BASIS FOR TREE CLASSES 185 



can be overcome by proper methods of measm-ement (§ 25). It 

 is the purpose of volume tables to show average volumes for sound 

 trees. Since defective logs or trees will be scaled as if sound in volimie 

 table construction, they are suitable for this purpose. 



157. Total versus Merchantable Heights as a Basis for Tree Classes. 

 Where cubic contents, either total or merchantable, are the basis of 

 tree volumes, the total height of the tree to tip of crown is the only 

 serviceable basis of classification by height (§137). Where the volume 

 of the tree is desired in merchantable units of product, such as board 

 feet, the height desired in practice is the merchantable length of the 

 bole or height of the top of the last log. Timber cruisers commonly 

 use the number of logs of given length in a tree, and not the total height 

 in feet, to obtain the contents. The practice of basing height on the 

 merchantable length of bole is most useful where the proportion of total 

 length used is most variable, as in large hardwoods or heav^'-limbed 

 conifers, and where there is an evident variation between actual top 

 diameters utilized. Total heights in dense stands of tall old trees are 

 hard to see and measure while the top diameter limit is usually visible. 

 This basis is used almost universally in the estimation of old-growth 

 timber of all species. 



The same height basis must be used in timber estimating as is used 

 in the tables, if volume tables are to be employed. Hence the method 

 of measuring heights in cruising will be either determined by the existing 

 tables, or else the tables must be constructed on the basis desired for 

 the estimating. The measurement of trees for the construction of vol- 

 ume tables should therefore include both the total and merchantable 

 height, to permit of constructing tables on each basis for use as desired. 



158. The Coordination of Merchantable Heights with Top Diam- 

 eters. The use of volume tables to determine contents of standing 

 trees requires the determination in the field of but two dimensions, 

 namely D.B.H. and height, and is based on the assumption that the 

 volume of an average tree of these dimensions gives the average volume 

 of the trees of the same sizes in the stand to be estimated. Where 

 total height is used as the basis, there is little opportunity for error in 

 applying the volumes in the table, since but one point on the tree can 

 be measured for height, namely the tip. But where merchantable 

 height is the basis, a second variable is introduced, the top diameter. 

 The volume now depends, not on one definite factor of height as before, 

 but on securing coordination between these two variables, i.e., height 

 of merchantable top, and diameter of merchantable top, in the apphca- 

 tion of the volume table. 



The choice of top diameter limits has been discussed. But the 

 effect of this choice upon the merchantable length (the height), in 



