DIAMETERS IN VOLUME TABLES 



149 



the more pronounced this swelling, and the further it extends up the 

 trunk. Tree volumes must be averaged on the basis of their diameter 

 in inches. If this diameter is taken at some point on the butt swelling, 

 a tree with a rapid butt swelhng will have a far smaller volume than 

 one of the same stump diameter and a gradual swelling, as is illus- 

 trated in Fig. 24 by trees A and B. But if these diameters were taken 

 at a point above the butt swelhng the two trees would properly fall 

 into different classes. Since it is necessary to put in a single class 

 trees whose volumes are as nearly similar as possible (trees A and C), 

 the practice of classifying these trees by their diameter on the stump 

 is inaccurate. The height of stump itself is also a variable. Tables 



^ 



A B C 



Fig. 24. — Comparison of stump height and breast height as points of measurement 

 to determine the diameter of standing trees. 



based upon " diameter at the stump," which do not indicate at what 

 height this diameter is measured, are difficult to apply and unreliable. 

 For very large trees with excessive butt swelling such as cypress, 

 or many West Coast species, the diameter classes should be based 

 upon measurements taken above this swelling. A standard form of 

 universal table used on the Pacific Coast is based on a butt measure- 

 ment to be taken 1 foot above the point where the butt swelling ceases. 

 The disadvantage of measuring at a variable height is considered as 

 offset by the merit of avoiding this variable factor of butt swelling. 

 In cypress, one tj^pical table was based on diameter at 20 feet from' 

 the ground and cruisers customarily estimate cypress trees from the 

 diameter obtained above the butt swelling. 



