Thinning and Pruning Forests

Thinning

Forest trees often grow too close together for the development of timber suitable for harvest. Crowded trees compete for light, moisture, and nutrients, resulting in slow growth and weakened condition. To avoid overcrowding and competition, a stand of trees should be thinned when young to increase the growing space available to each tree. The primary goal of thinning is to produce fewer large, healthy trees for logging rather than many small, weak trees.

Thinning Purposes There are five basic objectives may be met through implementation of a thinning program:

• Increase the growth of the remaining trees, particularly the diameter of the tree rather than the height. This allows trees to reach a marketable size more quickly.
• Select the most favorable species for growth by removing the unwanted or low value trees.
• Reduce vulnerability to insects and diseases by improving the health of remaining trees.
• Select for the highest quality trees by removing broken, deformed or forked trees.
• Utilize or sell trees that are cut.

Timing of thinning As noted, thinning is generally most successful when conducted on younger trees. Older, large trees typically do not respond to thinning as much as young trees, as their growth has already been suppressed due to competition. This is particularly true for species that require relatively more light to thrive. Shade-tolerant species are more likely to respond to thinning at older ages. The following chart shows the shade tolerance of several common Pacific Northwest tree species.

Thinning is typically initiated when the branches on neighboring trees begin to overlap and while the tree crown still accounts for a high percentage (over 40 percent) of the tree height. This rule applies as the trees mature, requiring several thinnings as the forest grows. Depending on the species of trees, thinning should occur every 5 to 15 years.

Late summer and fall are the best times to conduct thinning operations. This allows trees to develop during the spring and early summer growing season. Also, cut trees can attract insects that have the potential to kill the remaining trees. Working later in the year will help suppress any infestation. Thinning can be carried out in winter, however, care must be taken to avoid compaction and erosion of soils during the wet season.

Thinning intensity The number of trees removed, the thinning intensity, is an important factor in managing the forest. If the intensity is too light, the competition among trees is not reduced enough to result in substantial growth of the remaining trees. If thinned too much, the land is under-utilized and you will not maximize your harvest. Wind damage and sun scald can also be issues in heavily thinned forests of certain species or on highly exposed sites.

Thinning guides are available to help determine appropriate spacing. Professional foresters can recommend appropriate guidelines for your forest area.Contact the SCCD Forester at (509) 535-7274 or other professional for assistance.

Additional thinning advice is available from the Washington State Department of Natural Resources.

Pruning

Pruning branches can produce healthier and more valuable trees, while also improving other aspects of forest health.

Pruning Purposes Removing branches from the lower part of the tree will allow clear wood to develop. This wood can be much more valuable for use as veneer, plywood and finished materials following harvest. Pruning lower branches, particularly dead limbs, helps reduce the chance of fire climbing the tree and reduces access to potential damaging animals, such as porcupines.

Pruning timing Pruning is best conducted in the fall or winter. As with thinning, this will reduce the potential for insect infestations in the debris. The chance of damaging the bark is also reduced by not pruning during the spring and early summer growing season.

Pruning Techniques Branches should be removed to a height of 15 to 20 feet, depending on the overall height of the tree. Be sure to leave a substantial crown, at least 2/3 to 3/4 of the total height. Limbs should be cut as close the the branch collar as possible, making sure not to scar the tree. To avoid having the branch break and possibly damaging the trunk of the tree, use three cuts to remove a branch: 1) make an under cut about 12 to 24 inches from the branch collar, cutting about 1/3 of the way through the branch; 2) make a top cut all the way through the branch about 1 inch out from the under cut (this will remove the weight of the branch to make it easier to create a clean final cut; and 3) make a final cut just beyond the branch collar.

Additional pruning information is available from the Washington State Department of Natural Resources.

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