14 FOREST TREE DISEASES. 



urally gathers here, and spores find the most favorable 

 conditions for germination. 



Resin is not the only means of natural protection. 

 All trees have the faculty, though in varying degrees, 

 of healing over wounds and growing over pin knots and 

 stubs. This is, of course, a very slow process, and the 

 tree is subject to infection until it is completed. 



Purely mechanical destruction is not disease, but it 

 may produce disease. Porcupines, for instance, will 

 live off the tender bark of young pines (Jeffrey pine, 

 lodgepole pine) when the snow is too deep for them to 

 get any other food. As long as they destroy only small 

 patches of living bark no material damage is done. In- 

 significant injury caused by mechanical destruction of 

 small parts of vital organs is soon overcome in a healthy 

 tree; the remaining sound tissues simply take over the 

 surplus work to be done they work " overtime." But 

 when not enough bark remains to perform the functions 

 required of it, nil parts of the tree above the point of 

 attack soon show signs of suffering. The same holds 

 true for insects. A tree scarcely suffers from the attack 

 of isolated insects ; but when they appear in great num- 

 bers and practically girdle the trunk the tree is seri- 

 ously injured. 



Mechanical destruction is brought about by man, by 

 animals, by lightning, fire and storm, cloudbursts, heavy 

 snowfall, etc. Carelessness in felling timber, removal 

 of bark for roofing of cabins, unnecessary and extrava- 

 gant blazing, tapping of Jeffrey pine for re^in and 

 abietene are only a few examples of the destructive ac- 



