PROTECTION OF FORESTS FROM FIRE 237 



amount with age. Accordingly, some trees which are 

 very resistant to fire when mature are exceedingly sensi- 

 tive when young. Good examples are the Eastern and 

 Western white pines, the red pine, the Western larch, 

 and Douglas fir. The cork in the bark acts as a non- 

 conductor, and protects the living tissues from overheat- 

 ing. 



Some species exude from the bark a great deal of 

 resin, which catches fire and increases the intensity of the 

 heat. A good example is lodgepole pine, which often 

 exudes resin over a considerable portion of the trunk, 

 and increases the damage by fire. Other trees have soft, 

 flaky bark, which catches fire readily. Like the resinous 

 trees, these are killed at the point burned by the heat 

 generated in this way. Shallow-rooted trees may be 

 killed by surface fires when the heat of the burning 

 humus is great enough to injure the insufficiently cov- 

 ered roots. 



The living parts of a tree are more sensitive to intense 

 heat at some periods of the year than at others. The 

 most sensitive period is during the early part of the 

 growing season, when active cell division is taking place 

 and new cells are being formed, which are tender and 

 naturally sensitive to abnormal conditions. This is very 

 well shown by the damage of late spring fires. Thus, a 

 surface fire in May or June may entirely kill hardwood 

 trees which in the early fall would successfully resist a fire 

 of equal severity. 



Living tissue is killed when it is heated to 54 C. 



