A Brief History of Oak Woodland in the Midwest in General, and Southern Wisconsin in Particular

This is the first post in a series of three about Wisconsin oak woodland history, health and restoration. In this first installment, we’ll cover an overview of oak woodland types in our region and how human activities have impacted them from pre-settlement times to today. In order to keep this brief and digestible, this post simplifies complex ecology and history. 

A lot of people take our woodlands in the Midwest for granted and many people don’t give them much thought. However, we’re at a critical time for our oak woodlands. Many are nearing a breaking point - a point of ecological collapse from which restoration will be difficult, if not impossible. So what is going on? To understand that, you need to understand what oak woodlands were like two centuries ago or more, and how human-caused changes have created the more depauperate woodlands we see today.

Pre-Settlement Woodlands of the Midwest

In pre-Euro-American settlement times, the most common woodland types in the region were:

Savannas - The Wisconsin DNR calls these “oak openings” with as few as one oak tree per acre. Those scattered trees were usually bur oak or white oak, but black oak savannas occurred on sites with sandy soils.

Savanna restoration at Pheasant Branch Conservancy.

Oak Woodlands - A very open canopy, no more than 50-80% canopy cover. A more diverse assemblage of oaks inhabited these woodlands, with some hickory and rarely other tree species.

A white oak woodland are in the Stone Barn Savanna complex in Nachusa Grassland.

These were very open woodland types with a lot of sunlight reaching the ground layer vegetation. As a result, there was abundant energy to drive photosynthesis, which allowed for a rich flora of wildflowers, grasses and shrubs. These plants then served as the food source for pollinators, grazing insects and mammalian grazers such as deer, elk and occasionally, bison. These insects fed a great diversity of songbirds and game birds such as turkey, quail and grouse, and large predators such as wolves, bobcats, mountain lions and black bears.

These were the dominant “forest” types in the southern 1/3rd of Wisconsin, all of Iowa, northern Missouri, and the northern 3/4th of Illinois and Indiana. In this post I’ll just be focusing the portion of Wisconsin south of the Tension Zone to narrow this discussion to our local conditions. These were fire-regulated systems, with frequent, low-intensity fires with a burn return interval of roughly 1-3 years. Oaks, being amongst the most fire tolerant trees in our region, out-competed fire-intolerant trees. Across most of the landscape, trees that were not in the Quercus (oak) genus, were not often seen.

This map from the UW Extension illustrates the Tension Zone in WI, south of it, in lime-green oak woodlands were the dominant "forest" type.

On Finley map of pre-settlement vegetation types, the brown and orange are oak woodlands and savannas respectively. See this link for a more detailed PDF.

More dense canopy forests were uncommon in southern Wisconsin and restricted to areas where the topography and hydrology reduced the frequency and intensity of fires. These were limited to areas such as floodplain forests, areas with lake and wetland complexes, and steep north facing slopes in the Driftless region, particularly where wetlands or streams were at the bottom of that slope. These small forested areas varied quite a bit in composition so it can be difficult to generalize about them. Overall they were more often inhabited by trees that were more tolerant of shade, but intolerant of fire such as maple, basswood, cherry, walnut, elm, aspen, hackberries and others. Some oaks were present as well, particularly red oak, and occasionally white oak. Though there was more diversity in the tree canopy, the ground layer, which received much less energy from the sun, was accordingly less productive and therefore had less animal and plant diversity and abundance compared to more open woodlands. Still, a healthy and intact hardwood forest is much more rich than most woodlands today.

On a landscape level, these more shaded closed-canopy forest communities were mere pinpoints or ribbons on a landscape dominated by open prairie, savanna and woodland. So why are most of our wooded areas dense with trees and brush today?

Oak woodlands, savannas, prairies, and wetlands in our region were maintained by frequent, low-intensity fires. Fires lit by by the Ho-Chunk, Potawatomi, Sauk and Dakota Peoples who lived across this region could at times burn for days, and travel across hundreds of square miles. As a result, much of the landscape likely burned nearly every year. Woodlands may have burned every 1-3 years. Most people today are familiar with prairie fires, but there was little to stop these fires from simply continuing into woodlands and wetlands. These fires kept woody species suppressed, especially fire intolerant trees. 

A modern prescribed fire backing through an oak savanna.

Oaks are on average more tolerant of fire than other genera of trees. This is exemplified by bur oak which has a number of adaptations to fire. First, they have a large acorn with a lot of stored energy, which allows them to set down roots into the ground quickly upon germination. Second, bur oaks rapidly put on root mass. Many people believe that oaks grow slowly, but this isn't strictly true. Oaks put on much of their growth below ground in a massive root system. The benefit of this is that if the above-ground portion of the tree is destroyed by fire, animal activity, or weather, they have substantial reserves underground to re-route rapidly. There are reported cases where oaks have been burned over regularly for decades. These trees never got larger than a shrub above ground, but could have a root system as large as a mature tree. In the event that there was an absence of fire for just a few years, these ‘grub oaks’ could grow rapidly to a mature size where they were less vulnerable to fire. This brings me to the final adaptation bur oaks have to fire: thick insulating bark that, once mature, can withstand the frequent, low-intensity fires that were common in the region.

