Southern Mesic Forests of Wisconsin

This post originally appeared as part of an article for Blue Mounds Area Project Newsletter Fall 2021

BMAP formed an exciting new alliance with the Botanical Club of Wisconsin this summer. The result was a four-session outdoor Natural Communities of Southwestern Wisconsin class, the brainchild of Micah Kloppenburg, BMAP ecologist, and Kevin Doyle, DNR botanist and member of the Botanical Club of Wisconsin.

“The UW-Milwaukee field station offers a class called Vegetation of Wisconsin – where they travel around the state,” said Doyle.  “I felt the Botanical Club was uniquely qualified to provide similar classes for our area.”

The classes were led by: 

-Doyle, who provided overviews of each plant community to put plant ID into context so that people can better understand what pressures plants are responding to and how to encourage native plants to thrive. 

  • Stephanie Lyon, Assistant Professor of Biology at UW-Stevens Point, who provided reproductive strategies of plants in the context of their environment.
  • Mary Ann Feist, Senior Academic Curator of the Wisconsin State Herbarium, who offered specific details to help with plant ID.

The first class explored the ecosystem of Southern Mesic Forests, in a pocket of sugar maple-basswood forest on the east side of Blue Mound State Park on May 15.


In pre-settlement times, oaks were dominant in the southern part of the Driftless area because southern mesic forests, dominated by sugar maple and basswood (then covering about 18% of the Driftless Area, and only 3% of the region south of the Military Ridge) are not as well adapted to fire as oaks. They don’t put as much energy below ground as oaks do, so they can’t re-sprout as effectively, and their relatively thin bark is more susceptible to the fire. Today we find sugar maple forests on steep east- and north-facing slopes that were protected from the frequent fires that swept in from  predominant westerly winds. 

“One reason we get sugar maples on slopes is because they are fire protected, but it could also be because there is often water seeping out of these slopes providing nutrients, and in this case dolomite from the limestone provides minerals,” said Doyle. “And it’s also the slow breakdown of all the material on the forest floor that isn’t getting burned up.”

Maples allow only about 1% of sunlight to make it to the forest floor. Compared to an oak woodland, there is not a lot of structural complexity and almost no shrub layer. Wind (not fire) is the most important factor in southern mesic forests because it creates openings in the canopy when a tree comes down. 

Maple saplings can survive a long time waiting for an area of light to open up in a way that oaks can’t, although red oak can sometimes be found in the mesic forest. “Red oak has more shade tolerance and can hang on for an opening in the canopy,” said Doyle. “Also red oaks grow straight up to reach the light like sugar maples. White oaks tend to stretch out searching for light, and bur oaks, with low-growing branches, are at the far end of shade intolerance.” Basswood and ironwood are also common in the Southern Mesic Forest.

The floor of this type of woods is dark, closed in, and fairly wet, Stephanie Lyon noted. Once mature maples become the dominant life form on the landscape, they maintain this through massive seed production, a strategy that helps ensure that the deer won’t get them all. 

In the spring, the canopy closes in fast, so the forest floor plants may need a long time to build up the resources to reproduce. Spring ephemerals like trout lily can take seven years from seed to flowering. Most of these herbaceous plants have well-developed underground storage systems.

“They might come up as vegetative plants for several years, using the narrow window of light to photosynthesize and then die back down, waiting for a year when they can get big enough to produce flowers. Then they have to get pollinated,” said Lyon. 

“Some plant species like those in the buttercup family use wind to pollinate,” said Lyon. “They flower very early when there is more possibility of wind.”

Some plants time pollination to when the first insects are just starting to emerge. “As with the last several years in particular we can get a random hard frost in early May, and the timing with insects is tough,” Lyon said.

 Most plants in the understory that are insect pollinated must be generalists. A lot of them have fairly open, bowl-like flowers in yellows and whites to attract whatever is around. That tends to be a lot of small, ground-nesting native bees who have made a nest in an exposed patch of soil.

Jack-in-the-pulpit can actually be a male “Jack” or a female “Jill” — depending on conditions. “They are a fine example of the waiting-it-out strategy,” said Lyon. “They can change year by year. Their vegetative form produces a leaf, and takes what resources it can, then dies back down. When they have more stored energy, they come up as female. Jacks are usually single-leaved individuals and tend to be smaller than the Jills. A lot more energy goes into the female end of reproduction.”

Nutrient Cycle

Sugar maple leaves are different than oak leaves. Oak leaves have more carbon and lignin, and they burn better, so when fires are coming through, oak leaf litter sustains the burn. Sugar maple leaves have a higher nitrogen-to-carbon ratio, and they break down a lot faster. 

Another source of nutrient comes from downed woody debris. Downed trees release a slow trickle of energy back into the system. This benefits soil microbes and fungi that the plants form symbiotic relationship with and need to survive.

“When I’m looking for a high quality southern mesic forest, I am looking for those downed trees and snags,” said Doyle. “Nineteen snags per acre is a good average for an old growth sugar maple forest. You will notice in a managed forest, the downed trees are often absent because the timber harvest machinery is taking everything. You’ll even see stands that have really big trees, but the ground looks like it’s been vacuumed. Not good. Different micro-habitats that provide nutrients and moisture are a critical part of the plant community.”


Earthworms: Earthworms are the biggest threat to the mesic forest because they are not native to Wisconsin. They were brought in by European settlers. Today there are different kinds of earthworms that affect different parts of the soil, but the bottom line is that they munch up all the decomposing material in the upper levels of the soil, and they speed up the nutrient cycling too fast for plants to get to them. They also eliminate a lot of the essential fungi and soil microbes either by breaking them up or eating them. Now, a lot of the relationships that plants once depended on are gone. 

“Wormed areas are easy to spot,” Doyle said. “You can feel it with your feet. Wormed areas feel more like walking on pavement.” 

There are not a lot of management opportunities with earthworms. However, studies are also finding that there are connections being drawn between deer and earthworms, indicating that if you control the deer, the earthworms will decrease as well. “It’s likely that the deer are adding nutrients in their pellets, and earthworms are surviving on that,” he said.

Invasives: Garlic mustard and honeysuckle are breaking up important connections between the plants and soil microbes and fungi because their leaves contain chemicals that change the soil’s nutrient recycling. “It’s not necessarily a crowding out (although in some cases it is),” Doyle said. 

Grazing: Deer, of course, play into this by creating openings for invasives to come in – adding nutrients to the soil that earthworms take advantage of.

“Basically everything in Wisconsin that could be grazed for agriculture has been grazed,” said Doyle, “and there are impacts from that, including soil erosion and compaction.” Livestock browse preferentially, and even after they are removed, the Southern Mesic Forest seed bank is affected. When the native seed source is gone, but there are a lot of nutrients available, invasive generalists move in. “When I see a heavily-grazed site, there are a lot of prickly ash, grasses, sedges, and ferns,” Said Doyle. “The other grazer is deer, and they don’t eat a lot of sedges and grasses or ferns; those are the plants that are left.

he Botanical Club brings amateur and professional botanists together to study our state’s plant biodiversity. You can learn more about this active and informative organization online by googling Botanical Club of Wisconsin.

Doyle said he considered this summer’s class an exciting pilot project. “Micah and I saw a joint opportunity for Botanical Club members to provide more services to those interested. Perhaps in future years we can expand it.”

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