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Finding history among the trees

Finding history among the trees

U.Va. researcher and graduate student Dan Druckenbrod extracts and examines samples from trees at Monticello.
Photos by Fariss Samarrai
U.Va. researcher and graduate student Dan Druckenbrod extracts and examines samples from trees at Monticello.

By Fariss Samarrai

What most people call a tree, Dan Druckenbrod calls history.

He stands in the forest below Monticello, the home of Thomas Jefferson, looking for old trees.

“Just because a tree is big, it doesn’t mean it’s as old as a smaller tree nearby,” he said. “White oaks grow slow, so they can be much older than the faster-growing red oaks.”

Druckenbrod spots a white oak that looks like it has seen some years, with missing and gnarled branches near its canopy. He begins coring the tree, seeking its center with a hand-cranked boring device. Soon, he will know how old the tree is and where its place is in the land-use history of a famous American’s property.

“Different trees have different climate responses,” Druckenbrod said. “They have different life spans and growth rates. They are affected by the amount of light and moisture they get. Reconstructing their life history is somewhat of an art, but it’s rationale-based.”

Druckenbrod, a U.Va. doctoral student in environmental sciences who will be graduating in May, compares the weather records kept by James Madison and Thomas Jefferson with the climate history stored in the trees surrounding the old plantations at Montpelier and Monticello. He’s curious about the history of the forest, how old the trees are, what the climate has been like during the lives of the trees and how the land has been used — whether the forest has been logged and replanted, or if it came back naturally.

“There is 300 years of land use at Madison’s Montpelier,” he said. “I’m interested in how the use of the land there has impacted the environment. I’m looking for lessons in land use from the past.”

Druckenbrod, 28, recently published his first scientific paper — in the Bulletin of the American Meteorological Society — detailing his reconstructions of precipitation patterns at Montpelier in the late 18th century. The first chapter of his doctoral dissertation on forest responses to climate, the paper is eloquently written, a fascinating combination of human and natural history.

For more than a year Druckenbrod and colleagues cored dead trees in the old-growth forest around Montpelier and analyzed the long, thin plugs for climate history in the time of Madison. Madison kept extensive weather diaries for seven years while living on the property, and Druckenbrod has compared the natural archive from the trees with Madison’s carefully logged observations.

“They agree well,” he said.

Where Madison saw heavy rainfall, Druckenbrod found accelerated tree growth, a “signal” of moisture. Where Madison recorded drought, Druckenbrod found indications of slow growth, a sign of low moisture. What he found that surprised him, though, was that over a tree’s long life, 200 or more years in some cases, the moisture signal shifted from May in Madison’s time, to June in succeeding decades.

“This may be a physiological response to climate by the tree as it ages. It may be because of a shift in land-use patterns, such as land clearing which may have impacted the seasonal precipitation in the area. Or it may be that the summer peak of rainfall has actually shifted one month later,” Druckenbrod said.

He also found that a drought Madison recorded in 1792 shows up in the tree rings from the Montpelier forest and corresponds with an El Niño event now known to have occurred at the time. El Niño, a periodic warming of surface water in the tropical-Pacific Ocean, often causes a lowering of precipitation in the eastern United States.

Druckenbrod has now turned his attention to the forest below Jefferson’s Monticello. Archaeologists at Monticello are interested in reconstructing the forest history there, and they hope to determine the age of timbers from a log cabin that existed during Jefferson’s lifetime. This information can provide a clearer picture of how the plantation operated and how the land was used. Druckenbrod is again comparing the history he finds in the trees and timbers with the detailed weather diaries kept by Jefferson.

Druckenbrod also is, as a matter of curiosity, trying to determine the age of his parents’ farm home in rural Pennsylvania, the house where he grew up. A large timber from the house rests on a table in his lab along with cores and the cross-sections of trees from Montpelier and Monticello.

“I spent a lot of time in the woods growing up,” he said. “I grew up in a house that dates back to the 1780s, and my parents always collected antiques. I’m interested in both natural and human history.”

Druckenbrod’s co-authors include U.Va. environmental sciences professors Michael Mann and Hank Shugart, and University of Arkansas researchers David Stahle, Malcolm Cleaveland and Matthew Therrell. The paper can be found at:
http://ams.allenpress.com/amsonline/?request=get-toc&issn=1520-0477&volume=084&issue=01

 


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