Assessing the Health of Oak Woodlands, a Guide for Land Owners and Land Managers

In my last blog post, I presented an abbreviated history of oak woodlands in our region, from their ‘pristine’ pre-settlement condition to the unfortunate reality of the present: that most of our woodlands are degraded from the impacts of nearly two centuries of settlement, development, and neglect. In this post, I’m going to give you some tools to evaluate the health of your woodland, and you won’t need a degree in ecology to do it. By asking the questions below, you can start to get an idea of the health of your woodland and develop a vision for how it can improve. While this guide is specific to southern Wisconsin, it should be a useful guide anywhere in the upper Midwest, south of the Tension Zone.

Woodland Structure:

How far can you see into your woodlands?

During the growing season, does it become a green wall that you can’t peer into, or can you see for quite a distance? Early settlers reported that you could see for up to a mile through woodlands at that time. They were able to drive a wagon or carriage through oak woodlands or even ride a horse at a gallop! If this all seems hard to imagine, then your woodland is too dense with brush and trees.

If the edge of your woodland is a 'wall of green' like this, that's not normal or healthy.

How much sunlight can you see through the tree canopy?

Is it 90% leaves and 10% blue sky? Or is it 50/50? A typical target for woodland restoration is to have between 50% and 70% canopy (50-30% blue sky!), and for savannas, we like to see less than 50% canopy.

This woodland canopy is about 75% full, this is within the natural range of variability for a woodland, but some minor thinning might stimulate ground layer vegetation and oak sapling growth.

What kind of trees are your oldest trees?

OK, this does take a little botanical knowledge, but we’ll try to keep it as simple as possible. If you can identify an oak vs any other kind of tree, you should start to see some patterns emerge. If a site has historically been an oak woodland, you should see that the largest trees are oaks. A typical pattern is that the largest and oldest trees are oaks, but the more numerous and younger trees are other species such as cherry, boxelder, mulberry, elm, hackberry, walnut, maple, and basswood, among others. Many times, there is a pretty big gap in age with the oaks being over 100 years old, and all other tree species behind less than 50 years old. These younger trees do not belong in an oak woodland, and are taking up growing space, sunlight and water that would be better used by young oaks, wildflowers and grasses.

Sometimes you might see other trees like maple, basswood or cherry that are nearly as large, but they may still be much younger since they grow faster than oaks (at least the above-ground portion of the trees grows faster).

Lastly, there are certainly cases where the largest maples and basswoods are as large, or larger than the oaks, and red oak are the most numerous oak species. In these cases, you probably have a site that was a closed-canopy, mixed-hardwood forest to begin with.

In this woodland the two largest trees are white oak (left) and red oak (right), these are also the oldest trees around 100 years old, maybe a little older. The many skinny 'poles' are sugar maple and are much younger, less than 30 years old, and there is also some elm and cherry which are likely less than 50 years old.

How big are the largest trees in your woodland?

Tree growth rates vary a lot between species, so it’s not easy to compare the age of an oak versus a maple based on size. Even within a species, growth rates can vary wildly based on soil type and moisture availability, competition, site aspect, environmental stresses, and land use among other factors. With that in-mind, here are some rough estimates that I use to assess ages of white and bur oak are:

• A 16” diameter at breast height (DBH) oak is about 70 years old.
• A 20” DBH oak is about 100 years old.
• A 24” oak is probably over 150 years old.

If you have multiple oaks which are a little over 2’ in diameter, that probably indicates they sprouted around the time of Euro-American settlement. This may have been the result of fire-suppressed “grub oaks” sprouting after regular fires ceased, or as new seedlings that sprouted after the original woodland on the site was cleared. Oaks larger than this are probably pre-settlement, “old growth” oaks. But again, growth rates vary a lot from site to site; on dry sites, a 24” bur oak oak may be more than 250 years old!

If your largest oaks are under 20” (and your site is not particularly dry), it’s likely that the site has been logged in the last century. Sometimes your biggest trees are 12-16” DBH, that typically means these trees are roughly 40-70 years old. Also look for multi-stemmed trees. If you see a lot of multi-stemmed trees that is usually a sign that the area was clearcut and these multi stemmed trees are resprouts from the root of a tree that was cut.

This ~4-foot diameter bur oak is easily over 300 years old.

Do you see any young oak trees?

