When people think of Arboretum restorations, they likely envision prairies, especially Curtis and Greene Prairies. That makes sense, given the historical importance of Curtis and the exceptional quality of Greene. However, we know from historical accounts and old aerial photos that prior to European arrival and the conversion of land for settlement and agriculture, the upland areas that are now the Arboretum would mostly have been oak savanna.

A few old, large-diameter oak trees with broad, open-grown canopies still offer evidence at the Arboretum of those old oak savannas. Therefore, instead of prairie, oak savanna is often the target plant community for our current restoration projects. Less than one percent of the Midwest’s original oak savannas remain, so restoration and management are critical for preserving oak savannas and the species that depend on them.

Some of the Arboretum’s old savannas are very degraded – overgrown with dense thickets of invasive shrubs such as honeysuckle and buckthorn and the uber-invasive vine Oriental bittersweet. This means that oak savanna restoration can be messy, especially in the early stages. But sometimes it’s necessary to make one mess to take care of another, and that holds true for oak savanna restoration. It is also a long-term process and may take years to achieve the desired vegetation structure (the plants’ vertical and horizontal arrangement in a space) and plant diversity that supports a healthy ecosystem.

I’ve written about our savanna restoration in previous articles, but the basic steps of the process include: 1) forestry mowing and cutting invasive shrubs and vines; 2) thinning undesirable trees; 3) treating stumps, resprouts, and new seedlings with herbicide; 4) introducing prescribed fire; 5) adding native seed; and 6) repeating these steps as necessary.

Step one – forestry mowing – creates the biggest mess, leaving battered stumps and a thick layer of shredded woody debris on the ground. Sometimes it is perceived as destruction of the “forest,” but it’s actually a highly efficient way to remove invasive shrubs and small trees from a degraded savanna. Although an area may look unpleasant immediately following forestry mowing, the woody debris decomposes within a few years, and a good prescribed fire can speed that up. While messy, forestry mowing sets the stage for the subsequent steps – and ultimately the success – of  savanna restoration. You can see signs of forestry mowing from recent winters in the Lost City Forest (someday renamed Lost City Savanna?) near trail marker L4, in Noe Woods (someday Noe Savanna?) near E5, and at the Grady Tract near X6.

Mowing invasive brush makes it easier for staff and equipment to enter the interior of a project area for step two – removing undesirable trees such as black walnut, box elder, sugar maple, and other species that aren’t typical of an oak savanna system. Sometimes we also thin out oaks if they are too dense. There are areas where trees were removed and thinned in many parts of the Grady Tract, such as south of U1 and V1, south of X3, and north of Y1 and Y8. Most of the tree removal in those areas was done ten to twelve years ago.

Removing undesirable trees and leaving scattered oaks opens up an overgrown forest, providing a savanna-like structure. Fewer trees means more light reaches the ground, which helps increase the growth and diversity of native plants in the understory. Removing invasive shrubs and excess trees also improves access for staff to carry out steps three to five (herbicide treatment, prescribed fire, and adding native seed) – all of which further increase native plant abundance and diversity.

The Grady Tract areas mentioned above show how fires and seed additions over the last ten years have increased native plant abundance and diversity. On the other hand, native vegetation sometimes rebounds on its own after decades of being suppressed by invasive shrubs and trees, such as in the Lost City Forest south of L3. An impressive array of native plants appeared shortly after forestry mowing, with no additional seeding needed. I wish it was always that easy!

We’ve found that it often takes about ten years after the initial “mess” of restoration for an area to more closely resemble the historic oak savannas, with improved structure and increased native plant abundance and diversity. Restored savannas also benefit animals, such as insects that rely on native plants and savanna-specific species such as red-headed woodpeckers that have high conservation value. If you happen to see a recently forestry mowed area and you’re curious about the goal, I encourage you to visit some of the Grady Tract areas I’ve mentioned to see what we are working toward. For the Arboretum, having more oak savanna, and the species that depend on them, is worth the messy first steps.

—Michael Hansen, land care manager