Thinning Forest Stands
Thinning is one of the most beneficial activities most eastern Washington forest owners can perform to improve the health and vigor of their woodlands and reduce the potential for damage from catastrophic fire. Here are some guidelines that may prove helpful:
- Save the biggest and the best. Retain the tallest trees with larger diameters and large healthy crowns and stems.
- Remove competitors. Trees with below average diameters, shorter crowns, disease, insect, or other problems should be removed from the upper level canopy to benefit more desirable trees left behind. Don’t make the mistake of thinking that you are promoting the growth of lower level trees by removing the larger, better trees growing above them.
- Retain the best suited species. On most eastern Washington sites, pine species and larch are preferable to Douglas-fir and grand fir, which are more susceptible to drought and fire.
- Retain trees for wildlife. Dead or hollow trees should be retained, unless they are in an unsafe location. Consider leaving a few small clumps of trees un-thinned and un-pruned to provide habitat diversity.
- Remove enough trees. A common error is to remove too few trees, so that the resulting stand is still overcrowded and stressed. Use the D+8 formula as an approximate guideline for spacing. Nature is not uniform, so don’t strive for exact spacing between all 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.
- Thin at the right time of year. In eastern Washington, August through December is the best time to thin your forest stands. Avoid creating any pine slash between January and July to reduce the potential for Ips bark beetle [link] infestation. Ips bark beetles are native forest insects that overwinter and breed in pine slash with branches and bole wood over 4” in diameter. When large amounts of slash are present, populations of Ips can grow and become a serious problem. It is best to burn or dry out slash in the sun so it is not a suitable host for the Ips Beetle.
- Be wary of fire hazards. Slash left from thinning and pruning activities can be a fire hazard if significant accumulation occurs. Slash may need to be piled, burned or chipped. State law requires that excessive slash must be removed from within 100 feet of a public road and 500 feet from buildings.
For additional information on thinning for forest health check out the following:
Guidelines for thinning Ponderosa Pine for improved forest health and fire prevention
Thinning and Pruning (streaming video)
Variable density thinning for wildlife and wood production
Photo by: Nicole Strong, Oregon State University Extension
D+8 Rule of Thumb
Pick ten “best” trees which you plan to retain after thinning. Measure their circumferences at 4 ½ feet above the ground, using an ordinary tape measure.
Divide by 3 to estimate the diameter (for example, a tree 12 inches in circumference is approximately 4 inches in diameter).
Add diameters and divide by 10 to get the average diameter.
Add 8 to this number. This value should be the approximate spacing in feet between the remaining trees after thinning.
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: 4, 5.3, 4, 4, 3, 5.3, 8, 4, 8, and 3.
Adding these numbers and dividing by 10 gives an average diameter of 4.8 inches.
Adding 8 and 4.8 gives you 12.8. So when you thin your stand you should leave approximately 13 feet between the trees that will remain.
Heading using the h3tag
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