Sustainable Farming Association of Minnesota, United States
Oak savannas are unique and highly biodiverse ecosystems. Once covering millions of acres across North America, oak savannas are now on the brink of disappearing. Remnant savannas have suffered from the removal of fire and large grazers from the landscape, often persisting in a degraded, woody-encroached state. Restoring savanna remnants is challenging because we lack an understanding of how current restoration tools can be layered to mimic historic disturbance dynamics. To fill that knowledge gap, this study evaluated the outcomes of ongoing oak savanna restorations that have received increasing levels of restoration effort: 1) no management, 2) tree thinning, and 3) thinning + prescribed burning. We then tested whether a new tool, targeted cattle grazing, could be used to improve oak savanna restoration outcomes beyond what thinning and burning can achieve. To that end, we conducted vegetation surveys at 70, 1-m radius plots per restoration level. We recorded shrub species, stem density, and height, as well as herbaceous species and cover. In the areas that had been previously thinned+burned, we established nine 1-acre grazing paddocks. We monitored vegetation changes in the grazing paddocks using both plot- and transect-based vegetation surveys.
In general, increasing restoration effort corresponded with shifts toward characteristic oak savanna structure and composition. Compared to the unmanaged reference, average canopy cover was 14% lower in the thin-only treatment, and 46% lower in the thin+burn treatment. Average sapling density followed a similar trend; the number of saplings per hectare was 53% lower in the thin-only and 83% lower in the thin-burn treatments. Herbaceous cover, richness, and diversity were all highest in the thin+burn zone, again showing a clear trend toward the desired oak savanna composition. Shrubs were the only vegetation type that did not shift in the desired direction. As restoration efforts increased, shrub density increased from 10 stems/m2 in the unmanaged areas to 15 stems/m2 in the thin+burn treatment (p< 0.001). This highlighted the need for an additional restoration technique to help reduce shrub stem density, such as grazing. By implementing targeted cattle grazing in the previously thinned+burned areas, we were able to successfully reduce shrub stem density by 48% (p< 0.001). Our research demonstrates how targeted grazing can be layered with thinning and burning to improve oak savanna restoration outcomes, and improves our understanding of interacting disturbances applied in a restoration context.