530 RELATION TO ENVIRONMENT. 



nothing of the great variety of animal life. Some forests are more 

 complex than others. Because of the great divergence of form, 

 character, and work among the members of a forest society, it is 

 one of the most interesting ones for study. 



1008. Different kinds of forests. We know that the con- 

 stituents of a plant community vary. Not only is there variation 

 in different years or periods, there is variation in different regions. 

 Regions which are so widely separated as to show great climatic 

 differences show great differences in the character of plant com- 

 munities. The same is true of the forest. Each different climatic 

 belt or region has its characteristic forest. For example, the forests 

 of the Hudsonian zone in North America are different from those 

 of the Canadian zone, and these in turn different from those in 

 the Transition zone. The forests of the Rocky Mountains and 

 of the Pacific coast differ from those of the Alleghanian, Carolinian, 

 or Austroriparian areas, because there are natural barriers 

 extending north and south in addition to the transcontinental bio- 

 thermal barriers. Finally, tropical forests are strikingly differ- 

 ent from those of other regions. Similar variations occur in the 

 forests of other regions of the globe. The character of these 

 forests depends largely on climatic factors. The character of the 

 forest varies, however, even in the same climatic area, dependent 

 on soil conditions, or success in seeding and ground-gaining of the 

 different species in competition, etc. 



1009, Thickets. According to Schimper thickets belong to 

 the woodland climatic type of vegetation (see Chapter XLIX). 

 Thickets are dense formations of shrubs. Warming says they 

 are the " unsuccessful attempt of nature to form a forest." 

 Examples in our country are the thickets of willows, alders, 

 hazels, etc., found in the eastern and northern parts of North 

 America, and the " chaparral " in the southwestern United 

 States in the arid regions. Here these " chaparral" thickets 

 are often composed of the mesquite, mimosas, acacias, etc. 

 The highest type of thicket, however, is developed in some of 

 the tropical or subtropical regions, especially of Africa, where 

 the climate is rather severe, there being a long dry season alternat- 



