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AMERICAN FORESTRY 



SURFACE FIRES 



Nearly all forest fires start as surface 

 fires. The dry litter on the surface of 

 the ground is ignited by a spark, per- 

 haps from a locomotive or a campfire. 

 At first the fire burns in a small circle, 

 gradually eating out in all directions. 

 If there is a wind, the fire burns with 

 greatest intensity on the leeward side, 

 and quickly assumes an oval form. If 

 the wind is very strong, the fire may die 

 out entirely on the windward side, but 

 it burns intensely on the other side, 

 soon developing a distinct front or head, 

 with side wings running diagonally with 

 the wind. At first the front of the fire 

 is very narrow, but it gradually widens 

 and takes the form of a broad, irregular 

 line. The front may reach indefinite 

 proportions, from a few hundred feet 

 to a number of miles in width. Irregu- 

 larities of topography and differences 

 in the amount of inflammable material 

 cause the fire to burn more rapidly in 

 some spots than in others, so that the 

 entire front becomes scalloped and ir- 

 regular. 



Ordinarily a surface fire simply burns 

 along the ground and does not get into 

 the tops of the trees. Sometimes, how- 

 ever, the flames reach up into the crowns 

 and scorch them, or even ignite them 

 here and there ; but such a fire still has 

 the character of a surface fire, unless 

 it actually burns through the crowns. 



The manner of burning, the form of 

 the fire area, the rapidity of burning, 

 and the intensity of the fire depend upon 

 the following conditions : 



( 1 ) The character and quantity of in- 

 flammable material. 



(2) The topography. 



(3) The character of the soil. 



(4) The condition of the atmosphere. 



Inflammable Material 



The severity of a surface fire de- 

 pends largely on the quantity of dry 

 material in the forest. If there is 

 an accumulation of leaves represent- 

 ing the fall of a number of years 

 the fire is much more severe than if the 

 litter is the result of the fall of only a 



year or two. The quantity of accumu- 

 lated litter is greatest with species hav- 

 ing large leaves and large crowns. 

 Maple and red oak, for example, make 

 a heavier litter than ash or birch ; white 

 ])ine makes a heavier litter than pitch 

 pine. The severity of a fire depends 

 further on 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 in- 

 crease 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 in- 

 flammable. 



Again, the condition of the litter and 

 debris governs largely the character 

 and severity of the fire. The most se- 

 vere 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 distributed, a surface fire 

 burns very irregularly. Still another 

 cause of the irres:ularity of surface fires 

 is the varying soil moisture. 



Topography 



A fire runs u])hill 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 pro- 

 portion to the steepness of the slope. 

 After passing the crest, a fire travels 

 slowly in its descent on the other side. 



Mechanical obstructions, such as 

 abrupt walls, narrow ridges, outcrop- 



