VEGETATIONAL TYPES OF POLAR LANDS 381 



than 9 C, with less than fifty days between spring and fall frosts, 

 with the subsoil in most places permanently frozen, with an annual 

 precipitation normally below 50 cm. (commonly and widely below 

 25 cm.) and largely in the form of snow which drifts and is packed 

 tightly bv the wind, with the soil generally moist in the summer, 

 and with sheltered salt as well as fresh water frozen over during 

 much of the winter. Both arctic and high-alpine regions typically 

 exhibit marked microhabitat effects and consequent variability from 

 spot to spot. Their slopes may also undergo ' solifluction ', which 

 is a slow flowing or creeping downwards of the comminuted surface 

 material over a frozen or other hard substrate, while in flatter areas 

 the sorting in relation to frost action of the surface soil into various 

 kinds of ' polygons ', most often with the finer material in their 

 centres, is extremely widespread especially in the Far North. 



In spite of its treelessness and generally dwarfed nature, giving, 

 to the layman, an impression of monotonous sameness, the vegeta- 

 tion of arctic regions varies very markedly from place to place. 

 This variation is often extreme in closely contiguous areas of diflPerent 

 habitats or, it sometimes seems, without involving any marked 

 difference in conditions — even suggesting that repeated readjust- 

 ments to disturbance outweigh any tendency to equilibrium. Indeed 

 one of the most striking features of arctic vegetation is its extreme 

 variabilitv from one small area to the next — in the absence of 

 sufficient growth to control the physical conditions of the environ- 

 ment, which conditions themselves often vary rapidly and even 

 drastically from spot to spot. Thus whereas in a forest, for example, 

 the vegetation largely determines habitat conditions (including the 

 microclimate), in the Arctic the vegetation is relatively impotent. 

 Here the struggle of plants tends to be with the inimical forces of 

 a harsh physical environment rather than with hostile competitors 

 as in more favourable situations, though there is still plentiful 

 competition between plants in the more favourable arctic habitats. 

 Such competition is particularly rife where growth is relatively 

 luxuriant towards the southern limit of the Arctic ; similarly in 

 high-alpine regions it is found mainly towards the lower limits, as 

 in the Antarctic towards the northern boundary. 



In the arctic regions land is ranged practically around the North 

 Pole, though unlike the situation in the southern hemisphere there 

 is none at the very highest latitudes. In spite of considerable 

 differences in flora, especially at the lowest latitudes of what we 

 recognize as the Arctic, the over-all picture of vegetation is closely 



