Climate Fact: Wildfire in the Southeastern United States
In Brief: Humans have been intentionally setting fire to the landscapes of the South for over 12,000 years, directly influencing the evolution of the region’s ecosystems and obscuring the role that climate variability has played over the same period.
People have been intentionally setting fire to the ecosystems of the southeastern United States for the entire post-glacial period of the last 12,000 years. Early cultures used fire to hunt ice age megafauna (Wooly Mammoths, Giant Sloths, Mastodons, etc.) and continued this practice with smaller game after they went extinct. By around 8,000 years ago, people in the South began using fire to maintain more favorable habitat types: edge habitats for deer, canebrakes where materials for weapons and lodging grew, open areas favorable to species like blueberries, and in Appalachia, fire tolerant oak communities for acorn production. Beginning around 2,300 years ago, burning was used to clear lands to adopt more sedentary and crop-dependent life styles. The frequent use of fire to shape the landscape continued by the European settlers, who used fire for many of the same reasons, with the additional purpose of creating better forage for grazing. It was not until the 20th century that human encouragement of fire in the South transitioned into human suppression of fire. Along the coastal plain, fire suppression likely contributed to the loss of old-growth longleaf pine ecosystems, considered America’s most biodiverse habitat type and now 95 percent gone. In the Appalachians, fire suppression has led to the replacement of fire-dependent oaks and pines by maples and beeches.
Versus the climatic influence, the direct human influence has likely had a stronger impact on wildfire occurrence in the South over the past 12,000 years. Despite this, some general statements about the climate fire relationship in the South can be made:
- Burning peaks in the spring and fall, when lack of leaves on deciduous trees allow sunlight to dry surface fuels and low humidity and high winds encourage fire ignition and spread. There is a lull in fire activity during the winter, particularly in the more northern sections of the South.
- As is the case throughout the world, the variability of precipitation is as important for fire occurrence as the total annual precipitation. For example, the DeSoto National Forest in Mississippi is one of the wettest locations in the South in terms of average annual precipitation, but this annual average obscures the significant variability that leads to periods with abundant moisture and ample plant growth being followed by dry periods when this growth dries and creates just the right conditions for a burn. The South is now wetter than it was 100 years ago, with most of the increase in precipitation coming in extreme rainfall events. The average duration of warm season dry episodes grew as well.
- In Virginia and West Virginia, fire is more frequent in the Blue Ridge province than in the much drier Ridge and Valley province to its west, as well as much more frequent than in the Appalachian Plateau to the west of the Ridge and Valley province. Dry conditions in the rain shadowed Ridge and Valley province do not allow for fuel build-up and the consistent and light rain events in the Appalachian Plateau mean that fuels rarely are allowed to dry.
- The warm, humid and possibly wetter conditions of the middle Holocene (about 6,000 to 3,000 years ago) correspond to a period of reduced fire in the South.
Seasons: Fall, Winter, Spring
Sources: Lafon, CW and Quiring, SM. “Relationships of Fire and Precipitation Regimes in Temperate Forests of the Eastern United States.” Earth Interactions 16 (2012): 1-15 and Batzer, DP and Baldwin, AH (eds.). “Wetlands of the Northern Gulf Coast” in Wetland Habitats of North America: Ecology and Conservation Issues. Berkeley, California: 2012. University of California Press and Mitchell, RJ and Duncan, SJ. “Range of Variability in Southern Coastal Plain Forests: Its Historical, Contemporary, and Future Role in Sustaining Biodiversity.” Ecology and Society 14 (2009): 17-33 and Lafon, CW. “Fire in the American South: Vegetation Impacts, History, and Climatic Relations.” Geography Compass 4/8 (2010): 919-944 and Folwer, C and Konopick, E. “The History of Fire in the Southern United States.” Human Ecology Review 14 (2007): 165-176.