Regenerating oak on dry sites in central hardwood forests with shelterwood harvest and one prescribed burn
Valued for timber and wildlife, oaks (Quercus spp.) face many regeneration challenges in the eastern United States. Decades of fire suppression have produced favorable conditions for tree species that grow well in humid climates, such as maples, and less favorable conditions for species that thrive under drier conditions, such as oaks. The Hoosier National Forest in southern Indiana noticed a lack of oak advance regeneration in the 2010s, when long-term research showed that clearcut harvests performed in the 1980s, which were intended to regenerate oak, were now more dominated by red maple, tulip tree, poplar, and black cherry.
Looking for a way to regenerate oak, United States Department of Agriculture, Forest Service staff worked together. Chris Thornton, then the Hoosier National Forest silviculturist, consulted Northern Research Station scientists Daniel Dey and John Kabrick to explore potential solutions. They brought in partners from Purdue University to investigate why oak were not regenerating on the Hoosier. In a recent article, the researchers describe how they tested a method developed in more productive sites in the eastern United States that typically involves multiple prescribed burns and evaluated whether oak preferentially regenerated on drier sites with fewer prescribed burns. Successful oak regeneration after one or even a few prescribed burns could make this method more cost-effective to implement.
Shelterwood and Prescribed Burn
Prescribed burn in the Hoosier National Forest research plots in 2019.
The main treatment applied in this study—a shelterwood harvest accompanied by at least one prescribed burn—originated from earlier Forest Service research studies conducted in humid forests in the eastern United States. This method involved prescribing a shelterwood harvest, which can include a series of partial harvests to increase light and resource availability, followed by one or more prescribed burns to favor oak regeneration over competitors.
The shelterwood at the Hoosier National Forest was established between 2012 and 2015 and included the removal of tree species that competed with oak, such as sugar maple, tulip tree, and American beech, and of poor-quality trees. Dominant and co-dominant oak and hickory trees were left standing. This shelterwood was performed across eight experimental units totaling 71 acres. One prescribed fire was applied in April 2019.
Oak Outperforms Other Species
Overall, oak seedling density increased after the shelterwood cut and prescribed burn. Oaks and hickories survived at a higher rate following the prescribed fire than other competing species; oaks and hickories averaged over 90 percent survival, while maple species averaged less than 67 percent survival. Oak seedlings also sprouted at a higher rate than maple seedlings, resprouting at a rate of nearly 93 percent for oaks compared to 67 percent for red maple. Altogether, this evidence shows that a shelterwood establishment cut and one prescribed burn could be enough to shift the regenerating species composition from maple to oak.
When considering site conditions, the researchers found that the shift in species composition from maples to oaks was strongest in sites that were less productive, which was indicated by higher soil aluminum concentrations, lower soil nitrogen and organic matter concentrations, and lower soil pH. This result could mean that oak regeneration may be more successful, and therefore cost-effective, when promoted in less productive, dry forests on the landscape.
Restoring Oak
Oak species hold high timber value and are critical to wildlife species, but regenerating oaks can be time and labor intensive. This collaborative research with the Hoosier National Forest demonstrates a way to successfully regenerate oak in central hardwood forests and shows that this method may be more cost-effective when applied in drier and less productive areas, where only one prescribed burn may be needed. Management activities on dry and less productive sites could help maintain oak species across forest landscapes with less effort than where repeated burning and other follow-up treatments may be required.