From fire scars in the stumps of old-growth oak trees, a team of researchers led by Illinois botanist William E. McClain has given us an amazing glimpse into the history of fire in the U.S. Upper Midwest. The team’s work, published in a recent issue of the journal Castanea, is the most in-depth study of the region’s fire history published to date—detailing the frequency of human-made fires over a 226-year period and revealing how a brief interval of fire suppression permanently changed the landscape.
McClain, who is based at the Illinois State Museum, began tracing the history of fire in Illinois oak trees in 1996. That year, he made the observation of a lifetime when he was invited to have a look at a stand of trees slated for auction in northwestern Hamilton County, at a site a few miles south of the village of Dahlgren.
“The woods was wonderful,” he said. “It had numerous trees that had old-growth features, such as the spiraling of bark on the trunk. We walked through an adjacent [lot where] trees had [already] been harvested. The fire scars in the stumps caught my attention immediately. I had never seen so many scars.”Living Records of Fire History
While researchers from the Illinois Department of Natural Resources, Illinois Nature Preserves Commission, and Illinois Natural History Survey, created maps and performed extensive ecological assessments of the study area, McClain prepared cross-sections from 36 old-growth post oaks (Quercus stellata) scattered across the Hamilton County study site. For each cross-section, he counted growth rings and fire scars.
Fire scars, similar to other marks and features found in the growth layers in the heartwood of trees, are records of natural history. Each year, a new layer of wood, or growth ring, is formed from cambium, the thin layer of living tissue between the wood and the bark. In trees that survive fire, the areas of cambium that die as a result of exposure to intense heat are overgrown by a new cambium layer. “This process continues each year until the wounded area is healed,” McClain said.
After healing, there exists a visible fissure in the heartwood, which is a distinguishing feature of fire scars. Fire scars frequently are dark in color and contain charcoal.
Tracing the Fire History of Illinois
While the ecological impacts of present-day wildfires are relatively well understood, much less is known about the historical role of human-made fire in shaping the ecosystems that exist today. This is especially true for places such as the Midwestern United States, where fire is now relatively infrequent but appears to have been common throughout long periods of the region’s history.Written accounts indicate that many fires were observed in Illinois in the 17th and 18th centuries. According to McClain, “During [these periods], many fires were started intentionally for hunting or other purposes. Lightning-caused fires were not as common.”
After comparing the chronology of post oak fire scars with the written accounts of fire, and after examining fire records dating to the 1670s from white oaks in Baber Woods in Edgar County, McClain and colleagues were able to confirm that the peoples who inhabited Illinois prior to the arrival of European settlers set fires that burned into the woodlands of Hamilton County about every two or three years. The team also found that a temporary period of fire suppression produced a dramatic and permanent ecological change locally.
Indeed, the fire scars in post oaks and fire records from nearby states indicate that fires were set routinely in the Upper Midwest, perhaps for hundreds of years, until 1850. Up to that point, open post-oak woodlands and prairies dominated the Midwest landscape. (Fires did continue longer in parts of Illinois and other states.)
But the disappearance of fire in the Hamilton County woods after 1850 caused a fundamental change in the region’s ecology. Post oaks are slow growing, relatively insensitive to fire, and intolerant to shade. Until 1850, they were able to thrive because frequent fire killed off faster growing, shade-tolerant species and kept the woodland open. Although the practice of intentionally setting fires resumed around 1885, by then the woodland had a greater density of trees. The lack of fire for 35 years had allowed shade-tolerant species to proliferate, giving rise to a forest with a dense understory.
According to McClain, this pattern of fire suppression and ecological change is now apparent elsewhere in the United States. “The lack of fire is considered to be the reason why mesic (requiring moderate amounts of moisture), shade-tolerant, fire-sensitive tree species are replacing the oaks in forests throughout the eastern part of the United States.”
“Oaks are not reproducing,” he added. The same is true for the post oaks in Hamilton County and for the white oaks in Baber Woods—the seedlings of these are not surviving, which means that they may soon disappear from the region entirely.
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