CHAPTER XXI 

 FOREST COMMUNITIES OF ANIMALS 



Forests have diverse forms, and living conditions in the different 

 types of forest present many gradations. Favorable climatic conditions 

 must always be present before forests can appear, for they require a 

 certain minimum of temperature and moisture. If we regard a forest 

 as an aggregation of trees at least 8 m. in height, forest growth re- 

 quires an average temperature for four vegetative months of at least 

 +10°. Such a mean temperature corresponds to an annual mean of 

 -f-3° in the north where warm summer alternates with cold winter and 

 to one of +8° in the south where the winters are much less cold. The 

 southern hemisphere has no abrupt change of seasons, and the de- 

 ciduous forest, which is characteristic in the north temperate region, 

 is much reduced. In the northern hemisphere, the northern limit of 

 forest is extended along the west coast of Europe and America by 

 warm ocean currents. The continents of the southern hemisphere, Ant- 

 arctica aside, do not reach far enough toward the poles to have their 

 forests limited by temperature as they are to the north before one 

 reaches the tundra, or as they are by the low temperatures of high 

 altitudes. 



A second condition for forest development lies in the presence of 

 sufficient ground and atmospheric moisture. There must be a minimum 

 rainfall of 50 mm. during each of the four chief vegetative months. 

 On long stretches of the west coast of the Americas from 40° N. lati- 

 tude to central Chile (except locally), and on the west coast of Africa 

 south of the equator, the rainfall is insufficient to provide the neces- 

 sary soil water for forest growth. Forests may be lacking even when 

 there is a rainfall of 50-100 mm. in the vegetative months, when the 

 average humidity is less than 50%, for example, in the North Amer- 

 ican prairie and the steppes of south Russia. Only stunted trees and 

 bushes thrive in regions where these vital factors approach the minimal 

 requirements for forest growth. 



The forest itself has an influence on the temperature and moisture 

 existing within it. Warming as well as cooling take place much more 

 slowly than in open country. In the forests of north Germany, the 

 mean yearly temperature range during the course of the day is 2.1° 



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