PROTECTION OF FORESTS FROM FIRE 229 



the character of the leaves. A layer of resinous softwood 

 needles burns more rapidly and with a hotter fire than 

 does a layer of hardwood leaves. 



The amount of dry wood on the ground influences 

 largely the severity of a fire. In some types of forest 

 there are a great number of fallen dead trees, which litter 

 the ground, and thus increase the fire danger. This is 

 well illustrated in the lodgepole pine forests of the Rocky 

 Mountains. In localities subject to windfall there is 

 likely to be a large amount of fallen timber, while fires, 

 disease, and insects leave standing dead trees and snags, 

 which are easily ignited. After lumbering in the old- 

 fashioned way, the ground is covered with a mass of tops 

 and rejected logs, which soon become dry and highly 

 inflammable. 



Again, the condition of the litter and debris governs 

 largely the character and severitv of the fire. The most 

 severe fires occur where the material is thoroughly dried 

 to the mineral soil. When the material is only partially 

 dry the fire is slow, and the litter is not completely 

 burned. 



Since the ground litter is, as a rule, unevenly dis- 

 tributed, a surface fire burns very irregularly. Still 

 another cause of the irregularity of surface fires is the 

 varying soil moisture. 



Topography. — A fire runs uphill with great rapidity, 

 because the heated air-currents draw the flames upward. 

 If the litter is evenly distributed, the velocity with which 

 a fire will run up a slope is in direct proportion to the 



