To many Wisconsinites, the state’s northern forests are sturdy and vast swaths of land that resist the forces of the outside world and symbolize the state’s natural inheritance. But for well over a century, human activity has removed virtually all of the fabled old-growth trees in Wisconsin and continues to shape what contemporary residents think of as the venerable old North Woods.

When glaciers last retreated from Wisconsin about ten thousand years ago, they left behind soil conditions conducive to growing different kinds of trees, which gradually spread across the state. The population and distribution of tree species in Wisconsin’s forests were pretty stable from about 2,000 years ago until the 1800s, according to David Mladenoff, a professor of forest and wildlife ecology at the University of Wisconsin-Madison. It’s only the last 200 years that have been really dramatic in terms of the pace of change.

In an October 8, 2015, talk in Madison presented by the Wisconsin Academy of Sciences, Arts and Letters, Mladenoff explained how and why the state’s forests have changed since the Ice Age — termed the Wisconsin glaciation — and what challenges lay ahead. This talk was recorded for Wisconsin Public Television’s University Place.

When it comes to the big picture, Mladenoff said, it’s important to understand that change is always happening in a forest, even if people associate these landscapes with a certain permanence. He focused on how settlement and farming across the state rearranged landscapes, and how voracious logging beginning in the 1840s rapidly changed the ecology of the North Woods. As American expansion and growing Midwestern settlements like Chicago devoured the state’s pines for lumber, northern Wisconsin became hospitable to different varieties of trees and animals.

For his talk, Mladenoff used data from surveys of the state’s forests to create detailed maps and visualizations depicting how the species balance was altered, and how factors like the sizes of trees changed as well. He also got into some of the finer points of the different ecosystems at work in forests around Wisconsin, the mechanisms by which human activity has changed them, and looked ahead to the ways in which climate change might change the forest’s balance of species yet again.

Key facts

• The balance between oak savanna-dominated southern Wisconsin and the pine- and hemlock-dominated northern forests changed only a little from 2,000 years ago to the 1800s, though not all that dramatically. Historically, southern Wisconsin’s forests have burned more often — through both natural and human-lit fires — than forest farther north. This burning helps to explain the prevalence of oaks in southern portions of the state, as these deciduous trees thrive in the conditions created by fires.
• Before logging began, hemlock covered more of the landscape than other pines. White cedar, tamarack, and other conifer species also were much more plentiful than they are today.
• Lumber production in the North Woods peaked in the 1880s, crashed during the Great Depression and picked up a bit during World War II, though never again got anywhere near the 19th century levels. By the end of World War II, commercial loggers had pretty much consumed the “original” forest that existed before their arrival. From the 1930s to the present, forests across Wisconsin have been dominated by young trees ranging from 5 to 10 inches in diameter.
• In the North Woods, logging left behind a lot of “slash” — branches and fallen trees the loggers didn’t clear or harvest These materials would dry and encouraged more fires, often ignited by trains passing on railroads and other human activities. These fires burned off organic matter on the forest floor as well as topsoil, which slowed down some tree species’ ability to reproduce.
• The conditions after late 19th and early 20th century fires — lots of light, soils that forest ecologists refer to as “bare mineral soil” — were good for aspen and paper birch trees. Aspen came to dominate the area by the 1930s, but has tapered a bit since.
• Certain forest bird species, including the blackburnian warbler and pine warbler, were far more common in the North Woods before logging began. With the current dominance of aspen in the area, it’s become a better habitat for other birds, especially grouse.
• Today, recreation is the biggest use of these forests, and “northern hardwoods” like sugar maple and yellow birch take up the biggest portion of Wisconsin’s forest area, though oaks are gaining on them.
• Human-driven climate change poses the greatest challenge to the Wisconsin’s northern forests into the future. Over the next 100 years, modeling predicts the state will warm by 4 to 9 degrees Fahrenheit. Wisconsin has already warmed by about 1.4 degrees Fahrenheit over the past century, with the biggest difference being that nighttime average temperatures aren’t as low as they were previously. The warming will harm several northern tree species’ ability to reproduce, and warmer winters could also create difficulties for the logging industry.

Key quotes

• On how people view forests: “When we go out to our favorite forest, for example, I think we expect it to be, you know, kind of the same. That’s what we like about it. And so we don’t really expect change.”
• On the post-glaciation growth of Wisconsin’s forests: “The forest didn’t move back up here as whole communities that we think of them now. The pine and oak and paper birch forests didn’t all come up together, for example.”
• On the environmental impact of logging: “Not only was it the terrestrial habitat, the forests themselves, that were treated in such a destructive way, but the aquatic habitats as well had very bad treatment, and especially streams…. You can’t do such big things on the land without also affecting aquatic systems.”
• On the significance of fires that followed logging in northern Wisconsin: “The heat of these fires was really unprecedented. They were not a kind of fire that had ever really happened in these systems before… If anything tried to grow, it often would burn again.”
• On today versus the 1930s: “It’s changed somewhat, but really not that much. We really still have a pretty young forest, even though there’s been almost a century of recovery since the ’30s. Obviously some of the forest we use, and it’s harvested and remains young that way.”
• On the Wisconsin forest’s recovery since the peak logging era: “With what we’ve gone through, it’s important not to minimize the successes that have occurred with the forest, because it is regrown in a very large extent, and that really is a success story. Compared to the turn of the century… it’s pretty dramatic. It’s largely forested. And these forests are still growing, still recovering. This isn’t something that happens in just a few decades. They’re still not as diverse as they were in the past.”
• On forest management: “It is important to have different strategies for different ownerships. In part, what one does can sort of compensate, one for the other. You can’t grow everything and do everything the same, and that’s probably a good thing.”
• On the big challenges of management: “We can’t think about how we’re going to manage the forest without taking climate change into account.”