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.