Botanist, Michigan Natural Features Inventory

Connecting the Dots: Restoring Midwestern Oak Savanna Ground Layer Plant Communities, from Toledo to Madison

Grasslands such as prairies and savannas are threatened globally, and Midwestern oak savannas are among the most endangered ecosystems in the North America. Protracted fire suppression is a primary threat. Without fire, key aspects of ecosystem structure that differentiate savannas from forests are lost, including greater light availability associated with low canopy and understory tree and shrub density, and a diverse ground layer dominated by dense herbaceous vegetation rather than leaf litter. Importantly, ground layers are composed largely of species uniquely adapted to savanna conditions, many of which are of high conservation value. Managers and researchers have often found that prescribed fire is slow or insufficient to restore structure, diversity, and composition, and restoration requires fire in concert with mechanical thinning of shrubs and trees to overcome decades of woody encroachment. Despite this progress in understanding, most studies have been geographically limited, spanning one site to a single landscape. A generalizable framework is still needed to describe how variation in fire and thinning affects restored oak savanna structure, diversity, and composition, and how restoration varies across environmental gradients such as soil productivity. We collected plant community and environmental data in 100 oak savanna restorations across the southern and western Great Lakes basin from Toledo to Madison, including sites in Michigan, Ohio, Indiana, Illinois, and Wisconsin. These restorations represented different management histories (burn-only, thin-only, burn-and-thin, unmanaged controls). We found that the combination of thinning and burning is often associated with more favorable restoration outcomes, including increased native plant species richness and abundance, especially for forbs, woody species, and species of high conservation value. These responses were mediated through increased light availability more often than reduced leaf litter, and occasionally varied across soil productivity gradients. By connecting the dots between management history and plant communities across landscapes that vary in environmental conditions, these results provide a foundation for a generalizable framework for the restoration of Midwestern oak savannas.