Fire as a Biological Factor 43 



proven, almost to the exact date, by counting the annual rings 

 in old, fire-scarred trees. Unquesionably fire has been and is a 

 most potent factor in disturbing the more or less well-established 

 equilibrium among forest societies, and especially among the 

 members of such societies. Invasion, succession, and re-estab- 

 lishment of equilibrium take place each time upon the virgin 

 and sometimes practically sterile soil left by a very destructive 

 fire. The character of these processes is determined by the 

 severity of the fire, the degree of fire-resistance of the species, 

 the.productive, locomotive and propagative qualities of the sur- 

 viving species on the ground, or in the vicinity, efficiency of wind 

 and other carrying agencies, and the fighting qualities of the 

 offspring with, or fitness to, conditions of their immediate en- 

 vironment on the one hand, and the members of their own or 

 associated species and generations on the other. This may be 

 expected to apply not only to forest trees, but to shrubs, herbs, 

 and all the lower orders of the plant kingdom, even down to the 

 bacteria, both parasitic and of the soil. The new societies may 

 thus be very different from their predecessors. In the case of 

 the Douglas fir, a single characteristic, namely, its notably high 

 fire-resistant powers, probably due to its thick, non-inflammable 

 bark, in the course of ages may have had a far reaching influence 

 in the determination of its enormous range and present promi- 

 nent status in a large number of West American forest associat- 

 tions. 



While not excusing carelessness in handling fires it may be 

 said with good reason that many of the forest fires for which 

 responsibility has been attributed to man, have been originated 

 by lightning. Observers have not infrequently noted fires being 

 started in the forest by the discharge of atmospheric electricity 

 during thunder showers, and trees showing the scars of a bolt are 

 often found badly burnt. However, in this as in many other 

 directions. Nature provides her own check to destructive forces. 

 If it were not for the fact that the most violent disruptions are 

 usually followed by falling rain, and resulting fires thus ex- 

 tinguished, forest fires originating from this cause would unques- 

 tionably be much more numerous and destructive. Also, the 

 fire damage in mountainous regions is lessened by the fact that 

 the discharges are largely concentrated upon trees on high ridges 

 and exposed peaks. As fire will not readily spread down hill 



