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Northwoods Researcher Finds Tree Ring Sizes Match Historic Lake Levels, Giving Window Into Past

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Ben Meyer/WXPR
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Records of water levels on many Northwoods lakes often only go back a few decades, if they exist at all.

But one researcher has figured out a way to see the story of lakes going back hundreds of years.

That history, and a clue about the future, is as simple as tree rings themselves.

“We’re proposing using these trees as an Excel spreadsheet, as a way to get at [the history of] these lake levels,” said Dom Ciruzzi, a UW-Madison graduate student working at Trout Lake Station in Boulder Junction.

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Credit Ben Meyer/WXPR
Dom Ciruzzi inspects the rings of a century-old tree under a microscope at UW-Madison Trout Lake Station in Boulder Junction.

Ciruzzi was looking at a slim cylinder of wood, the size and shape of a drinking straw, under a microscope.  It was a core sample of a century-old red pine.

Each tree ring in the sample corresponds with one year.  The bigger the ring, the more growth that year.

Ciruzzi’s research found the ring sizes of trees near lakes match almost perfectly with the water level in the lake that same year.

“This relationship between tree growth and groundwater is so strong that you can reconstruct groundwater and lake levels from tree cores,” Ciruzzi said.

To reach that conclusion, Ciruzzi spent hour after hour in the field, armed with a tool to take tree cores.  He looked for red pines 100 or 200 years old near lakes and near groundwater monitoring wells.  With groundwater depth data and core samples in hand, Ciruzzi was able to analyze the data.

The more samples he examined, the more convincing the results were.

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Credit Ben Meyer/WXPR
A straw-shaped core sample of a red pine tree.

“Among all of these different variables and factors that I’ve found and related to tree growth in northern Wisconsin, every single time, depth to groundwater has been the best predictor of tree growth in this area for red pine trees,” Ciruzzi said.

He found trees near lakes don’t just rely on falling rain.  Their roots tap into groundwater for hydration, too.

Higher groundwater levels lead to better growth.  Those same higher groundwater levels also correspond to higher lake levels.

That means tree rings show, with a high level of accuracy, lake levels year by year.

“I’m trying to get this concept of trees as sensors, basically,” Ciruzzi said.  “They’re essentially collecting data over their lifetime in their rings.”

By looking into a microscope, Ciruzzi looks into the past, which can help give clues about the future.

“Something that we’re interested in is the trajectory of these lakes,” he said.  “What will these lakes look like in the future?”

Each groundwater reading and each tree core get Ciruzzi a little closer to a full picture of the Northwoods.

“It just seems like the start of a new sort of understanding about how these trees are growing up here,” he said.

Ciruzzi has found that inexpensive home equipment works just as well as pricey lab tools to measure and document tree rings.  That means just about anyone can see the history of their lake in a tree core.

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