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



Although young trees are also exposed to the attack of insects, it 

 is clear that a colony of bark beetles will prove far more injurious 

 in killing a merchantable tree than in killing a small pole. Moreover, 

 certain insects seem to have a predilection for larger sizes. The 

 probabilities of attack increase with the length of the cutting cycle. 

 Besides, the older the tree and the more it has been exposed to 

 wounding the more liable does it become to attack by wood-boring 

 insects which materially reduce the value of the timber. 



The timber contained in trees killed by lightning, as long as they 

 are not destroyed, and in those killed by bark beetles may be utilized, 

 and, with increasing timber values, will be utilized before they 

 deteriorate. 



The few veterans which have withstood the many dangers of earlier 

 life do not go on living forever; they finally succumb like the rest. 



It is still an open question whether forest trees are theoretically im- 

 mortal and die only through the devastating influence of severe 

 storms, lightning, insects, certain diseases caused by fungi, such as 

 Armillaria mellea and Fomes annosus, or because the root system of 

 the veteran finally has exhausted all available resources of the soil 

 within its reach. As we are interested here only in the future of cut- 

 over areas in relation to the length of the cutting cycle, it is unneces- 

 sary to enter into a discussion of this question. The cutting cycle for 

 any one species will in all probability never be long enough to raise 

 hidividual decadence from old age to the rank of an influencing factor. 

 We should bear in mind, however, that individual decadence is not 

 in itself deterioration unless decay sets in. 



The importance of the reduction in the timber value of the tree 

 through the agency of fungi, on the other side, can not be overempha- 

 sized. This reduction in timber affects either the prospective timber 

 values — that is, the increment — or the present stock, or both. In the 

 first case, the fungi hi question (mostly Pyrenomycetes and rust fungi) 

 inhabit living tissues of the foliage or of the young bark. The con- 

 tinuous drain on the assimilates of the foliage either hi the leaves 

 proper or on their way down through the bark is evidenced by a 

 decrease in increment of the tree, which hi long cutting cycles will 

 represent a very considerable loss in timber values. In other words, 

 trees affected with foliage or bark diseases will be far from yielding the 

 timber we might expect from sound trees. It must be mentioned 

 that losses in prospective values are not alone due to fungi; mistle- 

 toes and leaf-inhabiting insects are responsible for enormous deficits 

 in yield. The economic role of the fungi, mistletoes, and leaf-inhab- 

 iting insects in our virgin forests is highly important and will remain 

 so for a long time, on account of the difficulties connected with their 

 control and even more on account of our very limited knowledge con- 

 cerning their life histories and specific action. More intensive studies 



