Backyard Forest Stewardship in Western Washington
Thinning
Photo by: P. Duniho. The stand of trees on the left has not been thinned. The stand on the right was thinned, allowing for a more open appearance and healthier residual trees.
Thinning is one of the most beneficial tasks most Washington forest owners can perform to improve the health and vigor of their woodlands and reduce the potential for damage from catastrophic wildfire.
Why should I thin?
Many forest stands in Washington are overcrowded. Growing too closely causes stressed and unhealthy trees -- which can result in several problems. When trees compete for sunlight, nutrients and moisture, results are:
Poor growth and health. Overcrowded and stressed trees have poor diameter growth and small crowns. Stressed trees are more susceptible to forest health problems than vigorous trees.
Poor wildlife habitat. If too little sunlight reaches the forest floor, shrubs and forbs, which are beneficial to wildlife, are shaded out.
High wildfire hazard. When crowns of adjoining trees touch, fire can spread quickly.
Unattractive forest. Many people find a well-spaced forest stand more inviting, accessible and attractive than an overcrowded stand.
When should I thin?
When branches of adjoining trees touch, it is time to thin. Short tree crowns are also a sign of overcrowding. Healthy trees should have approximately 40% of their total height in live green branches. Here are some guidelines that may prove helpful:
Save the biggest and best. Retain the tallest trees with larger diameters and large healthy crowns.
Remove competitors. Trees with below average diameters, shorter crowns, disease, insect, or other problems should be removed from the upper level canopy to benefit their more desirable neighbors. Do not make the mistake of trying to "release" poorer, lower level trees by removing larger, better trees growing above them.
Retain trees for wildlife. Dead or hollow trees should be retained, unless unsafe. Consider leaving a few small clumps unthinned and unpruned to provide habitat diversity.
Watch out for fire hazard. Debris (slash) left from thinning and pruning may result in a fire hazard if a significant accumulation occurs. Slash may need to be piled, burned, or chipped. State law requires excessive slash to be removed from within 100 feet of a public road and 500 feet of buildings. Consult the Safe Debris Burning section of this publication.
Remove enough trees. We know that most forest landowners really love their trees, but a common error is to remove too few of them, resulting in a "thinned" stand which is still stressed and overcrowded. After thinning, branches from the crowns of adjoining trees should be several feet apart with open sky visible between trees. You can use the diameter spacing D + 6 rule:
Diameter spacing (D + 6) rule
For example: let’s say you have measured 10 “best” trees you plan to retain after thinning. By using the procedure described above in steps 1 and 2, you estimate the diameter of each tree as follows: 8.7, 7.3, 8, 8.7, 7.7, 8.3, 9, 7.3, 8.3, and 8.7. Adding these numbers and dividing by 10 gives an average diameter of 8.2 inches. Adding 8.2 and 6 gives you 14.2. So when you thin your stand you should leave approximately 14 feet between the trees that will remain. Use this formula as an approximate guideline. Nature is not uniform, so do not strive for exact spacing between all of the leave trees. Retention of the best trees should take precedence over exact spacing. If you want a "park like" appearance, you may wish to space the trees further apart. |
Selling Your trees
In some cases, thinning projects remove enough trees of sufficient size to make selling the trees commercially an option. Neighbors can work together to create an economically viable project.
In recent years, markets for small diameter trees have improved. In most areas, small-scale logging operators are available. If conducted properly, thinning projects can have positive environmental and economic results.
If you are contemplating such a project, it is advisable to seek professional advice from a consulting forester. WSU Extension maintains a directory of consulting foresters. You may also want to read: Managing Your Timber Sale.
Learn more:
Thinning: an Important Timber Management Tool
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