These tables are based on the assumption that trees having the same diameter and merchantable height will have similar, though not, necessarily identical, rates of taper in the saw log portion above the first log, and that all differences in volume in trees of the same diameter and merchantable height may be attributed largely to taper variations which occur in the first log. These volume differences are provided for by 26 separate tables, each of which shows volume according to diameter classes and an index to first log taper, called form class.
Definition of form class. Form class is the percentage ratio between the diameter, inside bark, at the top of the first 16-foot log and the diameter outside bark at breast height (4 ½ feet above the ground). For example, a tree with a first-log scaling diameter of 16.0 inches and a breast-high diameter of 20.0 inches has a form class of (16 / 20 x 100) 80 percent.
Upper-log tapers. Taper is the decrease of diameter in a log or a tree stem from butt to top. The average u per-log taper rates shown in table 2 were originally compiled from ocular estimates of taper on about 2,000 trees of assorted species in the, anthracite region of Pennsylvania, and later modified and extended on the basis of ocular taper estimates made on about 20,000 additional trees throughout the South. Although taper differences exist between and within species, measurements of felled trees summarized in table 3 show that the values given in the tables, which are based on these average tapers, may be applied with reasonable accuracy to a number of species in a wide range of localities.
The following example illustrates how the taper estimates in table 2 were used in compiling the tables. A form-class-80 tree with a diameter of 20.0 inches will have a first-log scaling diameter of 16.0 inches. In a tree having two 16-foot logs, second-log taper is shown by table 2 to be 2.1 inches. Scaling diameter of the second log is therefore 16.0 minus 2.1 or 13.9 inches. The two logs scale 181. and 133 board feet, International 1/4-inch, respectively, or 314 board feet for the tree. Volumes for all trees in even 2-inch diameter classes and full log lengths were computed for each form class in this manner. Volumes for odd diameters and half log lengths were obtained by interpolation.