The Process of Oak Woodland Restoration

If you’ve read my previous articles about the history of oak woodlands in the Midwest and how to assess the health of oak woodlands, it should now be clear that most woodlands in our region (and particularly, in southern Wisconsin) are in pretty bad shape and need active management to restore them to ecological health. In this article I’ll explain how we go about doing that.

Site Assessment, Site History Research and Planning:

Look at the Maps

I always prepare for a site evaluation by looking at modern areal imagery for a site and topographic maps. You can use Google Maps, but many counties have GIS websites that help you get more detailed imagery, the property lines, and sometimes other valuable layers such as soil type or historic areal imagery. Just do an internet search for “(your county’s name) GIS” and you should find what you’re looking for. This information gives me a basic understanding of the topographical, geological and hydrological aspects of the site. I can also see what the current vegetation cover conditions are, which gives me some idea what to expect when I get on-site.

This areal image of Cherokee Marsh in Dane County, WI shows a lot of detail useful to understanding the ecosystem. Property lines can be seen in yellow.

On-the-Ground Site Assessment

A lot of the basics of site assessment are covered in my previous article. Evaluating things on the ground allows me to understand the current site conditions and have some idea of the history of the site. Documenting the plant and animal species you find on a site is a key part of the assessment. Having a thorough plant inventory for the site will tell you a lot about the original natural community on the site, how much it has been impacted, and give you some guidance as to how to go forward as you try to re-establish a healthy natural community.

A detailed plant survey from many moons ago. Thanks for your help Lane.

Site History Research

In Wisconsin, we have a remarkable resource available to us in the form of scans of the original land surveyors’ notes from the 1830’s. So go to Wisconsin Public Land Survey Records website and look up what those surveyors found in your area. (Some instructions are available on the website to help guide you through the process.) Though they often don’t have a lot of detail, these notes are invaluable in developing an understanding of what the immediate area was like at the time of settlement. (Hint: there weren’t nearly as many trees, and the ones that were there were probably oaks.)

Then I start to look at historic areal imagery. These go back as far as 1937 and, for Wisconsin, can be found at the Wisconsin Areal Imagery Finder. Go ahead and compare that imagery to your modern satellite photo. You will probably see that in 1937 there were not nearly as many trees as there are today!

If you wish to see areal imagery from later in the 20th Century, through today, there are a couple possibilities. Some county GIS websites have these images. (Dane County DCI Maps is such a great resource!) Using Google Earth Pro desktop, you can get imagery as far back as the 1990’s and sometimes earlier. If you’re up for a trip, the state transportation department or a state geography library will have hard copies of the areal images taken from the 1930’s to the 1990’s. In Wisconsin, this means a trip to the Geology Library in Science Hall on the UW campus.

A scan of original surveyors notes from the Wisconsin Public Lands Survey Records website.

A 1937 areal image of what is now Edna Taylor Conservation park in Madison, WI.

Developing a Target Community Type, and A Plan to Get There

Now that you have an understanding of the history of the site and its current conditions you can start putting together a plan to restore it going forward. The first step is to develop a target natural community. Sometimes, the goal should be to return the site to the natural community type found on the site before settlement. Other times the reality of current conditions make establishing a different natural community more practical. For example, surveyors’ notes and 1937 photos may indicate that your woodland was originally an open savanna of bur oak and white oak. However, today you may have numerous straight-trunked red oak along with your white oak in your woodland, and bur oak are now absent (due to logging). In a case like this, it may be better to work with the mature red and white oaks that you have to establish an oak woodland, rather than to cut down a lot of red oaks and then plant bur oaks to recreate a savanna.

One thing I consider strongly is ground layer vegetation. If you find any prairie species, it’s best to open the site up as much as possible to an open savanna condition. Similarly, if you find savanna plants, you should at least open the canopy to a very open woodland condition (50% canopy cover or less). Planning your target community becomes an act of balancing the model of the original plant community with the reality of what you are dealing with right now.

Woodland Restoration Work, on the Ground

Clearing Invasive Brush

The first step on the ground is always clearing invasive brush. Non-native species such as common buckthorn, bush honeysuckle, and white mulberry get most of the attention. Japanese barberry, multiflora rose and Oriental bittersweet are (currently) less common invasive shrubs, but equally deserving of elimination.

