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  • Oak Savanna Restoration in the Kinnickinnic Valley

    By Joe Wilberg

    Oak savannas once covered almost 6 million acres of Wisconsin’s landscape. Today, the State Natural Heritage Inventory lists only 500 acres that resemble oak savanna. According to John Curtis, “an oak savanna with an intact ground layer is the rarest plant community in Wisconsin.”

    The original land survey notes from the 1840’s indicate that prairies, oak savannas, and oak forests bordered the Kinnickinnic and its tributaries. Oak savannas depend upon occasional grass fires to survive and flourish – and it is the absence of natural fires that has made savannas so rare today. Burr oaks have thick bark that make them very resistant to being hurt by ground fires.

    Curtis, in The Vegetation of Wisconsin, defined savannas as a ground layer of grasses and herbs in combination with widely scattered trees whose branches shade no more than half of the land’s surface.

    Healthy oak savannas have just two layers: a short grassy layer, and a tall layer of a widely spaced trees. Savannas do not have many shrubs-the ‘middle layer’ that is common in other plant communities, like forests. This is why savannas are often described as having a park-like appearance.

    Oak trees grown in a savanna are twisty and gnarly with large, low branches. In contrast, oaks grown in forests are usually straight, tall, and have few low branches. When you are in the Kinnickinnic watershed, watch for oak trees like those in the photograph to the left – these grew in the open, and can indicate an area that was once savanna.

    These savannas are situated on limestone bluffs that flank the Kinnickinnic River, and are aptly called goat prairies since their steep slopes suggest only a goat could traverse them. There are flat areas above the bluffs however, into which the oak savannas extend.

    Invading species like juniper (also called red cedar), buckthorn, and prickly ash have grown between the old oaks, shading and crowding out the once-prevalent prairie grasses and herbs. Using remaining prairie plants and open-grown oaks as a guide, my task is to map the original extent of these savannas and to develop appropriate guidelines for restoring and then managing them.

    First, we remove brush and invasive plants by cutting them down and piling them up. This autumn we plan carefully controlled, prescribed fires. These activities simulate the grazing and fires that naturally maintained oak savannas before they were converted to agricultural lands. Small grass fires every few years will help keep brushy species from re-invading.

    Monitoring plant communities for changes in their species composition is essential to restoration success. Of special interest on one conservation easement property are kittentails (Besseya bulii), a prairie plant that is endangered.

    I counted 185 kittentails, mostly along one of the bluffs in the savanna on one particular property. Kittentails is the only plant found in oak savannas and no where else. It will be interesting to see if their populations improve as the savannas are restored. Below the bluffs, on a dry river terrace, is a vigorous population of marbleseed (Onosmodium hispidissium) listed as a “watch” species in Wisconsin.

    It is a wonderful feeling for me to walk through the savannas, forests, and floodplains, because each time I do I learn something new. A significant benefit is knowing that I am helping protect this beautiful valley.