Minnesota scientists probe tree rings for clues to climate history

The towering red pine that lived for nearly 250 years Up North is no more.

But its remains hold valuable clues that reveal the ups and downs not only of the tree's life, but of its surrounding environment.

Most of us know that a tree's age can be determined by counting its rings. But three scientists at the University of Minnesota say that's just the beginning. Those rings also bear witness to floods, drought and other milestones, making it possible to track climate, weather and natural disaster trends spanning centuries.

"Right here you can see the birth," Kurt Kipf­mueller said, running his fingers across the surface of a large stump extracted from Voyageurs National Park. Tracing a circle at the stump's center, he noted: "It goes back to 1769."

Kipfmueller and his colleagues — Dan Griffin and Scott St. George — act almost like medical examiners performing an autopsy, inspecting the dead trees under microscopes, searching for tell-tale clues.

The Minnesota team represents one of the most prominent research groups of its kind in the country. They conduct their examinations in a pine-scented lab on the university's West Bank campus, surrounded by stacks of tree slabs cut into cross sections.

Where others see dead wood, these guys spot treasure.

"Trees are like nature's bookkeepers," St. George said. "Every year they are influenced by different aspects of their environment — the weather, forest fires, our actions and the actions of animals. Because wood is so sturdy, information that gets written down now will maybe last for 10 years or 100 years or even 1,000 years."

At their cluttered lab, each scientist is known for his specialty: Kipfmueller's the fire guy; St. George is into floods; Griffin's the drought expert.

They gravitate to different kinds of trees, too.

Pine trees are what Kipfmueller knows best. He has examined hundreds of trees in the Boundary Waters Canoe Area Wilderness and has a seemingly endless catalog of ring patterns in his head.

Griffin focuses on oaks, stemming from his experience working in California studying drought patterns. Meanwhile, St. George favors burr oaks, found throughout Minnesota and Canada.

They teach separately, but often regroup in the lab. Dressed casually in jeans and hiking shoes, they look like they could hit the trail at a moment's notice. "We're very serious sometimes, but we also like to have a little bit of fun," Kipfmueller said.

One recent afternoon, the three playfully ribbed one another while sharing stories about how they came to work together.

"I'm from Canada," St. George said.

"We don't hold that against him, though," Kipfmueller interjected.

"Well, I'm Southern, and they do hold that against me," said Griffin, an Arkansas native and the newest member of the team.

All three studied at the renowned Laboratory of Tree-Ring Research at the University of Arizona in Tucson, which was founded by A.E. Douglass, an astronomer who is considered the father of dendrochronology — the scientific study of tree rings.

Steve Leavitt, the Arizona lab's acting director, says the tree guys in Minnesota are poised to contribute greatly to the science community's understanding of climate issues.

"All three of them are well-accomplished and well-known in the world of dendrochronology," Leavitt said. "In terms of solving research problems and now, with three [in Minnesota], they have a serious critical mass — necessary for all kinds of success in the field."

Like the trees themselves, the men are determined to leave their mark.

Into the woods

But even in their own back yard, they've got a lot of work ahead of them. "[We] don't know much about the climate history of northern Minnesota," Kipf­mueller said. "We could stand to learn a lot more, especially since things are changing pretty rapidly."

The historical record embedded in the wood is as varied as the trees themselves.

"Some trees will tell you a story that is mainly about the weather," St. George said. "Trees that are growing in stressed environments, they'll tell you stories about wet years and dry years throughout their lifespan." Other trees have something to say about the people who lived around them.

In the summers, the work takes the three men out into the field — sometimes to remote parts of the continent.

Kipfmueller makes two trips a year to the Boundary Waters area, paddling deep into the woods by canoe. St. George has trekked to Montana. Griffin has been dispatched to California to study drought patterns. He recently published findings that suggest California's 2014 drought was the worst in 1,200 years.

"We're like raccoons. We'll go into rivers, houses, buildings — anywhere that you have solid wood preserved," St. George said. "Sometimes you find a tree that grew at the right time at the right place. It's like the perfect piece of a puzzle."

Take that old red pine, for instance.

The centuries-old tree has five fire scars, indicating that it witnessed several forest fires. By analyzing the rings, Kipf­mueller said the fires occurred fairly regularly between 1801 and 1846. After that, there are no more fire scars. No one knows for sure why the fires abruptly stopped, but Kipf­mueller says there are a few possibilities.

"One theory is it's because there are no people [living] there to burn," he said. "There's no more Native American burning." Indians may have set fires to boost berry production, he said.

Another theory is that European settlers put out the fires during what is called a "fire suppression period" and the practice continues today. Or maybe it was a lack of lightning in the area that lessened the frequency of fires.

"Right now, we're trying to be cautious," Kipf­mueller said. "It's really speculative. We don't have enough information."

The red pine's rings have yielded many important clues.

But for now, this case remains under investigation.

Allie Shah • 612-673-4488