Managing an oak-hickory forest

             The oak-hickory forest type requires frequent disturbance in order to maintain itself.  Disturbance in the form of fire promotes competitive seedlings and decreases competition.  Disturbances that affect the forest canopy, such as harvest or severe wind damage, provide higher light levels for early height growth.  Without these disturbances, shade tolerant species such as sugar maple and American beech regenerate.  Over time, these species will replace each overstory oak one by one, creating a beech-maple forest type.  This forest type does not provide the multiple benefits an oak-hickory forest provides, especially when considering wildlife habitat and mast production.  For example, oak trees produce about 90% of the wildlife food mast in an oak-hickory forest, with ash, hickory, beech, walnut, persimmon, black cherry, and maple producing the rest.

            Research across the eastern United States has reported consistent failure of oak-hickory regeneration after cutting treatments have been made over the past 20 years.  Researchers consistently site the absence of desirable advanced regeneration as the biggest factor.  And in circumstances where desirable advanced regeneration was present, they were typically outgrown by rapidly growing species which were also present in the understory.  They are also out competed by light seed species which can often seed in after the treatments, such as fast growing yellow poplar.

            Today, prescribed fire and the shelterwood method are the two silvicultural tools used to re-establish the oak-hickory forest.   The shelterwood method is when a partial overstory is left after a harvest to provide shelter and seed for seedling establishment.  The shelterwood method has two to three phases, or cutting cycles.  The first cut removes undesirable species that may cause problems in the future stand and stimulates seeding of desirable species.  The second cut removes a part of the overstory after advanced regeneration is present in adequate numbers.  The third cut removes the remaining overstory, allowing advanced regeneration to take its place.  Prescribed fire is used before and after the initial cut to kill competing species, develop a seed bed, and promote desirable species. 

            A two-aged shelterwood cut can also be used to improve aesthetic quality.  This method permanently leaves large, residual trees scattered across the landscape.  The trees can be used as a seed and food source, and can then be girdled in the future to create wildlife dens.  This method gives the forest a park like appearance with a developing understory beneath it.

            Prescribed fire has shown great success in treating the understory of oak forests.  In an unmanaged stand with oak-hickory objectives, prescribed fire will often be the first step to re-creating a valuable forest.  The initial burn will decrease understory densities by killing shade tolerant competitors.  The second burn is conducted 2 to 5 years later, or after adequate oak regeneration is present.  It serves two purposes: to help develop a high root: shoot ratio, and to further decrease the number of competitors.  After regeneration is established, burns can be conducted at intervals of 4 to 20 years depending on site and stand characteristics, finances, and motivation.

            Regeneration cuts, also called clear cuts, were commonly used as a method to regenerate the oak-hickory forest.  Regeneration cuts remove all standing stems over 2 inches in diameter at once, with the main objective being the creation of a new even-aged stand.  One necessity for the use of a regeneration cut is the presence of advanced regeneration before harvest.  In the case of oak species, if advanced regeneration is not present and competitive, they rarely have a chance of becoming dominant trees in the next stand.  The altered disturbance regimes of the past century have increased the density of forest stands, which decreases the numbers of oak-hickory regeneration in the understory, leaving the effectiveness of regeneration harvests in today�s oak forest debatable.

            Group selection is also used to regenerate oak-hickory forests while increasing biodiversity at the same time.  Group selections are like mini clear cuts, but need to be fairly large to regenerate oak trees.  Two acres is the minimum.  They have the same problems that regeneration cuts have, but they are more aesthetically pleasing, provide wildlife habitat, and increase biodiversity.  Hand thinning the understory with chain saws and using prescribed fire before harvest can supply adequate amounts of regeneration before the cut is implemented.

            Many landowners are often tempted to use a diameter-limit cut, which removes all trees over a specific diameter, allowing the mid-story to occupy the future stand.  This practice is called high-grading.  It does not favor the development of desirable regeneration or favor high quality trees for the future, leaving behind undesirable trees with poor form and value.  Repeated diameter-limit cutting in an oak-hickory forest will eliminate oak seed sources, decrease timber value, create patchy and irregular mixtures of open and crowded areas, reduce vertical structure, decrease growth rates and volume production, and decrease biodiversity.  Diameter-limit cutting trades long-term production potential and other values for immediate financial gain.  A sustainable forest is always considered a long-term investment.

 Alternatives to prescribed fire

                 Prescribed fire is not the only tool that can be used to reduce the density of forests and promote healthy oak regeneration.  A timber stand improvement cut (TSI) is a common approach to thinning out over stocked stands and crowded understorys.  TSI work is done with chainsaws, is labor intensive, and increases fuel loads.  It is often used in conjunction with prescribed fire as the first step in an unmanaged forest.  TSI kills the larger trees that will be unaffected by prescribed fires, and increases fuel loads when a hot fire is desired.  

              Herbicides are also used to kill competing vegetation.  They are most useful to eradicate invasive/exotic species and to treat stumps after TSI work to prevent re-sprouting of undesirable species.  When used to clear the forest understory, they often kill more than undesirable species, can degrade water supplies, and have health concerns when large amounts are applied.

            Both of these alternatives are viable options for forest management, but neither provides the benefits of fire in an ecosystem.  Fire recycles nutrients and makes them available for plant uptake immediately.  It also reduces the chance for future catastrophic fires by reducing fuel loads in a controlled fashion.