Estimating Individual Tree Volumes
Tree volume tables are used to determine volume from DBH and merchantable height measurements. It is important that you be aware of the specifications (stump height, top diameter, bolt length, etc.) used in constructing the table to ensure that you use the table appropriately.
Volume tables generally are specific for each type of wood product. Volume estimation for various products is discussed below.
Sawtimber and Veneer Trees
Trees that are large enough and of high enough quality to produce logs that can be sawed into lumber are referred to as sawtimber. To qualify as sawtimber, trees should have at least one eight-foot bolt, should be at least 10 inches DBH, and should have a top diameter inside bark that is the larger of either eight inches or 50 percent of tree DBH. Minimum sawlog top diameter inside bark for a tree with a DBH of 20 inches is therefore 10 inches. (These diameter specifications are typical; individual buyers may have different specifications.) Sawtimber trees must not contain too many defects such as excessive branchiness, decay, scars, bulges, bark distortions, holes, branch stubs, and crookedness.
Individual trees of many species (black walnut, sugar maple, elm, red oak, white oak) that are of exceptional quality, have at least one eight-foot bolt, are at least 16 inches DBH, and contain bolts that have a top diameter inside bark of at least 10 inches often can be sold as veneer trees. Logs harvested from these trees will be sliced or peeled to produce veneer. Such trees are more valuable than sawtimber.
The basic unit for estimating volume for both sawtimber and veneer trees is the board foot. A board foot is a piece of wood of any shape that contains 144 cubic inches of wood (12 inches X 12 inches X 1 inch, 6 inches X 6 inches X 4 inches, etc.).
The Scribner log rule is one method for estimating sawlog volume in Minnesota. Table 3 describes tree volumes obtained by applying the Scribner rule to the logs in trees of various sizes.4 As an example, Table 3 indicates that a tree with a DBH of 22 inches and 32 feet (four bolts) of merchantable height will yield approximately 290 board feet. If there were 10 percent defect in the tree, total tree volume would be reduced to 261 board feet. A reasonable range for board foot volume estimates per acre is 1,000 to 15,000 board feet.
There are several other log rules used in different parts of the United States. Some purchasers of veneer quality trees (especially buyers outside Minnesota) may buy material on the Doyle rule. These buyers may either remeasure each tree to be sold using the Doyle rule or apply a conversion factor to change Scribner volumes to Doyle volumes.
Pulpwood
Trees that are too small or too poor in quality to be sold for sawlogs are often sold for pulpwood. Ultimately, these trees are chipped or ground up to manufacture products such as paper, hardboard, and various types of structural board. A large number of tree species growing in Minnesota can be sold for pulpwood. However, in some areas of the state, it is difficult to find buyers for certain species because of the long distance to the nearest mill that uses the species.
Minimum DBH for pulpwood trees is 5 inches. Minimum top diameter inside bark is the larger of either 4 inches or 50 percent of tree DBH. Minimum pulpwood top diameter inside bark for a tree with a DBH of 12 inches is therefore 6 inches.
The basic unit for estimating pulpwood volume in trees is the cord. A standard cord is the equivalent of a stack of wood eight feet long, four feet high, and four feet wide (128 cubic feet) (Figure 5). The wood content (excluding bark and air space between stacked pieces) of a cord varies from about 65 to 95 cubic feet depending on the diameter, roughness, and crookedness of the pieces. An accepted average value in the Lake States is 79 cubic feet of wood per cord.
Table 4 provides tree volume estimates for pulpwood. According to the table, a tree with a DBH of 10 inches and 24 feet (3 bolts) of merchantable height will yield approximately 0.10 cords. If the tree contained 10 percent defect, tree volume would be reduced to 0.09 cords. A reasonable range of cord estimates per acre is 5 to 45 cords.
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