26 
BULLETIN 871, TJ. S. DEPARTMENT OF AGRICULTURE. 
to determine just what the limits of the intermediate class should be, 
so in order to preclude any error in judgment the procedure as first 
stated of establishing just two classes, dominant and suppressed, to 
include all the trees, is followed throughout. 
The method of obtaining the volume of the tree in cubic feet 
requires a little explanation. Each tree was considered as a perfect 
cone over the stump, at which the age count had been taken, in 
order to obtain directly comparable figures for the different ages. 
Figures from normal trees showing the relation of the diameter 
breast high to diameter of butt at stump height (1.5 feet) were plotted 
and curved, the strongest portion of this curve lying between 10 and 
50 inches diameter breast high. From this curve a table expressing 
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400 
Fig. 1.— Comparison of average volumes of incense cedar on the optimum and intermediate areas. 
the relation of the diameter breast high to diameter of butt at stump 
height for each inch class was read. It was then a simple matter to 
secure the diameter outside the bark at stump height for any tree, no 
matter how irregular the stump might be, due to wounds or other 
factors, and combining this with the height to work out the total cubic 
contents. Loss of volume caused by wounds or other factors was dis- 
regarded. In other words, each tree was treated as if it was absolutely 
normal. Let it be emphasized again that the volumes obtained were 
not meant to be an exact expression of the actual volume of each 
tree to the last cubic foot but merely had to be directly comparable 
to each other for the various ages. 
In considering the trees with decay, each separate focus of dry-rot 
is termed an infection, and there may be two or more infections in 
