300 George E. Nichols, 



it is conceivable that even on a bare rock surface, through the 

 gradual amelioration of the habitat by biotic factors, the succes- 

 sion of plant associations might progress still further, and that 

 the vegetation here might ultimately attain the condition which 

 characterizes the climax association-type of the region. But, as 

 a matter of fact, on bare rock outcrops the succession seldom 

 proceeds further than the coniferous forest stage. In other 

 words, the coniferous forest can be regarded as representing the 

 edaphic climax association-type of the rock outcrop successional 

 series: it is a permanent association-type, though ordinarily less 

 mesophytic than the regional climax association-type (in this 

 connection, see Nichols '17, pp. 310-317). In its optimum devel- 

 opment, the coniferous forest association-type of the rock out- 

 crop series in the lowland may resemble very closely the climatic 

 climax of the mountains, and indeed it may be quite as mesophytic 

 as the regional climax type. Balsam fir is the predominant tree, 

 while white spruce, paper birch, black spruce, white pine, red 

 maple, yellow birch, and mountain ash are more or less abundantly 

 represented. But, as has already been suggested, such may be 

 the effect of the limiting edaphic factors that in many places the 

 succession halts at a much earlier stage than this. 



C. THE ASSOCIATION COMPLEXES OF GLACIAL DRIFT 



Extensive outcrops of bare rock are seldom encountered in 

 the lowland. The most widespread type of substratum here is 

 glacial drift. The drift, to perhaps a greater degree than any 

 other type of substratum, is well adapted to rapid colonization by 

 plants. So favorable, indeed, were the original conditions here, 

 and so rapidly has the succession of plant associations ensued, that 

 the drift everywhere has long since become covered by forests. 

 It is only where the original plant cover has been destroyed, 

 either through the agency of stream or wave erosion, or else as 

 the result of human activity or fire, that the earlier phases of the 

 succession become apparent. The early stages of primary suc- 

 cessional series on drift can be reconstructed by analogy, after 

 a fashion, from the study of primary successions on other sub- 

 strata and of secondary successions on the drift. 



Coniferous forest locally an edaphic climax. — Disregarding for 

 the present the earlier phases of the succession, suffice it to state 

 that eventually there may arise on the drift a type of forest essen- 



