SPREAD AND SEVERITY OF FOREST FIRES 255 



of forest fires cannot be influenced by silvicultural treatment 

 although those relating to the presence and condition of in- 

 flammable material are susceptible to partial control. 



The most important factors governing the rate of spread 

 and the severity of fires are listed below and separately dis- 

 cussed. _ The spread and severity of fires are so interrelated 

 as to warrant consideration together. 



Chief Factors Influencing the Spread and Severity of Forest Fires 



Inflammable material: 



Amount, Dr5Tiess, Density. 

 Topography: 

 Atmospheric conditions: 



Wind, Precipitation, Humidity. 



Inflammable Material. — Fires are dependent both for their 

 severity and rate of spread upon the available fuel^ supply. 

 This consists of the humus, litter, ground cover, underbrush, 

 fallen limbs and trees, tops left after logging and the standing 

 trees large and small. In other words all the vegetable prod- 

 ucts living or dead may furnish fuel for the flames. 



Amount. — The severity of the fire is more directly de- 

 pendent upon the amount of fuel than is the rate of spread. 

 A fire may run rapidly xmder certain conditions even if the 

 fuel supply is scanty, but such a fire caimot be a severe and 

 damaging one. 



Dryness. — While in a forest there is normally a super- 

 abundance of potential fuel, this is not available for con- 

 sumption imless it is dry. Dr)mess then is often, though not 

 invariably, the key to the amount of available fuel. Certain 

 parts of the forest, such as the living trees and shrubs, rarely 

 if ever dry out sufficiently to burn readily. Green foliage 

 unless of a resinous nature burns with difficulty even in pro- 