If you don’t see young and middle-aged oaks, that means the oaks are not reproducing. If there are no young oaks to replace the old oaks, before too long the keystone species of this community will be lost.

Young oak trees, like this chinquapin oak sapling, are now among the most rare plants on the landscape.

Ground Layer Vegetation:

Is the ground layer in your woods “green” or “brown”?

Once the growing season is in full swing, it is not natural or healthy for there to be “brown” areas in your woodland that only have dead leaves or bare soil. A healthy woodland will be “green” with plants growing on or over every inch of soil surface by late-spring. If there are areas of barren soil, this is usually a sign that the tree canopy or brush layer is too dense and not allowing light to the ground for groundlayer wildflowers and grasses to grow. Bare soil can also be the result of compaction or erosion, or browsing by over-abundant deer.

Here buckthorn has shaded out all of the ground layer vegetation. Historic grazing probably had already weekend the ground layer flora. Excessive native trees, especially sugar maple, can have the same effect (see above).

Are there many spring ephemeral wildflowers in your woodland?

Spring ephemeral wildflowers go through their entire life cycle in roughly the first 2 months of the growing season. They are among the first plants to sprout in the spring; they flower before trees have fully leafed-out and then go dormant before the heat of summer with a long-lived perennial root that lies dormant until the following spring. Most commonly recognized spring ephemerals include trillium, spring beauty and Virginia bluebell. Do you see many flowers in the early spring, but then the entire plant turns yellow and disappears by early-summer? Many woodlands that have been heavily grazed over the years have lost their spring ephemeral component, either to direct consumption, or trampling or erosion. If you still have some spring ephemeral wildflowers that’s good - if you have a carpet of them, that’s great!

If you have lots of flowers blooming in the early spring, that's a good sign that grazing was less intense, and there is a greater chance for more native plants to come up with restoration work.

Are there flowers blooming in your woods throughout the entire growing season?

Many woodlands have a burst of wildflowers in the spring, but then nothing blooming mid summer… and then just a few goldenrods and asters blooming in the fall. Or maybe you just have flowers blooming on the edge of your woods, but not the interior? That’s not normal. A healthy woodland should have a variety of species blooming throughout the entire growing season. If you have few wildflowers in your woodland overall, that’s likely a result of over-grazing and erosion. If you have spring ephemerals and some fall bloomers, but not much during the bulk of the growing season, then that is typically the result of too much shade from the tree canopy or shrub layer.

This is a healthy woodland (Somme Prairie Grove) in August in the middle of the drought of 2012!

Do you encounter a lot of thorny plants?

If you go-off trail in your woodland, do you quickly get poked by lots of thorny raspberries, gooseberries, and prickly ash? An abundance of thorny brambles and shrubs indicates a history of heavy grazing. The mechanism here is simple, cows don’t want to eat thorny things. In an environment where grazing pressure is intense, less palatable plants will have a competitive advantage. Over decades of grazing, these thorny plants dominate. More recently, browsing by over-abundant white-tailed deer have maintained the status quo. Though these thorny shrubs are native, a diverse assemblage of plant species is important for woodland health. So having a small number of thorny species taking up a large amount of the growing space is not a healthy situation.

You can see why cattle would not want to browse on wild gooseberry! Seeing a lot of these plants, or raspberries, or prickly ash, is usually a sign your woodland was heavily grazed in the past.

Do you get a lot of seeds stuck to your clothing?

These plants are adapted to have their seeds distributed by large mammals. So it follows that, a woodland with a lot of sticky-seeded plants has experience a heavily grazed past, and/or an over-abundance of deer in the present. With the exception of the notorious burdock, most of the sticky-seeded plants in our woodlands are actually native. These includes stickseed (the worst!), enchanters nightshade and wood avens. These weedy natives are much more common in our woodlands today than they should be.

The notorious Stickseed, if abundant, is a sign of grazing impacts, and/or over abundant deer.

Soils and Water:

Soils are perhaps the most important component in our ecosystem, and yet their value is rarely recognized. Healthy soils provide many ecosystem services such as purifying surface and ground water, providing good conditions for seed germination, and providing habitat for organisms ranging from bacteria to badgers. Unfortunately, land-use practices such as row-crop agriculture, grazing, mining and urban development destroy or severely damage soils and often lead to erosion. Let’s take a look at the soils in your woodland:

Does your soil have a lot of rocks, or a lot of organic matter?

