220 JOURNAL OF FORESTRY 



Poiyporus auiarus, the cause of the very common dry rot or pin 

 rot of incense cedar, follows this species throughout its range and is 

 responsible for the marked, if not wholly justifiable, depreciation of 

 this otherwise valuable timber. The fungus is very aggressive, but it 

 is strictly confined to incense cedar and constitutes no danger whatever 

 to the other associated timber species. 



Certain agencies causing loss would in themselves be given a 

 lower rating if they were not linked with others of greater importance. 

 Leaving actual killing out of discussion, the damage done to older 

 trees by fire consists generally in more or less deep burns through 

 which an easily determined amount of timber is mechanically destroyed. 

 Once the fire has stopped, the injury has reached its maximum, it is 

 not progressive in itself. But the protecting bark is gone and the next 

 fire finds ideal conditions for further and greater damage. What is 

 worse, the wounds caused by fire very frequently reach the heartwood 

 and expose it to the attacks of heartwood-destroying fungi. Thus the 

 relative importance of fires not fierce enough to kill is to be gauged 

 not only by the actual amount of timber values destroyed, but also by 

 their relation to subsequent injuries. 



In the same way lightning cannot be rated merely on the basis 

 of mechanical destruction of timber values. Lightning is one of the 

 chief causes for forest fires and must, therefore, also be made, at least 

 indirectly, responsible for the secondary damage. 



It is clear that the basis for standardized rating must be the timber 

 species and that every factor liable to produce cull must be treated 

 separately. In pure stands this will be simple enough, but the fact 

 must not be lost sight of that here aggressive cull factors such as 

 certain insects or fungi are far more liable to do extensive damage 

 than in mixed stands where the chances of infestation or infection 

 are reduced in the ratio of representation of species. 



The relation of the loss factor to representation of age classes and 

 to cumulative risk must find its place in the rating. It is true that 

 young trees before the formation of heartwood are immune against 

 the ravages of heartwood-destroying fungi. Nevertheless, these young 

 trees, which are subject to a number of diseases impairing their incre- 

 ment — in other words, suppressing their growth — gradually move up 

 into the higher age classes when the very values resulting from this 

 increment and stored in the shape of heartwood are subject to destruc- 

 tion through infection coming from leftover diseased trees, through 



