A savanna looks like “trees in a prairie” or “prairie openings in a woodland.” A savanna has tree cover ranging from 1 to 20 or so trees per acre and allows at least 50% of sunlight to hit the ground. The dominant tree is bur oak, which has a corky bark that protects it from fire. Other trees, such as black oak, shagbark hickory, and white oak, may also be found.
The easiest way to identify a remnant savanna is to look for broad-crowned bur and white oak trees. Mature trees that grew up in a savanna have a mushroom shaped crown with a diameter that is wider than the tree is tall. In woodlands, the same tree species will grow taller and vase shaped due to competition for sunlight with their neighbors. Throughout southern Wisconsin today, you will find these broad-crowned trees in forests surrounded by smaller trees and brush that are growing up into the limbs of these ancient giants. Many of these remnant savanna trees are several hundred years old.
Southern Wisconsin historically had savanna cover of 20-40% depending on how you define a savanna. Generally, savannas occur in uplands, but lowland savannas can be found. In the original land surveys they might be identified as “barrens” or “scrub-oak” or “oak openings.” (see Resources)
The savannas of Wisconsin have deteriorated so profoundly and rapidly in the past fifty years that they have become one of the most endangered ecosystems on the planet. Most savannas today are thick with brush and younger trees. In most cases, the brush is European buckthorn and species of honeysuckle. But native trees such as cherry, elm, ash and boxelder can also be considered invasive in a savanna because they shade out young oaks and grow up through the branches of mature oaks, eventually killing them.
A healthy Midwestern savanna is extraordinarily diverse. Because of the variation of shade and sun, a significant number of species of both prairie and woodland ecosystems may be found in savannas in addition to a few species that are found only in savannas. However, the invasive species of buckthorn and honeysuckle combined with the onslaught of native trees “released” due to lack of herbivory and fire have pushed today’s savannas to the brink of collapse.
At the time of settlement, savannas were scattered throughout the region. Here is a quote from a traveler in the mid 1800’s upon his first encounter:
Lost as I was, I could not help pausing frequently when I struck the first oak-opening I had ever seen, to admire its novel beauty. It looked more like a pear orchard than anything else to which I assimilate it – the trees being somewhat of the shape and size of pear trees, and standing at regular intervals apart from each other on the firm level soil…Here too, I first saw deer in herds; and half- frozen and weary though I was, the sight of these spirited creatures, sweeping in troops through interminable groves, where any eye could follow them for miles over the smooth snowy plain, actually warmed and invigorated me, and I could hardly refrain from putting my rowls to my tired horse and launching after the noble game.
This condition was maintained by periodic fire and herbivory by bison, elk and deer. Today, former savannas are currently either trashy woodlands or crop fields. Restoring a savanna by starting with a pasture or corn field is a process that will take a least twenty years and starts simply by restoring a prairie and planting trees and shrubs in a fashion that they can be protected from fire in the early years. As trees grow and produce shade, savanna species are introduced bit by bit.
The process involved in taking a trashed-out woodland that was once a savanna to one that resembles the healthy state at the time of settlement is also a long one. In fact, it should not be considered a process with an end goal in mind, but rather a change in behavior that will last the lifetime of the land owner and hopefully beyond.
The early stages of oak savanna restoration involve removal of invasive woody species. Primarily this is done with mechanical means. The progress and quality of the restoration can be enhanced by careful reintroduction of herbivores – usually that means cattle or goats in today’s world, but bison and elk would be even better. In virtually any scenario, a degree of mechanical removal is necessary.
Unlike a remnant woodland, it is almost impossible to bring a savanna back to healthy state simply by removing woody invasives. In virtually all cases, only a few native ground covers are likely to come back on their own. More often than not, removing woody invasives will allow herbaceous invasives like Canada thistle, spotted knapweed and giant ragweed to come in. Therefore, we begin by seeding grasses so that we can control broadleaf weeds with herbicide. Once the grasses have become established and broadleaf weeds are under control, seeding of forbs and sedges can occur.
Once native ground covers become established it is time to reintroduce fire. From that point onward, a savanna restoration should be on a positive trajectory.