The “O-Horizon” is a layer of partially decomposed organic matter on top of the mineral soil. Do you have an O-Horizon, or do you have relatively intact leaf-litter over top of bare mineral soil? It’s important to consider that soil types can vary a lot across our landscapes. Even healthy soils may have a lot of rock, gravel, and sand since glaciation left many deposits of these materials across the landscape. Furthermore, in the Driftless region, bedrock is often pretty close to the surface. With that in mind, on average a healthier woodland will have better developed (less eroded) soils than a less healthy site.

Is your surface soil dark or light in color?

The “A-Horizon” is the upper layer of mineral soil, and will have a fair amount of organic matter mixed in with it It’s usually black or dark in color. This layer also has the most living organisms in it. A healthy soil should have a solid matrix of plant roots and soil organisms. Across the slopes and upland areas of your woodland, if you find that you have fairly beige soils with a lot of clay or rock at the surface, it’s likely that original O and A soil horizons in your woodland have eroded away.

There are many causes of soil erosion. By far, the greatest impact is from over a century of heavy grazing that most of our woodlands in southern Wisconsin have experienced. Conversely, in northern Illinois soil loss is less severe because there were fewer dairy farms and therefore fewer cattle grazing in the woodlands. Other causes of erosion include an overly-dense tree canopy which leaves no light for ground layer plants. These ground layer plants are critical to holding and developing soils; without them soil washes away and the system cannot recover. Furthermore, we now find non-native earthworms across most of the landscape, which can consume and diminish the O horizon, making it more difficult for many native plants to establish, and therefore accelerating erosion as well.

Do your oldest trees have a lot of exposed root flare or roots?

Root flare is the lowest part of the truck of a tree where it starts to spread out and transition to the roots. Having a little root flare visible is normal, and natural, and, in fact is an important guideline when planting a tree. With some exception, most trees will have only a gentle outward curve in the lower ~10” of the trunk, and will never establish roots above ground. If you start looking at the base of your trees, on some sites you can see old oak trees which record 8-12” of soil loss around their base.

Inversely, if you have trees at the bottom of a slope do they lack root flair? This may be because soil has washed down from higher up the slope and settled there, effecting burying the base of the tree. Roots that are exposed to air can develop bark to protect themselves. But tree trunks cannot shed their bark if buried, and trunk rot and tree death can result.

You can see some modest root flare on the red oak on the right, and significant root flare on the white oak on the far left. I would estimate that this site (which is relatively flat) has lost about 6" of topsoil since pre-settlement times.

Is there a lot of old junk in your woods?

If you have a lot of junk in your woods, ranging from bottles and cans to ancient farm equipment, it suggests that your woodland has perhaps been abused, but at least has been heavily used.

Wildlife:

Wildlife, by and large, respond to the plant community and geophysical structure of a site. While all wildlife will benefit from a healthier ecosystem, with increased diversity, numbers and health, certain species are more sensitive than others to ecological degradation. And certainly, some species are more visible than others. Here’s what you can look-out for:

Have you ever seen a bluebird or redheaded woodpecker in your woodland?

If you have, that’s great news! These birds, which were once abundant in our landscape, require open woodlands and savanna environments. So if bluebirds or redheads are nesting in your woodland, that suggests that you at least have the habitat structure that these stunning birds are looking for. If you don’t have them, consider what you can do to make your woodland a place they could call home.

Do you see many bees and butterflies in your woodland?

A healthy woodland should be home to plenty of pollinators. These pollinators need the above-mentioned flowers blooming throughout the growing season. Butterflies also need their larval host plants.

A monarch catapillar on poke milkweed, which is our true woodland milkweed species.

Conclusion:

Aldo Leopold once wrote:

One of the penalties of an ecological education is that one lives alone in a world of wounds. Much of the damage inflicted on land is quite invisible to laymen. An ecologist must either harden his shell and make believe that the consequences of science are none of his business, or he must be the doctor who sees the marks of death in a community that believes itself well and does not want to be told otherwise.

Now perhaps you can see some of the wounds in your woodland. But we can heal these wounds through ecological restoration. In the next article in this series, I’ll talk about the benefits that come with restoring oak woodlands to health.

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).