It is also important to understand that native shrubs and trees can become over-abundant as well, and large monocultures of these shrubs reduces the biodiversity potential of a site. So we often clear prickly ash, grey dogwood, and sumac, with the goal of reducing their populations, but not eliminating them entirely. Box elder is a native tree and not part of an upland woodland community, so remove this prolific species just like you would buckthorn or honeysuckle.

By removing these invasive non-native, and aggressive native woody plants, you eliminate them from competing with desirable native plants for sunlight and soil moisture. This also has the effect of making the woodland more open, and easier to move through, and work in.

We do brush clearing and tree thinning in the winter months to avoid harming desirable native plants and to avoid causing soil compaction or erosion. There are many ways to go about clearing brush, but our basic approach is:

• Cut each stem as low to the ground as possible.
• Treat each cut stem with concentrated herbicide. (For specific recommendations see our Weed Identification and Control Sheets linked at each species name above.)
• Clean-up is the most labor intensive part of this process. However, if the cut brush is widely scattered and does not inhibit moving through and working in the woodland, then you may not need to clean up. But typically, some clean-up is needed. This can be done by dragging brush by hand. But when possible, we prefer to use a tractor or skid loader with a grapple attachment, just be sure the ground is frozen and preferably snow covered so that you don’t disturb the soil by compaction or erosion.
• Piling and burning: Piles should be placed carefully to avoid harming high-quality ground-layer vegetation or sending too much heat into any trees’ canopies or trunks. While we’re out there clearing brush, we start with a small fire and slowly build it to a brush-eating bonfire. Fortunately, live ‘green’ buckthorn and honeysuckle both happen to burn very well. Keep the number of brush piles limited by feeding the fire as you work, and cut the brush to 6’ lengths, or shorter, to limit the footprint of the burn pile and allow for a more efficient fire.
• Follow-up treatment in the following growing season is critical, as some brush will resprout, and you will likely see many small invasive woody plants that you had missed during the winter brush clearing work, or that have sprouted since. Early-to-mid summer after these resprouting woody plants have fully leafed out is a good time to spot treat these plants with a basal bark herbicide application.

Cutting buckthorn with a brushcutter.

Each stump is treated with herbicide.

And the resulting brush is burned in a pile.

For larger projects, forestry mowing is an option. But this should only be done on firm, frozen ground, and follow-up resprout treatment is essential.

Removing Aggressive/Over-Abundant Trees

Typically we do this while we’re already out there clearing brush and the brush piles are burning. Restoring oak woodlands often involves removing a large number of young, small diameter non-oak trees that have filled in the woodland since pre-settlement times (and largely in the past 40 years). We will fell some of these trees, but larger trees (>10”) we typically girdle and leave standing to reduce the workload and create wildlife habitat.

An examples of thinning prescription might include:

• Remove non-oak trees that are in poor health; cherries with black knot disease, elms with dutch elm disease, or trees which have been damaged by other trees falling on them.
• Remove any trees impinging on mature oaks. We try to cut down any non-oak tree growing within 10-20’ of an oak canopy (many small trees will grow right up into the oak canopy). These keystone tree species need some growing space to be healthy.
• Remove any non-oak tree less than 6” or 8” in diameter. These are the youngest up-start trees that haven’t been in the woods very long.
• Canopy cover needs to be reduced to no more than 70% if you hope to get oak regeneration or shade tolerant wildflowers and grasses established. 50% canopy cover is probably more appropriate for restoring an oak woodland and less than 50% if savanna is your restoration target.
• For savanna restorations, typically the only trees we are leaving are oaks and hickories, and sometimes we are even thinning northern pin oak, shagbark hickory and butternut hickory if there is an abundance of these species.

An upcoming blog post will cover how to decide what undesirable trees to remove to restore an oak woodland in more detail. Stay tuned.

Judging by its open branching structure, this big white oak originally grew in a savanna. Many small hackberries will need to be removed to to preserve the health of this heritage oak.

