244 FOREST PROTECTION 



4. Injury to the productive power of the forest. 



5. Injury to forage. 



6. Injury to stream flow and industry. 



7. Injury to other property. 



8. Injury to hiunan life. 



Injury to Trees Containing Merchantable Material. — The 

 damage may range from slight fire scars at the base to com- 

 plete consumption of the tree. The latter occurs rarely even 

 in exceptionally severe fires. Frequently large trees are killed 

 outright. Death is caused by the killing of the cambium or 

 living layer between the bark and the wood. A temperature 

 of 54° C* (129.2° F.), is sufficient to kill the cambium. If 

 the cambium is killed all the way round, the tree is girdled 

 and dies. Dead cambiimi can usually be told by its dark 

 color in contrast to its light shade when in normal condition. 

 It is not necessary that the bark be burned off to cause death 

 of the tree. Heat which only scorches the outer bark is often 

 sufl&cient to kill the cambium. 



During the early part of the growing season when cell 

 division in the cambiima layer is at its height the cambium is 

 more sensitive to heat than later on in the summer or during 

 the dormant period.^ 



Power of resistance to fire differs for each species of tree. 

 These differences between species are due primarily to the 

 character and thickness of bark. Layers of cork, which act 

 as non-conductors of heat, are developed to a greater or less 

 extent in the bark. A tree which develops thick corky bark 

 is better protected than one with thinner and less corky 

 covering. Tree bark may be soft, flaky, easily inflammable 

 and apt to bum through to the wo«d underneath, or it may 

 be hard, ignited with difficulty and rarely burned through. 



Old trees of any given species are better protected against 



