Fire-scarred oaks reveal how Illinois changed under Native and settler Americans
A stand of aged and grizzled post oak trees has been telling scientists the remarkable story of the landscape around Illinois – as it shifted from Native American to settler - over the last 200 years. And it's a story punctuated by the rhythm of repeated forest fires, that left their scars in the tree-rings of these 226-year old trees. For the team at University of Illinois, it is the use of fires in shaping the landscape that is most striking.
The paper has just been published in the journal Castanea botanist William McClain, from the Illinois State Museum, and two colleagues from the Illinois Natural History Survey. It looked at the history recorded in sections of the trees, which were felled in 1996. ''I was just amazed at the fire scars in these trees,'' said McClain ''I knew that the information that was in these tree trunks was really, really valuable.''
The scars that so excited McClain were left on the hearts of the post oak trees by intense fires – which the trees appear adapted to survive, despite the damage done. And the fires were no accident; they appear to happen every 2-3 years in this region, when it was still settled by Native Americans. This tallies with accounts of Native Americans deliberately starting the fires to keep the landscape open – and so better able to sustain game-hunting on the resulting prairie and mixed woodland.
This managed landscape changed dramatically once European settlers arrived in numbers. After 1850, there was a two decade fire-break. That was enough to allow the fast-growing under-story to completely out-shade the post oak seedlings. The result was that the remaining non-agricultural land was dominated by a dense canopy of trees, such as maple, with the preexisting post oaks surviving in occasional stands.
There were further fires recorded in the tree rings after 1885, but these appear to be less intense. They were probably related to settlers clearing spaces in the dense forest, for livestock, and, ironically, to reduce fire hazards.
Co-author of the paper Greg Spyreas, of the Illinois Natural History Survey, said ''For hundreds, maybe thousands of years, this was a stable post oak woodland. And then you have a gap of a couple of decades where there were no fires and suddenly the whole system is completely different. It's amazing how, from Kansas to Ohio, these ecosystems completely depend on fire to be stable.''
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