Restoring Oak Savannas with Multiple Disturbances: Thinning, Burning, and Targeted Cattle Grazing

Abstract

Temperate savannas are one of the rarest and most imperiled ecosystems in the world. In the Upper Midwest (USA), over 99% of pre-colonial oak savanna area has been lost due to extensive habitat conversion and altered disturbance regimes. In this region, savannas were characterized by a grass-sedge-forb ground layer and widely-spaced oaks (e.g., Quercus macrocarpa). This open structure was maintained historically by frequent disturbances including fires (primarily Indigenous burning) and grazing by megaherbivores (e.g., Bison bison). Due to fire suppression and the near extinction of native grazers, savannas often persist today in a degraded, woody-encroached state. Restoring savanna remnants is challenging because we don’t know how to best apply contemporary restoration tools to mimic historic disturbances. To that end, my dissertation evaluated the response of oak savanna vegetation and wildlife to a gradient of restoration actions: 1) no management, 2) tree thinning, 3) thinning + burning, and 4) thinning + burning + cattle grazing. I found that thinning and burning successfully increased canopy openness, herbaceous cover and diversity, and savanna-associated plant species such as Ceanothus americanus, Andropogon gerardii, Galium boreale, and Agastache scrophulariifolia. Bird abundance and species richness also responded positively to thinning and burning, while butterfly abundance/richness and the activity of most bat species did not differ across the restoration treatments. Thinning and fire had the unwanted effect of increasing shrub density, particularly Corylus americana. I then evaluated if shrub density could be reduced using short-duration targeted cattle grazing under high stocking density. The results suggest that targeted grazing with Angus/Angus cross cattle can be an effective tool to reduce shrub density in the short-term, but repeated management is needed to prevent resprout. I documented no negative impact of cattle grazing on birds, butterflies, or bats throughout the duration of this study. Overall, the results of my dissertation suggest that combining multiple restoration strategies partially achieves vegetation goals and improves bird habitat without negatively affecting butterfly and bat communities. A key takeaway from this work is that restoration outcomes are not dictated by how many management approaches are applied, but rather, the nuances of how they are applied.