Natural Connections: Forests Have Layers

With snow still clinging to north-facing slopes, and tree buds waiting for warmer days to open, the trees in a forest can look like they are doing their own form of social distancing. I like to focus on the positive, and call this “see-through season.” As I’m walking familiar roads and trails, I can see through the forest to notice rock outcrops, pools, big trees, and even houses that I’ve never noticed before. By June, many of our forests will be dense walls of green, especially along the edges. Right now is a great time to observe their structure, and the shape of the landscape beneath them.

As I continue to share with you pages from Forest Lodge Nature Trail’s new interpretive booklet, I’d also like to remind you that the USDA Forest Service recently announced that this trail and many others are closed to the public, in alignment with current federal, state, and local guidance for social distancing. Happily nature is everywhere, and you can still make observations as you hike close to home.

Leatherwood is a distinctive member of the shrub layer in eastern North America. It’s yellow flowers brighten up the early spring woods. Photo by Emily Stone.

Forests Have Layers

Forests are composed of trees, of course, but that’s not all. There are five main layers to a forest. The canopy is where the crowns of tall trees meet and intercept most of the sunlight. Shorter trees form the understory as they grow toward the sun.

In some places, enough light filters through that the shrub layer is well-developed. This layer extends from about knee-high to a few feet above your head. These bushes, shrubs, and young trees provide food and shelter for wildlife. Deer browse on twigs that are easy to reach. Many songbirds hide their nests in the dense foliage. Insects take advantage of shelter from the wind and sun, and become food for birds and other animals. Pollinators buzz around flowers, and many critters eat the resulting fruits.

Nearby you’ll see the rounded shape and warm tan bark of a leatherwood shrub. Its twigs are extremely flexible, and its small, yellow flowers brighten up the forest in early spring. Maple-leaf viburnum grows here, too. Its twigs, leaves, and buds are oppositely arranged, which means they are paired along the stem. White flowers become dark purple berries that birds love. Beaked hazelnut also thrives in the shrub layer. Tiny, sausage-shaped catkins (a type of flower cluster) dangle off the twigs all winter long. Its nuts provide food to insects, squirrels, bears, and more.

Under the shrub layer, we find a carpet—or maybe just a few sprigs—of herbaceous plants in the herb layer. Sometimes, barely anything can survive under the deep shade of the trees. With soft stems, these ferns, grasses, and wildflowers stay close to the ground and provide habitat for ground-nesting birds, mice, and more. They must survive with very little light in the shade of the other layers. Some woody plants are here, too. The seedlings of maple trees can survive for decades while growing only a few inches tall as they wait for a gap in the canopy. The shiny, dark-green leaves of wintergreen are common in northern forests. It has bell-shaped white flowers that develop into fragrant red berries. They are food for ruffed grouse, deer, bears, chipmunks, and mice.

Below the herb layer is the forest floor. This is where dead stuff gets recycled back into soil by a team of decomposers like fungi, invertebrates, and bacteria. If you see a forest where the herb layer looks like a grassy lawn, then it probably has an earthworm invasion. Worms are not native to the north, because glaciers froze the soil. Plants here have adapted to growing through thick layers of dead leaves. Now when the worms decompose those leaves quickly, it changes the habitat and a lawn of Pennsylvania sedge takes over. (Be sure to put your extra fishing worms in the garbage!)

If you look off to the side of the trail, you’ll see a small depression. Depending on the season, it might be filled with snow, or water, or just mud and leaves. The wet ground prevents many trees from growing, so herbs and shrubs become more common. Small pools like this provide important habitat for insects. The airspace above them also becomes important habitat for bats. Bats use the open space to dart about and catch insects. They may also get a drink by skimming over the open water.

Some bats are in trouble, though. A fungal disease called white-nose syndrome is killing bats in North America. This cold-loving fungus infects bats while they are hibernating in caves. You can help by putting up a bat box in your yard, and by protecting important bat habitats like this forest opening.

Thanks to Wisconsin Master Naturalist Volunteers Thom Gerst and Kay Meyer for their help in drafting this booklet! Emily’s second book, Natural Connections: Dreaming of an Elfin Skimmer, is now available to purchase at www.cablemuseum.org/books. Or order it from our friends at redberybooks.com to receive free shipping!

For more than 50 years, the Cable Natural History Museum has served to connect you to the Northwoods. The Museum is currently closed due to COVID-19, but we're still building our new exhibit and bringing you educational content. Connect with us on Facebook and Instagram to see what we are up to.