256 FOREST PROTECTION 



tracted dry periods. The litter composed of fallen leaves and 

 twigs dries quickly and is easily kindled by a spark. Conifer- 

 ous needles burn faster and create a hotter fire than the litter 

 from broadleaved trees. Grasses and other herbaceous vege- 

 tation become as dry as tinder at certain times of the year. 

 During extended droughts the humus and even deep peaty 

 deposits in swamps may become thoroughly dry. Large 

 limbs and dead trees do not dry out as quickly as the litter but 

 ultimately may attain an exceedingly dry condition. This is 

 particularly true with standing dead trees from which the 

 bark has disappeared. 



Density. The uniformity and compact arrangement of 

 the inflammable material has great influence on the spread 

 and severity of the fire. Fastest spread occurs when the 

 material is of light inflammable character, and arranged uni- 

 formly so that the fire suffers no check but not so compactly 

 as to prevent the ready access of air. 



Severity of the fire, other conditions being equal, increases 

 with the density of material. Density makes for slow 'ad- 

 vance of the fire but keeps the fire in one place for a relatively 

 long time. This results in more complete consumption of 

 the material and in greater injury. For example, a fire burn- 

 ing through a field of scrub oak brush covered with dry leaves 

 will rush forward rapidly; but much material will be uncon- 

 sumed and injuries to standing trees may be slight because 

 the flames, though hot, do not remain long in one spot. If 

 this same brush is cut and left lying in an evenly distributed 

 compact layer on the ground and then burned at the same 

 degree of dryness as when standing, the fire will advance 

 more slowly, consumption of material will be more complete 

 and injury to the living trees greater. 



A ground fire burning in finely powdered humus with re- 

 stricted oxygen supply necessarily burns slowly but all dry 



