DRY-ROT OF INCENSE CEDAR. 37 



without adding much to the investigation, the chief aim of which 

 was to determine the age at which dry-rot became extensive and 

 far-reaching. However, the relative representation of trees in the 

 older age classes was maintained as far as possible, neglecting 

 entirely, of course, the few very old veterans always found scat- 

 tered through a virgin stand. 



The cull percentages given are indicative of the relative condi- 

 tions that will exist in the intermediate and optimum ranges, 

 although stands in the latter will on the whole be relatively more 

 free from dry-rot than our figures indicate. However, the Office 

 of Forest Pathology is now collecting figures for cull percentage 

 by a less intensive method over various localities, and these will 

 include not only cull due to dry-rot but to all other causes as well. 



Before drawing final conclusions there is one other factor which 

 must be considered in relation to dry-rot, namely, wounds or 

 mechanical injuries. 



MECHANICAL INJURIES. 



The mechanical injuries which must be reckoned with are those 

 caused by fire, frost, lightning, and the -breaking off of branches 

 as well as such miscellaneous factors as snow, falling trees, mammals, 

 and wind. These injuries are important from three standpoints. 

 First, they often afford an entrance to the heartwood of the tree for 

 spores of wood-destroying fungi; next, the growth processes of a 

 tree may be somewhat interfered with, resulting in a lessening of 

 increment and a consequent tendency to suppression; and, last 

 and least important, an actual loss in merchantable volume may 

 result from the mere presence of the injury. 



Spores of the dry-rot fungus (Polyporus amarus) must have an 

 entrance to the heartwood before they can germinate and develop. 

 As long as the tree is protected by a layer of bark and sapwood it 

 is immune from the ravages of dry-rot or any other heartwood 

 destroyer. Small superficial wounds are quickly protected by 

 resin exudation from the bark, which forms an antiseptic dressing 

 on the wound, safeguarding it from fungous spores until the wound 

 is finally healed or callused over. But incense cedar is poorly 

 supplied with resin. Normally it is found in a limited quantity in 

 the bark only, active in the inner bark, dry and hard in the outer 

 bark. Large superficial wounds may often prove to be serious. 

 If the bark is torn off over a large surface there is not enough resin 

 available to form a dressing, the sapwood dries out and cracks into 

 the heartwood, and these cracks offer an entrance for fungous spores. 

 The most serious type of wounds, of course, are those extending 

 deep into the heartwood, for then the heartwood is directly exposed 

 to infection by wood-destroying fungi for the entire period of time 



