24 



Geographic- 

 Distribution of 

 Communitips: 



Tundra Biome 



Tuiulra tyi)ically extends from trcc-liiie to the 

 line of perpetual snow and ice, both in the far North 

 and at liigher mountain elevations. It is essentially 

 similar in North America and luirasia (Herg 1950). 

 Very little tundra occurs on .Antarctica. 



CLIMATE, SOIL, AND TOPOGRAPHY 



Arctic regions in North America have an 

 annual preciptation less than 4 cm ( 10 in.), although 

 east of Hudson Bay it occasionally reaches 8 cm 

 (20 in.). Most of it comes as rain during the sum- 

 mer and early autumn ; snowfall is generally light 

 (Koeppe 1931). Humidity is high and evaporation 

 low during the summer. 



Mean monthly temperatures vary between e.x- 

 tremesof — 3.S°C and +13°C (—30° and 35°F). The 

 July isotherm of 5°C (41 °F) is sometimes used to 

 separate the so-called high arctic and low arctic, and 

 the isotherm of 10°C (50°F), which corresponds 

 closely to tree-line, to separate the low arctic from 

 the sub-arctic. Frost may occur at any time in the 

 North, but there is usually a frost-free period of about 

 60 days in the South. 



During the summer, the surface of the ground 

 commonly thaws to a depth of only a few centi- 

 meters : permanently frozen soil, permafrost, under- 

 lies (Ray 1951). The soil becomes wet and soggy, 

 and the accumulation of water in depressions forms 

 numerous shallow ponds. Freezing and thawing are 

 potent forces in arctic regions, since they may occur 

 daily for long periods of time. This action fragments 

 large boulders into small rocks ; forms polygon shapes 

 on level ground surfaces varying in diameter from 

 a few centimeters to several meters ; develops large 

 ground ice or peat mounds or smaller hummocks 

 (frost heaving) ; causes downward slumping of soil 

 on slopes to form terraces, or, a gradual creep of 

 rocks and soil downslope with the consequent round- 

 ing ofif of ridges and other irregularities in the topog- 

 raphy. The general moulding of the landscape by 

 frost action is called cryoplanation and is of eco- 

 logical importance because it makes the soil unstable 

 and limits the kind of vegetation that can develop on 

 it. 



During the winter, the soil freezes down to the 

 permafrost, except under streams, on stream banks 

 and narrow flood-plains, and in sandy areas. Lakes 

 are frozen for nine months of the year with ice a 

 meter or more thick. Under the ice, oxygen almost 

 disappears so that conditions are usually critical for 

 animal survival (Andersen 1946). Ponds less than 

 one or two meters deep freeze to the bottom. 



In the spring, absorption of solar radiation causes 

 the mean temperature of the surface of the soil to rise 

 above freezing three or four weeks before the mean 

 temperature of the air, and it is at this time that 



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