Why Wisconsin Forests Look the Way They Do

Sunlight filtered through the maple and aspen leaves as we gathered on the narrow, dirt trail. John Kotar, emeritus professor of forest ecology from U W-Madison, gestured widely as he described tree species, shade tolerance, soil moisture, and glacial history. With the title “Why Wisconsin Forests Look the Way They Do,” John’s field trip aspired to summarize his life’s research in just a few hours.

Several years ago, Kotar developed an ecological classification system for Great Lakes forests, and he published “A guide to forest communities and habitat types of northern Wisconsin.” His in-depth knowledge of our forests means that wherever we stop along the trail, he has something to say.

“I’ve been trying to think of a good analogy for the way I see the forest versus how a regular person sees it,” he said. “Imagine that you enter a room filled with people you don’t know. There is a wide mix of ages, appearances, etc. You just see people. Few details would stick out, and you might not be able to derive any information from the assemblage. Now imagine walking in to your family reunion. You scan the crowd and instantly know who everyone is, how they’ve changed over time, and how they relate to everyone else in the room. That’s how I – that’s how foresters – see the forest. Most people just see trees.”

So, we swatted mosquitoes and looked a little closer. Sugar maple, with its smoother leaf margins and U-shaped inter-lobes stood tall near the trail. Its many children, stunted from shade, but tough and long-lasting, carpeted the opening as seedlings.

The presence of sugar maple indicates that this soil has a fair amount of moisture and nutrients. Soil moisture and nutrients are the two main factors that John focused on to develop his classification system. On the ground, the small, leathery leaves of wintergreen quietly contradict the sugar maple. Wintergreen is an indicator of poorer soils.

The nature of this forest, and any natural community anywhere, is the result of multiple factors. John’s two factors are intimately related another set of factors often used by ecologists to understand the landscape: geologic history, recent disturbance and current climate.

Here, just south of Lake Namakagon in Northern Wisconsin, the pertinent geologic history revolves around glaciers. The Superior and Chippewa lobes of the most recent Wisconsin Glaciation stalled out here for a while, dumping tons of clay, silt, sand, pebbles, cobbles and boulders in lumpy hills called moraines. This mix of sediment sizes moderates both the soil moisture and the soil nutrients, resulting in sugar maple and wintergreen growing together.

Behind us, a red maple, with sharp, toothy leaves and a cluster of smaller trunks reminded us of the most widespread recent disturbance –logging. The forest in this recreational area has been logged a few times since the initial cut in the late 1800’s.

Dying paper birch on the edge of the opening also express the forest’s memory of a sunnier time. As a pioneer species, paper birch need full sun to get established. Then, as more shade tolerant trees grow up around them, they surrender their place, or hope for a fire. Shade-tolerant balsam fir forms thickets in the understory, and will rise to the top over time.

Our current climate, with adequate rainfall, cold winters, and hot summers plays a major role in determining which plants even have the option of living here. You won’t find a cactus in this forest, and neither will you find a baobab tree. Microclimates affect the forest on a local scale, with black ashes growing in low, wet spots, while oaks might claim a dry, sunny hill.

In the crisp fall sunshine, we wander down the trail a little. The forest changes gradually as we pass through an aspen grove, a balsam thicket, and more maples. Someone asks about how these forests have changed over time. While white pines were the most commonly logged species at the turn of the century, they weren’t necessarily a bigger part of the forest. Their ability to float on rivers to reach sawmills was their downfall.

Today, white pines tower above their neighbors on almost every combination of soil nutrients and moisture in Wisconsin. While we (we on this hike, and we as ecologists both) started off by describing which trees do best where, scientists are always trying to look deeper and ask “why?” or “how?”

Sometimes it is not just about one species, but how several work together. In 2001, soil ecologists discovered that white pine and other trees get some of their nitrogen from tiny soil arthropods called springtails. But first, a fungus, the bicolored deceiver (Laccaria bicolor), must kill and decompose the springtail using special enzymes. Then, then fungus forms a sheath around the roots of the white pine, and transfers the nitrogen and other nutrients to the tree in exchange for sugars produced during photosynthesis. About ninety-five percent of plants get some nutrients from fungi, allowing them to live in poorer soils.

Even small variations in the look of our forests – like the extra-vibrant fall colors in swamps, have explanations if we can figure them out. In 2003, a grad student found that in places where the soil was lower in nitrogen and other important elements (like swamps), red maple trees produced more of the red pigment anthocyanin in the leaves. This sunscreen pigment allows them to recover more nutrients from the leaves before they fall.

I love looking at a forest on the broad scale, and thinking about the giant chunk of ice that shaped my home. I also love finding out about these detailed and cryptic connections. But you don’t have to delve into the science to enjoy the fact that Wisconsin forests, especially in autumn, look the way they do.