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Jodi Forrester got the call while she was in the forest. The loggers were ready to go. So on a cold winter day in northern Wisconsin, she found herself riding shotgun in a harvester. Forrester, a research scientist in forest and wildlife ecology, watched as the loggers cut down the trees she and her team had carefully selected in the Flambeau River State Forest. Another huge vehicle, a forwarder, clambered behind, pinching the cut trees in its claw and moving them to where they were needed. All the while, the loggers played a little game, dodging between laundry baskets placed around the forest floor to catch leaves and falling debris. In the end, they managed to avoid all but a few.

It was not a typical job for the loggers. Instead of harvesting trees for timber, they were taking part in an experiment—the second phase of a research project on a large scale. Under the supervision of CALS forest and wildlife ecology professor David Mladenoff, Forrester and her colleagues had already been working for years to plan a forest experiment that would stretch over almost 700 acres. The loggers were there to implement that plan. Because all the wood they were cutting was going to be left in the forest as part of the experimental setup, the loggers were not able to remove any of it. It went against their nature.

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Light streaming through gaps in the forest canopy. The size and distribution of those gaps are a key structural element in determining the features of an old-growth forest. Photo by Jodi Forrester/Forest and Wildlife Ecology.

“Every once in a while, the loggers had to cover their eyes,” says Forrester with a smile. “There are a lot of beautiful, valuable trees in that forest, and I think they weren’t too sure about what they were being asked to do.”

But the loggers had agreed to the job because they knew it was part of an experiment that would push the science of forest management in Wisconsin forward. All the work, including the tough job of watching the wood get left behind, was being done in the name of science—specifically, in the name of bringing the characteristics of old-growth forests back to the state.

Old-growth forests have been a scarce sight in Wisconsin since the early 20th century. Clear-cutting in the late 1800s and early 1900s left few old-growth stands. In the Upper Midwest, most big trees had been cut down by the 1930s. In the place of those stands, younger second-growth forests emerged.

Starting in the 1980s, a push to promote and protect old-growth forests picked up steam. It started in the Pacific Northwest, where obligate species, such as the spotted owl, live only in old-growth forests. As the interest in these forests moved east, people in the Midwest began recognizing the valuable ecosystem services provided by old-growth forests, such as storing carbon, maintaining soils and fostering biodiversity in plants, animals and microbes by offering needed habitats.

In Wisconsin it wasn’t a matter of protecting old-growth forests, it was a question of creating them again, or at least some of the functions they provide. And that was no small task. Creating old-growth forests requires defining them, and even that can be difficult. It’s not just a matter of age—and age doesn’t always mean the same thing. A 40-year-old aspen forest would be old, notes Mladenoff; a 40-year-old sugar maple forest, on the other hand, would be quite young.

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Researchers established grids on the forest floor to mark the exact location form which various data (plants and other life forms, soil, temperature and water conditions, etc.) are gathered. Photo by Jodi Forrester/Forest and Wildlife Ecology.

“It’s not always the age that matters,” says Mladenoff. “Sometimes what really matters are the characteristics and features of the forest.”

With the features of Upper Midwestern old-growth forests unclear, Mladenoff and scientists at UW–Madison, other UW campuses and the Wisconsin Department of Natural Resources (DNR) in 1992 started Phase 1 of what was dubbed the Old Growth Project.

Phase 1 was a comparative study. The researchers looked at forests of various ages and histories—a total of 46 different areas—to determine what was unique to the older, unmanaged forests. They considered features like plant and tree species and sizes, woody debris on the ground, snags or standing dead trees, soil characteristics and forest wildlife. Different scientists looked at different aspects, the collaboration creating a complete picture of the forests.

After a decade of collecting and comparing enormous amounts of data, Mladenoff and his colleagues found that many of the features of old-growth forests had to do with two structural elements: the size and distribution of gaps in the forest canopy and coarse woody debris—sizable logs—on the forest floor.

Gaps are openings in the forest canopy caused when large trees fall. With sunlight able to reach the forest floor, these areas become places of regeneration and growth, and the diversity of understory plants is often higher in gap areas than in the surrounding forest.

Coarse woody debris, meanwhile, provides shelter for salamanders, insects and other small animals as well as food for fungi, insects and even other trees like hemlock and yellow birch. Logs also sequester carbon on the forest floor and reduce the amount of carbon dioxide returning to the atmosphere.

“We wanted to explore the importance of those two elements in more detail,” explains Mladenoff. “We wanted to know if creating those structural elements in second-growth northern hardwood forests could restore functional old-growth characteristics.”

Continue reading this story in the Fall 2015 issue of Grow magazine.