PATHOLOGY OF THE JACK PI-STE. 9 



INJURIES DUE TO OTHER CAUSES. 



In the absence of an adequate snow protection on the flat wind- 

 swept pine barrens of the Lake States, winter injury sometimes 

 results to young growth from long exposure to freezing temperatures. 

 Winter-injured seedlings of jack pine, however, recover more rapidly 

 than those of the more sensitive associate species and when in this 

 condition are not so apt to be attacked by secondary deteriorating 

 agents. It has already been stated that the deep root system of the 

 jack pine is unfavorable to some root-destroying fungi. In like 

 measure this is a safeguard against 'injury by wind. It is very 

 unusual to find jack pine blown down by the wind when the trees 

 are in a healthy condition. Very old trees sometimes succumb to 

 strong winds, but it is found that such trees are usually mechanically 

 weakened by wood-destroying fungi. The jack pine may be con- 

 sidered very windfirm. Porcupines and squirrels are known to do 

 considerable injury to jack pine during the whiter months when food 

 is scarce. The latter animal is much addicted to gnawing the galls of 

 Peridermium cerebrum in the spring during the period of the exudation 

 from the diseased bark of a sweet yellow liquid which bears the conidi- 

 ospores of the fungus. Since squirrels also gnaw the galls when the 

 seciospores are mature, they may be considered a factor in the dis- 

 tribution of this fungus. The bark of the galls is frequently com- 

 pletely gnawed away, killing the infection. 



In general, the jack pine is very sensitive to fire, which usually 

 causes the greatest injury in the typical dry sandy jack-pine plains. 

 In many cases fire injury in jack pine results from repeated burn- 

 ings, the tree having successfully withstood the first slight ground 

 fires. Fires in the more typical jack-pine forests pass through very 

 quickly, so that the thickened bark immediately at the base of the 

 tree affords sufficient protection until it is burned off by succeeding 

 fires, which frequently occur notwithstanding the meager ground 

 cover. The fact that the species frequently grows in orchardlike 

 stands or in isolated groups, more or less separated from one another 

 by free areas, greatly lessens the damage of the fire spreading from 

 one group to another. However, the low-spreading branches, which 

 often extend to the ground, increase the danger from crown fires. 



CONCLUSIONS. 



With reference to the prevalence and severity of its fungous ene- 

 mies, two distinct forest types for the jack pine may be recognized: 

 The pure dry sandy-plain type and the mixed type of moist protected 

 soils. 



The most important fungous disease of the jack pine is Peridermium 

 cerebrum Peck, the control of which in many localities is quite a serious 



