Commercial thinning is the principal TSI treatment in sawtimber stands.
Optimum stand density varies by forest type because of the different growing space requirements of the tree species found in them. Manage stand density by the crop-tree release method or with the help of a stocking chart (pg. 207).
A simple method for regulating stand density is to identify potential crop trees and remove any surrounding trees that are interfering with the crown of the crop trees. Based on your stand objectives, leave the most desirable species and best quality stems with dominant and codominant crowns.
When applying the crop-tree release method, leave crop trees at least 20 to 25 feet apart. If crop trees are scarce or unevenly distributed, leave two trees as close as 10 feet but treat them as one tree when thinning. Remove trees with crowns that encroach on the crop trees. (Figure 5-2)
Figure 5-2. Free to grow rating. Quadrants 1 and 3 are free to grow, for a FTG rating of 2. FTG rating = 2
The free to grow rating is a simple way to evaluate the level of crown competition. Ideally, crop tree crowns should be free to grow on all four sides; however, in dense, young stands, no more than two crowded sides should be released at once. Additional release can shock a tree, leading to epicormic sprouting and degraded tree form. Provide additional crown release in later treatments.
In general, do not remove more than one-third of the basal area from a stand at any one time. A heavier thinning may lead to wind damage, sunscald, and epicormic branching on residual trees. Epicormic branches (also called water sprouts) arising from dormant buds beneath the bark on hardwood trees create knots and lower wood quality.
A more sophisticated method for regulating tree spacing (that is, stand density) is to apply guidelines from a stocking chart if one is available for your forest type (see Chapter 2: Conducting a Woodland Inventory).
It is common practice to spray a spot or band of paint on trees to be removed during TSI.
Trees growing below the main canopy will not affect crop tree growth, but cut them if they are of marketable size or of undesirable species. Do not damage crop tree crowns, stems, or roots while thinning stands. Repeat thinning every 15 to 20 years.
During a commercial thinning, it may be necessary to remove more trees than you prefer to, including some high quality trees, to enable logging equipment to reach the trees to be removed and to provide enough wood volume to attract a logger. A professional forester can help you understand these tradeoffs and plan your harvest appropriately.