Intermediate Management

As pine stands grow and mature, individual trees compete for light, soil moisture, nutrients, and space. Many trees are crowded by faster-growing neighbors and die. Thousands of pine seedlings populate a naturally seeded acre. In a plantation, usually 500 to 800 seedlings are planted. However, at maturity only 50 to 100 large-diameter (greater than 20 inches) sawtimber pine trees remain. More than 90 percent of the trees die. Mortality is a natural progressive process in the forest. As trees grow larger and because of limited amounts of space and resources, the environmental constraints limit the number of survivors in the stand.

Thinning is a forestry technique that mimics the natural process of mortality. By applying cuts to immature stands, material that might otherwise die before rotation age can be used and growth can be concentrated on fewer, more desirable stems left in the stand. Thinning improves the condition in the residual stand by providing greater growing space and availability of sunlight for individual trees. Increased growth, vigor, and space for these trees produce a healthier stand. The residual or remaining trees may be selected to meet any landowner objective, such as wildlife, timber or aesthetics, but the result will ultimately reduce the time required for trees to reach a target diameter class. The decreased rotation length shortens the period of return on the landowner’s investment. If markets are available, some intermediate returns are possible from the thinned material removed.

When to Thin?

While there is no magic number of years to wait until applying a thinning, several indicators suggest when thinning should occur. As trees grow and mature, their crowns will begin to compete for available sunlight. Eventually the competition for crown space results in a forest with a closed canopy, where sunlight cannot reach the forest floor and where tree crowns end up with no more space to grow and expand. At this point thinning should be applied. A more quantifiable indication of when to apply thinning (based on crown structure) is when the live crown ratios are less than 33 percent. The live crown ratio is the height of the live crown divided by the total height of the tree. Live crown ratios decrease because sunlight is not getting to the lower branches, decreasing the photosynthetic area of the tree and resulting in mortality of lower branches.

Basal area per acre can be another good indication of when to thin. Basal area is a measure of stand density. Basal area is the sum of the surface area in square feet taken by an individual tree trunk at 4.5 feet for all individuals growing on an acre. As basal area exceeds 120 square feet per acre, individual tree growth declines and the stand becomes unhealthy. For most pine stands, a thinning that results in 70 to 90 square feet of residual basal area per acre represents an ideal stocking rate. A 14-inch tree is approximately 1 square foot of basal area.

What to Thin?

Which trees to remove will depend upon landowner objectives. For timber production, undesirable species, poorly-formed trees, and slow-growing individuals are removed. Wildlife considerations may leave some of the poor-formed or cull trees to provide habitat. How much to thin will depend on objectives as well, but it must be enough trees for loggers to be interested in the operation. A common mistake in thinning is to leave too many trees in the residual stand (too conservative). Generally 40 to 60 percent of the trees are harvested during a thinning. Several thinnings may take place before the stand reaches maturity. At final harvest, usually 80 to 100 mature trees per acre remain.

How to Thin?

Depending on when the stand is thinned, the cut can be deemed either a precommercial or commercial thin.

Precommercial thins are generally required in stands that are naturally regenerated where thousands of seedlings per acre are established. The goal of a precommercial thin is to reduce stocking to 400 to 600 trees per acre. There is no merchantable material removed during this thinning; thus it occurs at an expense. However, the removal of the excess seedlings will prevent tree stagnation and improve stand growth and development. To minimize cost, precommercial thinning should be conducted before the stand is 4 years old. Mowing strips across the stand, leaving seedlings in 1- to 2-foot-wide rows, will quickly reduce the total number of seedlings. Row width is dependent upon seedling density.

Commercial thinning occurs typically when the stand is 12 to 18 years old and can provide some intermediate return on long-term investment. Two methods are traditionally applied for commercial thinning: row and selection. A row thinning is the easiest thinning method to apply. With little regard to crown class, rows of pines are selected and removed, providing additional growing space for trees in the leave rows. A selection thinning requires a more acute eye. Selection thinning favors only the best trees by removing the inferior individuals and producing a consistent spacing between leave trees. The favored leave trees use the additional growing space and mature into the desired product size.

Summary of Thinning

Mortality is an inescapable function of nature. By actively managing pine stands, this mortality can be reduced by allowing select individuals to take advantage of additional space, sunlight and resources. The results from thinning are the improvement of forest health and growth and a reduction in the rotation length. Many of the pine stands affected by southern pine beetle are overstocked and growing poorly. A properly timed thinning will help keep pine stands healthy and growing and deter southern pine beetle susceptibility.