Conserving Large Oaks and Recruitment Potential while Restoring Midwestern Savanna and Woodland

Abstract

Restoring Midwestern oak savannas and woodlands over the long term requires balancing mortality of large oaks with recruitment of oaks into large size classes. At restoration sites in northwestern Ohio, we tracked survival of large oak trees (≥20 cm in stem diameter) and recruitment between 2002 and 2015 after initial tree thinning and prescribed burning treatments. The 24 study sites spanned a gradient of canopy cover from unrestored forests to restored woodlands and savannas. Of 141 large black oak (Quercus velutina) and white oak (Quercus alba) trees alive in 2002, 79% were alive 14 y later. Large oak survival varied among vegetation types, with restored savannas not losing a single tree, compared with 18% mortality in unrestored forests and 28% mortality in restored woodlands. At least some mortality in the woodlands was associated with a tornado that damaged two sites. Diameter distributions changed over the 14 y in all three vegetation types. Unrestored forests shifted toward proportionally greater large and fewer small diameter oaks. Meanwhile, restored woodlands, despite having the highest mortality of large oaks, still exceeded, via recruitment of new large oaks, the reconstructed pre-Euro-American-settlement tree density. Restored woodlands and savannas exhibited four-fold increases in oak recruitment potential (saplings 1 to 10 cm in diameter) between 2002 and 2015. Ecological restoration processes have been compatible with conserving large oaks and sustaining oak recruitment potential.