Midrotation Removal: A Beneficial Management Practice for Mature Oak Forests

Oak dominated forest types occupy 51.1% or about 12.4 million acres of Georgia’s 24.4 million forested acres. Oaks are some of our most important tree species as their acorns provide a food source and shelter for many wildlife species, the timber is among the most valuable of any tree species, and these forests help support a large recreational economy. Though many of today’s oak forests may appear in good condition, most of eastern North America’s oak forests lack young oak trees as U.S. Forest Service data documented a 35% decrease in volume of oaks between 5 and 6.9-inches diameter at breast height (dbh) between 1987 and 2007. Oaks are mid-successional species across most of the eastern U.S., and usually do not form the climax state of the forest. In the absence of disturbance, oak forest types eventually transition into less valuable (for wildlife and timber) shade tolerant species. Disturbance (fire, timber harvest, wind events, etc.) is vital to ensure that young oaks take the place of aging oaks in the overstory of oak dominated forest types.

Lack of disturbance in oak dominated forests often results in development of less valuable (for timber and wildlife) shade tolerant tree species in the under and midstory layers especially on higher productivity sites. These species cast so much shade on the ground (light levels often between 1 and 8% of full sunlight at ground level in stands with well-developed midstory layers) that they inhibit the growth and development of newly germinated oak seedlings into more competitive saplings termed advance reproduction. Advance reproduction refers to oak trees that are 4 feet tall up to one-inch dbh. Advance reproduction is the more effective form of oak regeneration to manage for when development of oak seedlings into larger size classes is a management goal. Oak stump sprout potential tends to be low in most stands as older and larger diameter oaks often produce few sprouts after cutting, and oak seedlings usually cannot compete after a harvest with faster growing species such as yellow-poplar, cherry and sweetgum. Most oak species develop best in intermediate shade conditions with light levels between 10 and 30% of full sunlight. Midstory removal can be used to create these light conditions and increase size and vigor of oak seedlings and advanced reproduction so they can compete with faster growing species after a regeneration harvest such as a shelterwood.

Midstory removal using herbicides and the hack and squirt application method is a low-cost, intermediate treatment that is usually completed several years (potentially up to 10 years) prior to a first harvest of a two or three stage shelterwood harvest sequence. The years after the midstory removal treatment allow oak seedling densities to accumulate (in many situations a low-intensity prescribed fire may improve seedling accumulation) and allow for development of seedlings into oak advance reproduction prior to a regeneration harvest. Herbicides such as triclopyr, glyphosate, imazapyr, or mixes of these herbicides are applied into cuts or frills made into undesirable trees just beneath the bark layer using a hatchet or machete and a small amount of herbicide or herbicide solution (typically 1 mL or less applied per cut—be sure to follow the herbicide label directions for appropriate mixtures and application rates) is applied using a handheld spray bottle. The herbicide kills the tree (including the root system) and sprouting is less likely to occur compared to cutting or mechanized removal. Midstory removal is most effective if conducted during late summer or fall when trees are translocating resources to their roots in preparation for the dormant season. Hack and squirt is easiest to apply on trees between 0.5 to 1 inch diameter up to 8 to 10 inches diameter. Depending on the herbicide used and the tree species, larger diameter trees may be more difficult to kill using this method. A goal for midstory removal to encourage oak seedling and sapling development is to remove approximately 20% of the stand’s average per acre basal area. Start with application to the smallest diameter undesirable trees (0.5 to 1” diameter) and larger diameter woody vines and then proceed with application to larger diameter stems until the 20% basal area target is met. Larger percent basal area removals may favor undesirable species as greater light levels at ground level will occur after application.

Landowners who own smaller acreages can easily complete a midstory removal treatment at their leisure or for qualifying landowners cost-share programs are available that pay a per acre rate for the practice such as the U.S. Department of Agriculture’s Natural Resources Conservation Service (NRCS) Environmental Quality Incentives Program (EQIP) Conservation Practice Standard 666 (Forest Stand Improvement). In either case, midstory removal can be a good first step to encourage development of young oaks in oak dominated forest types.