Controlling Herbaceous Invasive Species

As you are clearing the weedy woody component of a woodland, be sure not to clear so much area that you can’t mange the weeds that come up in that area in the following growing seasons. I describe garlic mustard control in detail in this blog post. Dames rocket, motherwort, and Japanese hedge parsley are similarly troublesome invasive biennials that should be treated while you are working to control garlic mustard. In sunnier sites, other weeds that might come up could include burdock, biennial thistles, Canada thistle and mullein.

Got Brambles?

Many sites with a history of grazing may have a lot of raspberry brambles and gooseberries. Again, these are native plants, having some of them is a good thing, but too many of them reduces biodiversity and makes it difficult to move through and work in the woods. The best way to keep these thorny native shrubs in check is to mow (or selectively cut) them right after they leaf out in the spring, and repeat this mowing as soon as they resprout for the entire growing season. The goal is to reduce their numbers, and allow growing space for other native plants, not eliminate them entirely.

Reintroducing a Prescribed Fire Regime

This can be a big turning point in woodland restoration. In sites with a lot of oak leaf litter, this may be done shortly after brush clearing. In sites with less oak leaf litter as fuel, you will need a few years to establish ground layer flora that can contribute to the fuel load. Prescribed burns can greatly help in the control of invasive plants such as garlic mustard and buckthorn seedlings. Often a burn will reveal and invigorate native grasses and wildflowers that have been struggling to survive, or invigorate new seedlings coming up from long-dormant seed. Burning off the leaf litter can also prepare a site for interseeding.

Fires in oak woodland are not as dramatic as you might think.

Reintroducing Extirpated Plants by Seed

Even the best intact woodlands will require some reintroduction of species that have gone locally extinct. Most sites will need a lot of species reintroduced. We usually start by spreading a commercial seed mix appropriate for the habitat type (woodland or savanna) and soil conditions (dry, mesic or wet). If you are DIY-ing your woodland restoration, you may want to buy some seed or trade seed (for other seed or volunteer labor) with local conservation groups.

Reintroducing by Planting Live Plants

Some species, particularly spring-ephemerals and the more shade-loving woodland species, are not available as seed, or don’t establish easily by dried seed. For these species you will need to plant live plants. We usually recommend doing this in small pods where you can keep track of them and water and weed them as necessary, and even put a small fence around them to exclude deer and turkey until the plants are better established.

Long Term Maintenance

Long term maintenance will depend on where you start. If you have a lot of garlic mustard, you will probably be managing it for a decade or more. Continued prescribed burns will help move everything in the right direction favoring native plants and discouraging invasive species. You will need to continue to monitor the site for resurgence of any invasive species, and positive or negative patterns in plant establishment. Hopefully you have a pretty thorough plant inventory by now. Start comparing the plant species on your site to nearby, healthier oak woodland communities. This should allow you to understand what species you are missing. Continual interseeding and exchanging seed with other sites will help increase plant diversity, which in turn will increase the value of the site to a variety of wildlife, and the resiliency of the site from everything from invasive species, to erosion, to climate change.

Benchmarks for Woodland Restoration

Here are three key things to look for to assess if you are seeing success in your restoration project:

• A healthy woodland will have a wide diversity of native wildflowers and grasses, with flowers blooming throughout the growing season, from April to October. Spring ephemerals should put on a show first thing in the spring, but having a variety of mid-summer flowers is the strongest indicator that you’re on the right track.
• The work you do should preserve the health of old oaks and other mature trees: reducing competition from other woody plants, increasing soil moisture and nutrient availability while reducing erosion.
• Oaks should successfully germinate amongst the ground layer vegetation over time, and survive to grow into the canopy. If you don’t see this happening, you probably need to open the canopy further to let more light to the ground, or work on controlling over-abundant deer that could be eating all your oak saplings every winter.
• Restoration work should stabilize soils, reducing erosion and starting to rebuild an organic-matter-rich surface soil. Never run equipment in such a way that it causes rutting, compaction or erosion.
• Wildlife return: You should see more pollinators, both bees and butterflies. You should see more songbirds, if you’re lucky you may get a visit from eastern bluebirds or the rare red-headed woodpecker.

When in doubt, conduct prescribed burns more frequently, and keep adding native seed and plants to areas where more native plants are needed.

In the next article, I’ll talk about why its so important that we begin the process of restoration in these woodlands that need our help.

A healthy restored woodland.