216 



FORM CLASSES AND FORM FACTORS 



80 ft., 



72 ft. 



If taken to a uniform or fixed top diameter, trees with a high form 

 quotient would be cut higher in the top and fall into a different merchant- 

 able height class than trees with a low form quotient. Therefore, for 

 trees of different form quotients, to attain the same merchantable top 

 diameter, trees with the lower quotients must be taller than .those whose 

 form quotient is high. Hence total and merchantable heights are 

 not interchangeable for trees whose form quotients differ. 



If taken to variable top diameters, this second variable will make 

 it practically impossible to distinguish form classes based on total 



height in the volumes 

 given, for these tops 

 would not vary in any 

 definite relation to 

 total height or form. 



As long as mer- 

 chantable rather than 

 total heights are used 

 in volume tables and 

 timber estimating, 

 form classes based on 

 actual form of the 

 tree cannot be used 

 to construct volume 

 tables in which trees 

 of different form 



are 



20" 20" 20" 



Fig. 37. — Effect of cutting to a fixed top diameter, upon 



merchantable height of trees having different form 



quotients. A form quotient of .60 requires either a 



shorter merchantable length or a taller tree than one separated, and the 



of .80. principle of averaging 



the differences in vol- 

 ume due to form must continue to be used for such tables. 



But for cubic feet, basic volume tables may be made up giving the 

 volume of each diameter, height and form class. Similar tables can be 

 constructed in any unit of volume, or for any log rule, from tables of 

 normal taper. In applying these tables, the method would be not to 

 attempt to tally each tree in its proper form class, but to determine 

 average form classes (§171) for stands or other subdivisions of the 

 forest, the volumes for which can be taken from this basic table to form 

 a standard volume table for the trees to which it applies. Not over 

 three such tables would be apt to be needed for any tract, however 

 large and varied. 



Methods of rapidly determining the form class of sample trees, in 

 order to apply such a system, are given in § 201, § 202 and § 203. 



