CHAPTER 4 



FUELS 



In forest fire language, "fuel" refers to any 

 material that may burn if it is ignited — grass, 

 needles, tree trunks, logs, muskeg, peat, or even 

 coal. It may be either dead or living. Fuel is 

 thought of in two ways: (1) as represented by 

 species or species groups (cover or timber type), 

 or (2) as represented by fuel types. Within a 

 cover type, e.g., white spruce-paper birch, fire 

 behavior is estimated according to how fast it 

 travels and how easily it can be controlled in an 

 average stand. A fuel type occupies an area in 

 which the vegetative material is classified ac- 

 cording to how fast fire will spread in it and how 

 easily the fire car be controlled, regardless of 

 the cover type. Fire control men prefer to use 

 the fuel type classification system as it is both 

 more precise and more flexible. 



To date, no fuel type classification system 

 has been established for Alaska. The cover type 

 classifications used and the relation of each type 

 to fire are described in this chapter. 



FUEL DESCRIPTION 



SIZE 



The initial advance of any forest fire is 

 usually through such fine fuels as grass, moss, 

 or dry leaves. Heavy fuels, such as down logs, 

 may slow the advance of a fire by being a bar- 

 rier between the flame and the fine fuels ahead 

 of it. Moisture content of large fuels does not 

 fluctuate with temperature and humidity as fast 

 as that of fine fuels. In general, the ratio of 

 surface area to volume determines how rapidly 

 the moisture content of a fuel fluctuates with 

 the change in such weather factors as tempera- 

 ture and humidity. This ratio is much higher in 

 a blade of grass than in a limb or a log. 



Mosses, lichens, and grass, in combination 

 or separately, are found throughout the vegetal 

 range of Alaska. "Moss" is a loosely used term; 

 as a general term it includes many species of 

 lichens, which are at least as fine as the moss 

 species and probably more flammable. Moss 

 grows nearly everywhere that any vegetation 

 grows, and is an extremely finely divided fuel. 

 A slight rise in temperature, a decrease in rela- 



tive humidity, and a spark are all that are 

 needed to ignite it. The influence of heat radia- 

 tion is similarly more rapid on ignition of moss 

 than on other fuels. 



The prevailing fuel types through which an 

 initial fire front advances in Alaska may be 

 made up of finer fuel particles and these may 

 have a higher rate-of-spread classification than 

 an average fuel type in most other States. 



CONTINUITY 



Within the vegetative zone, Alaska has a 

 nearly continuous expanse of ground fuels. 

 Mosses, grasses, and lichens are found in some 

 combination everywhere except on rivers, lakes, 

 or barren areas. In most other localities in the 

 United States, horizontal continuity is broken by 

 such factors as bare soil under brush stands, 

 roads, or cultivated lands. Needles under a well- 

 pruned timber stand support a much slower rate 

 of spread than does moss. 



Crown fires in Alaska are not unusual; the 

 nature of the timber stands presents an excellent 

 opportunity for crowning to occur. Cover type 

 descriptions point out the fact that the climax 

 spruce stands, both white and black, are typi- 

 cally close-grown with branches drooping to the 

 ground. The branches often support a heavy 

 growth of beard lichens, which adds greatly to 

 the amount of fine fuel that carries fire upward. 

 These conditions complete the pattern of com- 

 plete horizontal and vertical continuity simulta- 

 neously. In other words, if a fire gets under a 

 stand of spruce timber, the chances are excel- 

 lent that it will climb the tree, and, if much wind 

 is present, will spread from tree to tree through 

 the crowns. 



This situation is intensified by the fact that 

 spruce needles easily become detached when 

 heated and ignited, and float ahead to acceler- 

 ate the already rapid spread. In a spruce-birch 

 stand, the highly flammable birch bark further 

 intensifies the tendency for a fire to spread 

 rapidly through the crowns; decadent and over- 

 mature birch trees are particularly dangerous. 



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