210 FOREST VALUATION 



fires. This immunity is attained at an early age, but seldom 

 covers the period of reproduction, or the seedling stage of growth. 



On soils and in localities where fires burn only at long intervals, 

 most tree species are very susceptible to injury, and, as the 

 fires which do occur on these sites are unusually severe, practi- 

 cally the entire stand may be killed. The younger the stand, the 

 greater the per cent of mortality and consequent proportion of 

 value destroyed. 



The fire hazard is largely due to the inflammable character 

 of the forest floor. The entire mass of accumulated litter and 

 humus will burn when dry enough. Most of the undergrowth 

 is also extremely inflammable. Shade thrown by a dense cover 

 reduces the risk by preventing extreme desiccation, and keeping 

 out brush and grass. Of the materials forming the forest floor 

 and undergrowth, grass, when dry, is the most inflammable. 

 Pine needles come next, while hardwood leaves do not cause a 

 serious blaze. Humus or duff, when dry enough to burn, causes 

 a slow and very destructive ground fire, difficult to extinguish. 



The hazard is reduced by atmospheric moisture. Except on 

 the sandiest soils, belts of 'heavy rainfall will give rise to rank 

 vegetation. These are] regions of slight hazard. Over most of 

 the forest areas in this country droughts occur either annually 

 or at greater intervals, of sufficient severity to create an exces- 

 sive fire hazard. 



This risk is enormously increased by logging, because of the 

 great increase in litter and slash incidental to the operations. 

 As the liability is reduced in like proportion, the temptation 

 to ignore the hazard and abandon the attempt to protect the 

 residual forest values is very great. 



201. Wind. Wind resistance is greatest with small timber, 

 and becomes less with increasing bulk, weight and age. The 

 form, distribution and penetration of the roots of different 

 species determine their resistance to wind-throw. Shallow, 

 rocky soils, or those underlaid by hardpan or water, induce 

 windfall. Regions subject to frequent tornadoes present a 

 wind hazard which must be reckoned with in planning the 

 sequence of logging operations. This risk is excessive only for 



