236 THE STUDY OF PLANT COMMUNITIES * Chapter X 



tionships and the impracticality of expressing them as a function 

 of a single variable. This becomes even more obvious when influ- 

 ences such as length of day, winter temperatures, and the season 

 of precipitation are considered. 



CLIMAX REGIONS OF NORTH AMERICA 



The vegetation maps available for North America 230 ' 236 ' 268 serve 

 to emphasize by their similarities that the major vegetation types 

 are fairly obvious, but their differences in detail indicate disagree- 



FlG. 112. Alpine tundra in the Colorado Rockies— U. S. Forest Service. 



ment on the interpretations of climax relationships, especially 

 within formations. An understanding of the bases for different 

 interpretations can best be obtained by study of the many papers 

 dealing with local investigations of vegetation. There are, how- 

 ever, several of a more comprehensive nature, 230 ' 231 > 118 which give 

 more detail than can be presented here. 



The concept of climax formations and associations was discussed 

 earlier (Chap. 9). Classification of North American vegetation on 

 this basis is altogether logical, particularly if the point of view is a 

 dynamic one. The system shows modern successional and climatic 

 relationships but is based as well upon past history of the climaxes. 

 Although growth form is the apparent major basis of classification, 

 dynamic factors are given equal consideration. 



Below are listed the major climax formations of North America. 



