284 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1950 



must be taken to insure its stability. Otherwise it must be destroyed 

 and appropriate steps taken to prevent it from returning. 



BIOLOGIC SIGNIFICANCES 



Permafrost, by means of its low temperature and ability to prevent 

 runoff, is a potent factor that aids in controlling vegetal growth in 

 the Arctic and sub- Arctic (Mosley, 1937). Many places have semi- 

 arid climate yet have luxuriant growths of vegetation because the 

 pennafrost prevents the loss of precipitation through undergroimd 

 drainage (low evaporation is possibly as important). Such condi- 

 tions are natural breeding environments for mosquitoes and other 

 insects. 



Conversely, luxuriant growths of vegetation, by insulating the per- 

 mafrost in summer, prevent deep thawing and augment cold soil 

 temperatures. Hence those species with deep root systems, such as 

 certain trees, are dwarfed or absent, and nourishment available to 

 smaller plants is limited. 



Eaup (1941, 1947) and Griggs (1936) point out that much of Arctic 

 soil is unstable because of frost action (commonly associated with 

 permafrost) and 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 identi- 

 fied, 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 non 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 phe- 

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

 climate in material generally of heterogeneous composition. Perma- 

 frost is produced because, through a combination of many variables 

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



