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 

 70lum.es by ages would be much higher for the last-named areas 

 than for the first, which fact was strongly brought out by the resulti- 

 ng curves. These curves are presented in figure 1, the higher curve 

 jased on 461 trees representing the optimum area and the lower 

 i>ased on 340 trees, the intermediate area. The National Forests on 

 vhich these areas were located are also indicated. Thence, the trees 

 r or the intermediate area and for the optimum area were rated in 

 'egard to their respective curves, those with a volume higher than the 

 iverage given by the curve for the same age being classed as dominant 

 ind those with a lower volume as suppressed. At first, an inter- 

 nediate group was selected by designating an arbitrary volume above 

 md below the average volume, trees between these limits being 

 classed as intermediate. However, it was found that such trees 

 nclined either toward the dominant or the suppressed in their charac- 

 teristics, depending on whether they were above or below the average 

 n volume for the same age. Furthermore, it was exceedingly difficult 

 182803°— 20— Bull. 871—— 4 



