BIOLOGICAL FACTORS 



205 



paralleled by other less obvious losses. Such fires in the temperate 

 zones may destroy practically all the humus accumulated through 

 the years and necessitate the slow rebuilding of the soil before 

 forest can occupy the area again. Leaching and erosion, which 

 follow such fires, may delay revegetation for years. Thus the pro- 

 ductivity of the soil may be indefinitely impaired. 



FlG. 98. A subalpine flat denuded by intense fire that killed all trees and 

 burned off organic material down to mineral soil. The fire occurred many 

 years before picture was taken and it is obvious that it will be many more 

 years before the soil is sufficiently rebuilt to support forest.— U. S. Forest 

 Service. 



It is of interest that light, controlled burning has been found 

 beneficial for certain purposes. On some grazing land, certain 

 undesirable species may be kept down or eliminated to the ad- 

 vantage of more palatable plants. More vigorous growth of certain 

 forage types is sometimes obtained after light burning in the 

 proper season, probably because of the nutrients released and made 

 available. It would appear that under some circumstances fire 

 could be used as a beneficial tool. 117 



Man, like lower animals, transports seeds and fruits, but to far 

 greater distances and with resulting changes in vegetation of a 



