34 BULLETIN 871, U. S. DEPARTMENT OF AGRICULTURE. 



At these ages extensive decay, as represented by severe cull cases, 

 becomes common in the individuals of the respective groups. These 

 ages, using Meinecke's nomenclature, may be termed the "critical 

 age" and '"age of decline" for incense cedar — that is, the ages at 

 which suppressed trees and dominant trees, respectively, become 

 subject to extensive decay. Meinecke found a combination of severe 

 wounding and pronounced suppression both contributing to the criti- 

 cal age in white fir, but in incense cedar wounding is not necessarily 

 a factor. This will be brought out later when mechanical injuries are 

 considered. 



After the age of decline is passed, as shown by the percentage of 

 cull in the classes older than 240 years, the dry-rot is more extensive 

 in the dominant than in the suppressed trees. This means that 

 while the dominant trees are able to ward off the extensive develop- 

 ment of the dry-rot fungus for a longer period than the suppressed 

 trees, after the age of decline is once passed dominance ceases to be 

 a factor in resisting decay and, in reality, seems to favor it. This 

 may be due to the fact that in the old, overmature dominant trees 

 there is a higher percentage of food material (i. e., heartwood) for 

 the fungus to work on in relation to the total volume than in the 

 case of the suppressed trees. The fungus does not attack sapwood. 



Let us consider what the foregoing paragraphs mean from a prac- 

 tical standpoint. Roughly, we may place the critical age at 165 

 years and the age of decline at 210 years. This does not mean that 

 there is no loss from decay previous to these ages, or even that 

 there are no severe cull cases; but the latter are so rare, as shown by 

 Table IV, that they may well be regarded as exceptions. Since all 

 but a very few of the trees on the intermediate area are suppressed, 

 taking this area as representative for the intermediate range we 

 can not expect trees within this range to remain free from extensive 

 dry-rot after they have attained the age of 165 years. In the opti- 

 mum range this same age may be set for suppressed trees, while 

 dominants will remain relatively sound until the age of 210 years is 

 reached. On the optimum areas dominant individuals comprised 

 36.5 per cent of the total, but on the intermediate area only 7.6 per 

 cent. 



Xo relation was found between diameter breast high and dry-rot. 

 This could hardly be expected, considering that incense cedar is a 

 tolerant species in an uneven-aged mixed stand. 



In Tables II, III, and IV the comparison of the dominant and sup- 

 pressed trees in their relation to the percentage of cull due to dry-rot 

 has been emphasized to the neglect of other considerations in which 

 the same relation might be found in both groups. In Table V many 

 of the data given previously but separately for the dominant and 

 suppressed groups are combined. 



i 



