How to Calculate Timber Harvest
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Owners of small, private forests who are planning to sell some of their trees for lumber often need to estimate the volume of wood. A useful number to calculate in the Pacific Northwest where timber is priced by “thousand board feet,” or MBF, is the volume of board feet of merchantable timber in a particular parcel. This is a number useful in scheduling harvests and in financial and tax projections. While not as complex or detailed as the estimates calculated by professional foresters for commercial timber growers, these steps should give a small woodland owner a reasonable estimate of his forest's projected yield.
Select a group or “stand” of trees similar in age, size and species. Locate a place within this stand to use as a center point of your sample plot. There should be several typical trees nearby. Mark a circle from that center point with a 26-foot, 4-inch radius. This is .05 of an acre. Select five to eight trees within this plot.
Use a Woodland Stick, a four-sided stick used for measuring timber volume in the Pacific Northwest, to determine the diameter of the tree at breast height (DBH), or 4 ½ feet from the ground on the uphill side. Correct position is important when using this forestry tool. You must be 25 feet from the tree and hold the stick horizontally the correct distance from your eyes. Use side one to measure for diameter and write down your figure.
Use side one of the Woodland Stick to measure the height of the tree. Move back to 100 feet from the tree and be in a position to see the full tree. Hold the stick vertically the correct distance from your eyes and determine the height of the tree. Note this number as well. Do this for each of your selected trees within the plot.
Find the tarif number for each tree using these figures and the tarif access table for your tree species. Find the tarif tables for the Pacific Northwest in the Oregon State Extension Service publication EC 1609.
Consult the volume tables on the Woodland Stick using the tarif number and diameter of your tree to determine the number of board feet. The most commonly used table, the Scribner Volume Table, will tell you how much marketable timber you have per tree.
Multiply the average number of board feet per tree in your sample by the total number of trees per acre. This will give you an estimate of the number of board feet per acre. Multiply by the number of acres you plan to harvest for your total volume.
For more precise and detailed information on your timber quality, growth rate and marketability, consult a professional forester. Tree height can also be determined using a clinometer. You made need to measure several plots and average them if your forest is uneven. You may also need to adjust the plot size if you have too many or too few trees for a sample.
There are many variables in determining timber volume, including the age and health of the trees, density and how many species are mixed together. Finding stand volume is always an estimate, never an exact science, even for professional foresters.
- "The Woodland Workbook: Forest Measurement" Oregon State University Extension Service; 2002
- Washing State University: Woodland Stick Instructions
Margaret Mills has been writing for more than 30 years, focusing on articles about religion, forestry, gardening and crafts. Her work has appeared in religious periodicals including "Focus on the Family" and similar publications. Mills has a Bachelor of Arts in English from Northwest Nazarene University.