www.psu.edu/news/research/story/climate-scientists-increasingly-ignore-ecological-role-indigenous-peoples">debate continues among scientists over whether tree composition in forests in eastern North American historically have been influenced more by climate or by cultural burning, which is the intentional and controlled use of fire by Indigenous people to manage their environment. Now, a new study of southern New England forests by a team including a researcher from Penn State lends credence to the cultural burning hypothesis, suggesting that fire-tolerant vegetation - oak, hickory and pine - were significantly more abundant near Indigenous settlements over the last 5,000 years.
"The results of this study strongly suggest that Native Americans did, indeed, extensively use fire and other disturbances such as land clearing for villages, agriculture and trails, and both directly and indirectly promoted fire-adapted trees," said study co-author Marc Abrams, Penn State professor emeritus of forest ecology and physiology in the College of Agricultural Sciences. "These trees were very important to the Native American diet because of the mast - nuts and acorns - they produced. This mast also fed large and small mammals and birds, which then were hunted for food by the Native Americans."
To reach their conclusions, recently published in Land, the researchers employed varied analyses, comparing the locations of Indigenous settlements and climate over the last 5,000 years with relative tree abundances estimated from pollen and land survey records. They investigated evidence of long-ago burning, known as paleocharcoal data, and found that forests close to Native American villages in southern New England were populated by 86% to 91% fire-tolerant trees versus 66% to 82% outside the village catchment area. This increase was most evident in somewhat cooler areas where oak-pine forests transitioned to northern hardwoods.
They found that fire-tolerant trees - which possess fire-adaptive traits such as thick, insulating bark, deep roots and flammable leaf litter - were less abundant away from settlements. Correlative models, which predict the potential distribution of a species based on the observed association between its presence and environmental conditions, showed that the distance to an Indigenous settlement was an important predictor of fire-tolerant tree abundance in the 17th and 18th centuries, the researchers reported.
European colonization and settlement resulted in dramatic decreases in Indigenous populations and fire exclusion through land-use breaks and policy implementation, Abrams noted. Deprived of regular fire, forests changed over time, with increased tree densities and replacement of fire-tolerant vegetation with fire-sensitive, shade-tolerant vegetation such as such as maple, beech, birch and hemlock.
Regarding the ongoing debate about whether climate or cultural burning mainly has determined forest composition, Abrams suggested the findings of this study can't be ignored.
"These results indicate that pollen and charcoal analyses lack the specificity and resolution to detect low- to moderate-intensity understory burning used by Native Americans, leading some scientists to believe fire did not occur," he said. "In southern New England, climate was important in latitudinal changes in forest types, such as northern hardwood versus oak-pine, but climate cannot explain the increase in fire and pyrogenic trees near Native American villages."
The ecological process in which fire-maintained open forests, woodlands and grassy savannahs are converted to closed-canopy forests due to fire exclusion, leading to changes in forest-floor flammability and inhibiting fire-adapted species, is called mesophication. Abrams, who co-wrote the seminal paper on the concept in 2008, was interviewed and quoted about fire and oak and mesophication in an April 16 article in the Washington Post.
"Nearly 100% of these forests would have burned repeatedly over the last 5,000 or 7,000 years prior to European settlement," he said, adding that without fire, there is nothing to interrupt a process known as succession, in which open woodlands become forests, the canopy closes and mesophytic species - trees such as maple, beech, birch and hemlock that thrive in temperate, moist environments - dominate.
"During my long career, I often published on the importance of using understory fire to manage and sustain healthy oak forests and the negative consequences of excluding fire, which is the main scenario since the 1930s when the Smokey the Bear campaign started," Abrams said. "This is leading to the mesophication of oak forests where more shade-tolerant mesophytic trees start to take over and oak regeneration fails."
Stephen Tulowiecki, associate professor of geography and sustainability studies, State University of New York, was first author on the study. Brice Hanberry, research ecologist with the U.S. Department of Agriculture Forest Service, Rocky Mountain Research Station, contributed to the research.
The U.S. Department of Agriculture Forest Service supported this research.
