Oak savanna vegetation response to layered restoration approaches: Thinning, burning, and grazing

Temperate savannas are unique, biodiverse ecosystems that have undergone extensive habitat conversion globally. In the midwestern United States, 99% of historic oak savanna area has been lost. Most remaining patches of savanna are degraded due to woody encroachment following the removal of both fire and large herbivore disturbances from the landscape. Restoring degraded savanna remnants is challenging because we lack an understanding of how to best apply contemporary restoration tools to mimic historic disturbance dynamics. To that end, we evaluated the outcomes of ongoing oak savanna restorations that have received a gradient of restoration actions: 1) no management, 2) tree thinning, 3) thinning + burning, and 4) thinning + burning + cattle grazing. We assessed several metrics of restoration success including canopy, shrub, herbaceous, and non-native cover, herbaceous diversity, and plant community composition. We found that layering restoration ap-proaches achieved certain, but not all, structural vegetation goals. Compared to no management, thinning and fire successfully increased canopy openness, herbaceous cover, and herbaceous diversity, but had the unwanted effect of increased shrub cover. The addition of low-intensity cattle grazing did not improve structural outcomes. We also found that each restoration treatment left a unique signature on understory plant community compo-sition. Unmanaged and thin-only treatments were characterized by tree saplings and woodland herbs, while burned and grazed treatments were defined by shrubs and savanna-associate species. We conclude that rein-troducing multiple disturbances does not guarantee the successful restoration of disturbance-dependent eco-systems such as oak savannas. Restoration outcomes are not dictated by how many management approaches are applied, but rather, the nuances of how they are applied such as burn season and livestock density.

Automated summary

Temperate savannas are unique and biodiverse ecosystems that have experienced extensive habitat conversion worldwide. In the midwestern United States, the majority of historic oak savanna area has been lost, and the remaining patches are degraded due to the absence of fire and large herbivore disturbances. Restoring the degraded savanna remnants is challenging due to a lack of understanding of how to effectively mimic historic disturbance dynamics.