CLIMATIC FACTORS 167 



velocity of the wind. The zonation of the vegetation according to the 

 degree of wind exposure is outstanding. 



All of these "wind communities" are sharply circumscribed in area 

 and well differentiated from the surrounding turf communities (Fig. 

 91). They offer a fruitful field for the study of comparative ecology. 

 In addition to direct wind effects, exposed ridges often show indirect 

 influences upon the habitat, by changing the composition of the soil, 

 the soil fauna, the circulation of carbon dioxide, etc. These conditions, 

 however, have not been investigated. The soil on exposed ridges is 

 often less acid than adjacent soils protected from wind. Dense crowd- 

 ing in a community increases the wind resistance of the individual 

 members, be they trees, shrubs, or herbs. 



--- Birch 



w 



Fig. 93. — A, Wind timber line upon a bare ridge; and B, a temperature timber line 

 on a steep slope, Lapland. {After Hannerz.) 



Wind and Boundaries of Forest. — The treelessness of exposed 

 islands and sea coasts is well known. The relation of the polar forest 

 boundary to wind effects has been especially clearly shown by Kihlman 

 (1890). According to Skottsberg (1913, 1914) and Hauman (1926) the 

 treelessness of large parts of southern Argentina is due to the continuous 

 high winds. They cause, in western Patagonia, in Tierra del Fuego, and 

 on the Falkland Islands a maritime "tree Hne," beyond which only a 

 bare cushion heath exists. Morisson attributes the poverty of south- 

 ern Patagonia to the destructive force of the west and southwest 

 winds and the consequent impossibility of carrying on agriculture 

 outside the few protected harrancos. 



Szymkiewicz (1923-1927), from his studies of climate, concluded 

 that the cold regions of the earth must remain treeless wherever the 

 wind, 10 m. above the surface, attains a mean velocity of 6 m. per 

 second. 



High winds accompanied by freezing temperatures are especially 

 unfavorable for tree growth. Hence the peculiar course of the forest 



