48 BULLETIN 275, U. S. DEPARTMENT OF AGRICULTURE. 



must remember that by far the greater number of our trees, especially 

 after they have reached the age of about 80 to 90 years, are wounded. 



The first is the "age of infection/' which may be at 60 years or 

 below. Here the infection rarely leads to more than negligible decay 

 unless the tree is handicapped by quite unusually severe conditions, 

 such as very large old wounds. 



The second, at about 130 years, which we may term the " critical 

 age," marks the point after which a combination of pronounced sup- 

 pression and heavy wounding generally results in distinct decay. 

 This combination of deleterious factors is one commonly found in 

 virgin forests. Wounding alone is not sufficient to unfavorably 

 counteract thriftiness of growth. 



Another change comes about at 150 years, when even dominant 

 (that is, thrifty) trees become subject to extensive and intensive 

 decay. We might term this the u age of decline," because the inability 

 of the individual to throw off or keep in check the growth of the wound 

 parasite in its heartwood indicates a distinct decrease in resistive 

 powers, whatever their specific action may be, induced by age. For 

 thrifty but wounded white fir, such as we may expect to raise under 

 management, the age of decline is, therefore, the factor which will 

 influence the rotation and cutting cycles of the species, since for many 

 years to come the risk of inoculation will be more or less the same. 

 There will be fires as long as there are lightning storms. Wounding 

 through lightning and frost are inevitable. Besides, many trees are 

 already wounded. 



It is of interest to note that of a total of 160 trees only about 25 per 

 cent did not show any wounds except very slight lightning scars. All 

 the rest were wounded from some cause or other. Often a combina- 

 tion of fire, lightning, and frost cracks results in scarring a tree to such 

 an extent that almost in every foot some blemish will be found. This 

 is particularly true of the older trees which have been exposed to the 

 cumulative risk of many years. Any of these wounds, if large enough, 

 may offer an entrance to fungi. After the trees have reached the age 

 of about 80 to 90 years, more than 70 per cent of them are already 

 more or less badly wounded and therefore exposed to inoculation. 

 After they are about 106 years of age more than 80 per cent are 

 wounded. 



In the remote future, when all these wounded trees are removed 

 and when the risk of wounding for the trees growing up meantime is 

 minimized, a new age of decline may be established. What this age 

 of decline might be for unwounded white fir in managed forests we 

 can not tell from our material, because of 97 cases of decay only 6 

 were not to be traced to some wounding. It is obviously out of the 

 question to take even a clue from data of so scanty a nature. It 

 may simply be mentioned that the first case of this kind appears at 



