Tongass National Forest

Timber Stand Improvement

Variable Spaced Thinning

The Forest Service is experimenting with variable spaced thinning using a variety of approaches and spatial patterns.

Variable spacing can be achieved in several ways. One way is to specify a minimum and maximum distance for stem separation (trees left no closer than 4 feet and no farther than 20 feet). This method requires minimal pre-treatment data but final stand density depends heavily upon thinning toward an average stand density in stems per acre. Using this method spacing can be quite variable.

Another method is to specify diameter limit cutting where the large size stems would be left, no matter where they occur or how far apart. This method requires adequate pre-treatment data to prescribe for and achieve a desired stand density after treatment. Each of these two example methods has pros and cons.

The objective is to open up the canopy to encourage browse, create gaps and thickets. Through variable spaced thinning we can lessen tree competition and concentrate diameter growth on the leave trees.

Current pre-commercial thinning efforts on timber base lands use a compromise between minimum/maximum distance and fixed spacing. A target spacing is specified (14x14 feet) but a large variance (+/- 50%) is allowed to select the best tree. A large spacing variance of 50% allows trees to be selected no closer than 7 feet and no farther apart than 21 feet for a 14x14 foot average spacing. This approach requires little pretreatment data, allows young growth to be manipulated for tree size, quality and species while offering a wide range of spacing between individual stems. This is the present standard for pre-commercial thinning for timber objectives on Prince of Wales Island.