoal accumulation rates predictably increased during this time. But as the climate became wetter, hardwood forests took over. Fire levels on the landscape remained low through the two peaks of human habitation, until Europeans arrived and began cutting down forests and cultivating fields—at which point, charcoal accumulation rates shot up.
“This is an important paper because it tackles…the effects of Indigenous human populations upon ecosystems in the Americas,” said Jack Williams, a paleoecologist at the University of Wisconsin–Madison who was not involved in the study. “Getting this question right is important in part because it affects how we steward New England landscapes today to sustainably maximize their biodiversity, cultural, and economic resources.”
The new study appeared in Nature Sustainability.
—Rachel Fritts (@rachel_fritts), Science Writer