This spring, more than 40 percent of the western United States is in a drought that the USDA deems “severe” or “exceptional.” The same was true in 2013. In 2012, drought even spread to the humid east.

It’s easy to assume that a 3-year drought is an inconsequential blip on the radar for ecosystems that develop over centuries to millennia.

But new research appearing in the latest issue of Ecological Monographs shows how short-lived, but severe, climatic events can trigger cascades of ecosystem change that last for centuries.

Amy Hessl, professor of geography at West Virginia University, is one of the authors of the article “The legacy of episodic climatic events in shaping temperate, broadleaf forests.”

She said that climate warms, increasing drought conditions and earlier springs like that of 1774, could easily expose eastern forests to the kinds of conditions that changed them so abruptly in the 17th and 18th centuries.

“We tend to think of old growth forests as never changing and quite stable but in reality they are a legacy of prior climate conditions,” Hessl said. “Extreme events like frosts and droughts in the past shaped the forests – both the species and ages – of today. We can expect that extreme events in the future will continue to have major impacts on forests in the eastern U.S. at a regional scale.”

Some of the most compelling evidence of how ecosystems respond to drought and other challenges can be found in the trunks of our oldest trees. Results from an analysis of tree rings spanning more than 300,000 square miles and 400 years of history in the eastern U.S. – led by scientists at WVU, Columbia’s Lamont-Doherty Earth Observatory, the Harvard Forest, and elsewhere – point to ways in which seemingly stable forests could abruptly change over the next century.

The tree records show that just before the American Revolution, across the broadleaf forests of Kentucky, Tennessee, North Carolina, and Missouri, the simultaneous death of many trees opened huge gaps in the forest—prompting a new generation of saplings to surge skyward.

There’s no historical evidence that the dead trees succumbed to logging, ice storms, or hurricanes. Instead, they were likely weakened by repeated drought leading up to the 1770s, followed by an intense drought from 1772 to 1775. The final straw was an unseasonably devastating frost in 1774 that, until this study, was only known to historical diaries like Thomas Jefferson’s “Garden Book,” where he recounts “a frost which destroyed almost every thing” at Monticello and was “equally destructive thro the whole country and the neighboring colonies.”

Hessl received her doctoral degree in geography and regional development from the University of Arizona in Tucson, Arizona. She is interested in the interaction between ecosystem processes, climate variability and human activities in forested systems. She has studied the influence of climate and land-use history on fire regimes in the Pacific Northwest and Mongolia and has developed millennial length hydroclimate reconstructions for the eastern United States using the tree rings of ancient eastern red cedar.

She has published in BioScience, Climatic Change, Ecological Applications, Journal of Biogeography and the Professional Geographer, among other journals and has received support for her research from the National Science Foundation, the National Geographic Society, USDA Forest Service, USDI National Park Service, and the Joint Fire Sciences Program.

The full paper in the journal Ecological Monographs, “The legacy of episodic climatic events in shaping temperate, broadleaf forests,” is available at . Photos are available at

For more information or to arrange an interview with Hessl, contact Devon Copeland, Director of Communication and Marketing, Eberly College (; 304-293-6867).

Read about Hessl’s ongoing research in Mongolia here:



CONTACT: Devon Copeland, Director of Communication and Marketing, Eberly College of Arts and Sciences
(304) 293-6867,

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