Other management activities conducted by indigenous peoples such as wood harvesting, materials gathering, food foraging and agriculture also had an impact on the structure and ecology of these woodlands. It is clear that active management by indigenous peoples across the Midwest was what maintained a rich mosaic landscape of prairie, savanna and woodland. Before contact with Europeans, these people thrived with a suitable abundance of game, gathered foods, crops, medicines, building materials and other resources available for their use. Good stewardship by indigenous nations also provided resources for a diversity and abundance of plants, animals, fungi and all the other forms of life that inhabit a rich and complex ecosystem. We should be grateful for the stewardship the Potowatami and Ho-Chuck provided for this ecosystem for so many generations (and their continued leadership in environmental stewardship today). 

Changes in Land Use with Euro-American Settlement:

The Public Land Survey of Wisconsin began in 1832. Though there was scattered Euro-American settlement before this time, more settlers arrived in larger numbers from the mid-1830’s to the 1860’s. This change in control of the land from indigenous nations to colonizers of European ancestry had a number of profound impacts on the landscape. Some of the most impactful actions of settlers included:

1. Settlers partitioned up the land; developed roads, plowed fields. This had the effect of fragmenting natural communities into smaller units, reducing the ability of plants and small animals to disperse from one habitat patch to another.
2. New settlers actively suppressed fires. New roads and agricultural fields became unintentional fire breaks, preventing fires from spreading. Active suppression, including fighting wildfires, reduced fire's impact on the landscape further. Fires were effectively eliminated by the Smokey The Bear era in the late 20th century. This eliminated a key ecosystem regulator from the landscape.
3. Many woodlands were logged for building materials. This resulted in the loss of these woodlands as they were transitioned to grazing land or agricultural fields. 
4. Other woodlands were left uncut and used as graze land. This kept trees and brush suppressed, at least temporarily, but also greatly reduced the diversity and abundance of native wildflowers, grasses and shrubs in the woodlands.
5. Grazing dairy cattle was widespread across the entire landscape and was also very intense denuding all vegetation from the soil surface.

This photo from the Driftless region in the 1930's gives you some idea how bad erosion could be. All indications are that heavy soil loss was common throughout the Midwest at this time.

Dairy farming really picked up steam in Wisconsin in the 1860’s, peaking in the early 20th Century. As a result, it's likely that just about every woodland in the region was heavily grazed for over 100 years. The impacts of this grazing are apparent today in many woodlands:

• There is a low diversity of native wildflowers. For example, heavily grazed woods often have few spring ephemeral wildflowers such as spring beauty, trillium and trout lily.
• Heavily grazed woodlands often have an abundance of thorny shrubs, particularly raspberries, blackberries, gooseberries and prickly ash.
• Erosion can be observed by seeing the root flair and exposed root on old oak trees. Some sites have lost 6-10” of topsoil. As a result, these soils are of poor quality, and often rocky. This also means the seed bank for native plants has been washed away as well.

In the latter half of the 20th century the dairy industry in Wisconsin changed dramatically. The number of small dairy producers decreased over time and dairy production became more industrialized. The impacts on woodlands in the region is apparent. 

• First, you can often see in areal photographs woodlands that were fairly open in the late 1930 and into the 1960’s become more dense with trees in the 1970s and 1980’s. This trend continued to the closed tree canopies and dark woodlands we see today. 
• Second, you can see these effects on the ground. There are many non-oak trees in formerly oak dominated woodlands that are small in diameter (<10”) and less than 40 years old.
• Third, you do not see young oaks because they need more sunlight than is available to successfully germinate, grow and reach maturity.

Edna Taylor Conservation Park in Madison circa 1937. Overall, this is an example of a savanna environment. Note that this image was taken in June, with the leaves on the trees.

Edna Taylor Conservation Park in Madison in 1974. Notice how much more dense the wooded area is, with additional trees sprouting elsewhere in what used to be pasture.

Edna Taylor Conservation Park in Madison in 2014. Note that this is an early-April, leaf-off image, and yet the woodland looks much more dense than its 1937 counterpart! You can see restoration work has begun in the southern 1/3rd of the park but not the north.

Many invasive plants were introduced that have spread rapidly into these already compromised woodland habitats. Common buckthorn and bush honeysuckle are the most common examples of invasive woody plants in our region, though there are many others. These started to arrive with intentional introductions in the 1960’s through the early 1980’s. From isolated introductions they spread across the landscape. These invasive plants further impacted already degraded woodlands, further increasing shading and erosion and suppressing the growth of oaks, wildflowers, grasses and the wildlife that depend on these plants for survival. These conditions are ideal for other invasive plants such as garlic mustard, motherwort and dames rocket to establish and spread rapidly.

This "wall of buckthorn" is neither 'natural' nor healthy. But sadly this is the most common state of our oak woodlands today. This is an ecological dead end. Restoration work is needed to get things back on track.

Conclusion

We are roughly two centuries past a time when our oak woodlands were healthy and well maintained. Nearly 200 years of fragmenting, degrading and ignoring our woodlands have left them in a critical condition today. These systems are resilient and full of living things ranging from oak trees, to wildflowers, to rusty patch bumble bees continue to fight hard for survival. But we are now at an inflection point; these systems can’t take much more before they collapse. The old oaks will only live so long, and conditions are poor for their replacement.

We still have time to act. But we should act now. With proper management, we can restore our oak woodlands to health. This involves removing brush and excessive non-oak trees, controlling invasive ground-layer plants, and reintroducing native plant species that have been lost by seeding and planting. As for the wildlife, it really is a ‘if you make it they will come’ situation. Once there are food resources available for them they return remarkably quickly, especially birds.

In my next post, I’ll share some ways that you can evaluate the health of your woodland, even if you aren’t a trained ecologist. And following that we’ll talk about the many ways that restoring your woodland will benefit your woods, yourself and your community (natural or otherwise).