Ch. 14] FACTORS AFFECTING PERMAFROST 259 



that standard biological methods describing plant communities do not 

 apply. The normal associations have been greatly disturbed, special 

 communities for different frost forms can be identified, and above all 

 the plant communities must be described on the basis of their physical 

 habitat. 



Permafrost probably controls the distribution of some animal 

 species, such as the frogs or toads, that require thawed ground into 

 which they can burrow for the winter. Foxes can have dens only in 

 dry elevated places where the depth of thaw is 2 feet or more. Simi- 

 larly, permafrost affects worms, burrowing insects, and other animals 

 that live in the ground. 



Indirectly, permafrost, by exercising some control on types of vege- 

 tation, that is, tundra vs. forest, also determines the distribution of 

 grazing animals such as the reindeer and barren-ground caribou. 



FACTORS AFFECTING PERMAFROST 



Most major factors affecting permafrost are recognized quali- 

 tatively, but none is well-known quantitatively. These factors are 

 easily visualized by turning to the original definition of the term 

 permafrost. As permafrost is fundamentally a temperature phenome- 

 non, we may think of it as a negative temperature produced by climate 

 in material generally of heterogeneous composition. Permafrost is 

 produced because, through a combination of many variable factors, 

 more heat is removed from a portion of the earth during a period of 

 2 or more years than is replaced. Hence a cold reserve is established. 



Basically the process can be reduced to one of heat exchange be- 

 tween the sun, the atmosphere, and the earth. The sun, through solar 

 radiation (insolation) , and the interior of the earth, primarily through 

 conduction, supply practically all primary heat to the surface of the 

 earth (biological processes, natural or artificial fires, chemical re- 

 actions, cosmic or other radiations excepted). This primary heat is 

 dissipated to the atmosphere and to outer space by conduction, radia- 

 tion, convection, and evaporation. The atmosphere by warm winds 

 and precipitation also distributes secondary heat to the surface of 

 smaller areas. 



We know that earth temperatures at the depth of seasonal change 

 are in most places within a few degrees of the mean annual air tem- 

 perature, and that a geothermal gradient is established from the sur- 

 face to the interior of the earth. The geothermal gradient at any one 

 place is relatively fixed from year to year, though it varies from place 

 to place and has changed markedly during geologic time. It is gen- 



