Use of Tables in Timber Estimating

Procedure. Except for the necessity of determining form class, the procedure in applying these tables in a timber cruise is the same as required for volume tables which are based only on diameter and height. For most accurate results, trees in each desired species group should be tallied by diameter and height classes. An average form class ma then be assigned to each species group and volume obtained from the corresponding table. Because form class may vary widely for a given species, more accurate volume estimates will be obtained if separate form-class ratios are determined for each diameter class tallied as part of the cruising procedure.

Another method of cruising is to tally only the diameter of the trees in each species group. Tree heights are later determined by measuring sufficient sample trees to plot a satisfactory curve of height over diameter. Volumes for each diameter class are then obtained from a volume table based on diameter and height, and applied to the diameter class tally. The attached tables can be used with this method, provided the additional factor of form class is obtained for each diameter class.

Merchantable height. Merchantable height includes that portion of a tree from stump height to a point on the stem at which merchantability for saw timber is limited by branches, deformity, or minimum diameter. For smooth stems this minimum diameter is usually not less than 60 percent of tree diameter breast high in the case of the smallest sawlog trees, usually 10 inches, or 40 percent for large trees, 30 to 40 inches in diameter. This definition of merchantability or usable log length must be carefully observed. If height measurements include the small tops to which northern conifers are frequently utilized or the small tops of old-field southern pines made temporarily profitable by wartime conditions, the tables will overscale.

Determination of form class. Cruisers having no previous experience with tree form class will find it necessary to base it on direct measurements of tree diameter and first-log scaling diameter. On standing trees, measurement of diameter at the top of the first log may be secured by climbing or by the use of a light sectional ladder. Two centers and one base section of a window cleaner's ladder are recommended. Extended, these sections are about 17 feet long, and when telescoped they are easily transported. After practice, however, ocular estimates of form class of sufficient accuracy for most purposes are possible, and are much preferred because they speed up timber estimating. Proficiency in ocular estimates of form class can be developed and checked by the following procedure:1. Stand away from a tree in position to observe the bole from the point at which tree diameter is measured to the top of the first log. Study the taper in the first log, estimate the form class and record it. In making this estimate, avoid trying to derive it from ocular estimates of the actual diameters.

2. Check the estimate by measuring the form-class percentage as follows:

a. Measure tree diameter to the nearest tenth of an inch.

b. Measure the sealing diameter at the top of the first log to the nearest tenth inch.

c. Divide the scaling diameter of the first log by the diameter of the tree. The quotient, expressed as a percentage, is the form class.

3. Compare the results with the estimated form class. Repeat the test on additional trees until proficiency is acquired. A good start in ocular estimating can be developed by a few hours' practice.

Form class of swell-butted species. In swell-butted species, such as cypress and tupelo, volume would be underestimated if tree diameter were measured at breast height, because the upper-log tapers on which the tables are based were correlates with breast-high diameters of trees with normal butt taper. The upper-log tapers of swell-butted trees are less than in trees ot normal stem form having the same diameter. Because of this, tree diameter of swellbutted species should be measured about 18 inches above the pronounced swell. Measurement of first-log scaling diameter remains unchanged. In normal trees the two related diameters are 12Y2 feet apart, but on swell-butted species they may be only 6 to 10 feet, depending on the height of stump and the degree of swell.

Form class of trees chipped for turpentine. When the tree bole at breast height is deformed by turpentine chippii)g, tree diameter should be measured just above the highest face. Although the diameter will be smaller, the. form class will be higher and consequently there will be a compensating increase in volume. Top of the first log should always be measured at 16 feet above the point where the tree will be cut. Effect of scaling practice on form class. Although the Doyle log rule was designed for inside-bark scaling practice, it is customary on some private operations in the South to increase the scale, particularly for small logs, by including in log-diameter measurement one or both bark thicknesses to compensate for low values of the rule. Where such scaling practice is observed, the accompanying tables will give volume estimates needing no = correction if form class is based on first-log diameter according to local scaling practice and normal diameter at breast height. For example, a 20-inch tree with a normal form class of 80 percent will have an adjusted form class of 85 percent if an inch is added to the inside-bark scaling diameter of the first log to allow for bark thickness. Including bark thickness in the log diameter measurement will increase the form class for any tree and consequently the volume read from the table.

Form class averages. Advance knowledge of the form class of common eastern species will be helpful to cruisers unfamiliar with this term (table 4). These averages should not be used in lieu of local determinations unless a considerable amount of error is permissible. For example, second growth southern pines average about 78, yet the spread in form class ranges from about 65 for small, branchy, old field pines to 83 for older trees growing in dense stands.


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Most Recent Revision: 09/20/06
Private Forest Management Team
Auburn University, Alabama