22 BULLETIN 580, U. S. DEPARTMENT OF AGRICULIURE. 



seriously damaged each year in the north half of the yellow-pine 

 type. 



This poor showing of the injured trees would not be such a serious 

 matter were it not for the fact that even though an injured tree does 

 not die immediately, the needles, after repeated grazing, become 

 dwarfed and turn yellowish, and the whole plant becomes stunted. 

 If injury is continued the vitality is so weakened that a severe season 

 or an attack by natural enemies is very likely to kill the tree. It is 

 not uncommon to find saplings so impoverished by continued defolia- 

 tion that they can readily be pulled out of the ground by hand. 



The destruction of young trees is not such a serious matter where 

 more reproduction has become established than is necessary to insure 

 a complete regeneration of the stand. But where the reproduction 

 is more or less scattered, the loss of a considerable percentage of young 

 trees becomes decidedly important. 



SUSCEPTIBILITY TO FUNGOUS AXD INSECT ATTACKS. 



It is commonly supposed that young trees injured by grazing are 

 more susceptible to attack by fungi and by insects than are unin- 

 jured trees. The extent to which this danger actually exists, and 

 under what circumstances, is a subject which deserves careful con- 

 sideration. 



Dr. W. H. Long, forest pathologist for Arizona and Xew Mexico, 

 who has made a special study of fungi attacking western yellow pine. 

 has expressed the opinion that — 



The fungi and diseases known at the present time to cause serious damage 

 to yellow pine in the Southwest normally do not enter trees through ordinary 

 injuries caused by grazing. The spores of certain species of rust {Perider- 

 miutn spp.) may enter through wounds in the liviug bark of pines, and it would 

 be entirely possible for such spores to enter through wounds caused by grazing. 

 However, the total damage due to such a rust entering through grazing wounds 

 and finally girdling the trunks of yellow pine is probably very small. 



No heart-rotting fungus is known which is lapt to start growth through grazing 

 wounds other than in cases where the bark is removed or where the heartwood 

 and sapwood are exposed by breaking and twisting. Since injuries of this 

 character form a very small part of the total amount of damage, the danger of 

 fungus attack due to them does not require serious consideration. Injuries that 

 favor this kind of rot are caused by lightning, fire, deep blazes, etc. 



It might be presumed that mistletoe {Razoumofskya spp.) would be liable 

 to attack grazing wounds, but a knowledge of the circumstances under which 

 this parasite attacks tissue leads to the conclusion that this danger is relatively 

 unimportant, for while it is true that mistletoe berries start growth where 

 new tissue is exposed, such as on the calloused edges of blazes, and while the 

 wounds to tender shoots such as are caused by grazing might afford favorable 

 conditions for germination of these berries, the conditions are probably no 

 more favorable than they are on the tips of uninjured stems. 



