Scientists read centuries of history in tree rings

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    PLATTEVILLE, Wisconsin (Wisconsin State Journal ) — Some people use clocks and calendars to track time. Evan Larson and Christopher Underwood use trees.

Across a valley, they can recognize the contorted, reaching branches that distinguish an old oak from the more plentiful variety that sprung up around 1850 as European settlers moved into southwestern Wisconsin. A pencil-sized rod pulled from a tree trunk is like a weather almanac.

Larson and Underwood, both geographers at UW-Platteville, divine the history of the land from tree rings, a science known as dendrochronology used to reconstruct centuries of rainfall patterns across the Driftless region, records that underscore just how much wetter the region has become as the climate warms.

“Trees are nature’s best environmental recorders,” Underwood said. “These rings tell us much more than the age of a tree. They can tell us what the tree was responding to over time.”

Over a span of seven years, Larson, Underwood and their students sampled oaks across southwestern Wisconsin and northeastern Iowa, including some that have been growing since the 1700s. They sampled rafters from historic homes and barns built from locally harvested timber to push the record back another century.

The researchers have now turned their attention to cedars that contain data on the weather from 900 years ago, around the time that ancestors of the Ho-Chunk and Dakota people first inhabited the area.

Memories of the landscape
Most kindergartners know that trees add a ring each year.

But those rings can tell you much more than the tree’s age. Thick rings can signal a wet year; narrow rings a drought. Rings can also reveal information about temperature, volcanic activity or even if there were storms spawned by a hurricane.

Sunlight, rain and temperature can all affect how a tree grows, but rings from a single tree can only tell you so much. There are lots of other potential events that could affect a ring from one tree — a deer rubbing its antlers, a lightning strike.

When you look at multiple trees from different areas, you can weed out chance occurrences. Using statistical models, scientists can link growth patterns to specific climatological events.

“There are so many other things you can ask a tree,” Larson said.

By looking at the cells, scientists can piece together a history of first frosts. Certain oxygen isotopes in the tree’s cellulose are evidence of rain from hurricanes. Arsenic could be a marker of lead mining.

“It’s like a good novel that has multiple layers,” Underwood said.

Larson considers trees the memories of the landscape.

“When the people who lived there have moved on, either to a different place or the time has passed, those stories are forgotten,” he said. “But yet you have these trees that have been growing on the same spots for hundreds of years and they carry that story.”

Rediscovering geography
Colleagues since 2009, Larson and Underwood met in graduate school at the University of Tennessee in Knoxville.

Larson, 41, grew up in Milaca, Minnesota, where his parents made ash snowshoes and heated their home with wood.

“I spent my whole life working with wood,” he said.

He planned to be a fourth-grade teacher and football coach, but before his senior year of college he stumbled into a research project and spent six weeks studying the history of wildfires in a ponderosa pine forest of eastern Oregon. Two years later, he was enrolled in a graduate geography program at UT, where he continued his research on forest fires.

“That totally shaped my senior year and career and everything since,” he said.

A Tennessee native, Underwood, 46, studied environmental health in college. He was working on a master’s degree in forest products when he lost interest.

“It just didn’t speak to me,” he said.

He rediscovered geography when a paleoecologist handed him some juniper core samples that he used to reconstruct nearly 2,000 years of precipitation patterns in the Pacific Northwest.

“It put together all the sciences that I’d always loved,” he said. “It was all there and I never realized that until graduate school.”

‘Unknown gem’
Scientists have long used tree rings as a proxy for historical records of temperature, precipitation, fire, hurricanes and even volcanic activity. But with its relatively sparse tree canopy on the native prairies and extensive logging of forests in the late 1800s, the Driftless region has been something of a mystery.

“The Driftless area is an unknown gem,” Larson said.

They scoured the landscape and historic photos for oaks with wide canopies, relics of an era when the trees could spread their branches on the open savanna. They put out a call to landowners, though Larson said most of the trees they volunteered dated from the 1850s, when European settlers began altering the landscape.

“It is very rarely the biggest tree,” Larson said. “Generally the big trees are the ones that catch people’s attention.”

Using an increment borer, a hand tool developed in the 1800s, researchers can drill into a tree trunk and extract a thin dowel-shaped core sample, leaving the tree with an injury no worse than a paper cut.

The scientists also looked for historic buildings where they could sample timbers from trees that were cut in the mid-1800s. By matching patterns in the rings with those from living trees, they could figure out when that wood was cut.

Altogether they sampled 420 living trees, 15 stumps and 17 beams with rings dating as far back as 1631.

Wet times
What they found was a series of droughts in the 1700s and late 1800s far more intense than anything experienced in the era of modern weather records.

Larson said the data support models from the Wisconsin Initiative on Climate Change Impacts, which show Wisconsin has become significantly wetter over the past half-century and is likely to become more so as artificial warming continues.

“People of Wisconsin today have lived in unusually wet times,” Larson said. “We see climate change playing out in the rings of these oak trees.”

But Larson said the oaks also tell a cautionary tale for farmers and others who benefit from the current climate regime.

“It doesn’t tell you what is coming next. It does tell you what could,” Larson said. “What we think of as normal because of our human life span — it’s not if we look at the lifespan of a tree.”

12th-century cedars
Underwood and Larson have now turned their attention to cedars in the rocky outcrops in the Driftless region. With rings that date back to the early 1100s, these hearty trees contain records of another period of dramatic climate change and population shifts.

In that era, people known as Mississippians migrated north into what is now Wisconsin, mingling with the native Woodland people, said Bob Birmingham, a retired professor of anthropology and former state archaeologist. By about 1200, a new culture known as Oneota emerged, which Birmingham said may have arisen from the adoption of corn agriculture and contact with the Mississippians.

UW-Madison climatology models indicate this century was also punctuated by a sharp drought and much cooler temperatures, known as the “little ice age,” both of which Birmingham said would have affected agriculture and may have played a role in the rearrangement of the population.

Larson and Underwood hope they can combine the cedar and oak records to develop an even better understanding of the region’s prehistoric climate. Underwood likens it to composing music or writing a novel, with each species providing a different voice.

“Oaks are really good at telling us the story of drought. The cedars are really good at telling us something about temperature,” he said. “It’s just different layers, or different chapters of the environmental story of this landscape.”

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