DRY-ROT OF INCENSE CEDAR. 25 
The crown class, as determined by observation of the standing 
tree, expresses the past history, more or less strongly modified by 
conditions prevailing through a varying number of years previous to 
the time of observation; it may not give the real past history of the 
tree. " Dominance" and " suppression " are really incorrect terms, 
used for lack of better ones. They are based on the relation of the 
height of one tree species to others in the same stand. In this case 
height alone would be misleading. For example, consider a more or 
less second-story species in a mixed stand, in which category incense 
cedar falls. Practically all the trees would be included in the inter- 
mediate or suppressed classes when related to other species in the 
stand, thus entirely obscuring the true relation of the individuals 
within the second-story species. On the other hand, it is an exceed- 
ingly difficult undertaking, often leading to grave error, to attempt 
classification by the observation of individuals in a mixed stand 
with relation to other individuals of the same species. 
For our purposes we can not consider other tree species, but must 
compare individual trees with others of the same species. But here, 
also, height alone is not the deciding factor. Instead of giving 
dominance and suppression in the current meaning, these terms are 
expressed by the relation of the actual volume of the tree to the 
average volume of trees of the same age. Therefore, it was necessary 
to "curve" data collected on a number of trees to secure average 
volumes by age. Only trees of normal form with exact ages and free 
from severe wounds, malformations, and other seriously injurious 
factors which would interfere with the correct computation of the 
volume were used. Curves were constructed for the intermediate 
area and for the optimum area, since it was apparent that the 
volumes by ages would be much higher for the last-named areas 
than for the first, which fact was strongly brought out by the result- 
ing curves. These curves are presented in figure 1, the higher curve 
based on 461 trees representing the optimum area and the lower 
based on 340 trees, the intermediate area. The National Forests on 
which these areas were located are also indicated. Thence, the trees 
for the intermediate area and for the optimum area were rated in 
regard to their respective curves, those with a volume higher than the 
average given by the curve for the same age being classed as dominant 
and those with a lower volume as suppressed. At first, an inter- 
mediate group was selected by designating an arbitrary volume above 
and below the average volume, trees between these limits being 
classed as intermediate. However, it was found that such trees 
inclined either toward the dominant or the suppressed in their charac- 
teristics, depending on whether they were above or below the average 
in volume for the same age. Furthermore, it was exceedingly difficult 
182803°— 20— Bull. 871 i 
