CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 



243 



with water again, and succession is reinstated at the aquatic stage. 

 More often a burn results in pure stands of paper birch, which 

 eventually give way to climax. Wind throw and lumbering of 

 climax stands may also result in birch or aspen dominance but 

 sometimes are followed directlv by climax species. 



FlG. 115. Aspen stand (Populus tremuloides) at forty-five years of age in 

 northern Alinnesota. Its successional nature is clearly shown by the well- 

 developed understory of spruce and fir.— U. S. Forest Service. 



Transitions— The, northern border is abrupt, but the line is ir- 

 regular depending upon topography. Forest extends far into the 

 tundra in sheltered valleys, and tundra appears on the high ridges 

 well within the forest area. Timber line seems to be advancing in 

 Alaska, retreating in eastern Canada, and remaining more or less 

 stable in the interior. The southern transition is to deciduous for- 

 est in the east and to grassland in the west. From New England to 

 Minnesota, the transition is marked by pure stands of white pine 

 (Finns strobns), a subclimax of long duration. In the lake states 

 red pine (P. resinosa) and jack pine may also occupy similar po- 

 sitions on less favorable sites. Scattered individuals of white pine 

 especially tend to persist well into the climax. Through much of 

 the eastern transition, spruce, fir, and hardwoods may grow in 



