298 George E. Nichols, 



from the mantling of the rock surface by a mat of vegetation, 

 they cease to be confined to crevices and invade the areas 

 between. At first few and scattered, they gradually increase in 

 number and size and come to occupy the ground more com- 

 pletely. In the course of time, groups of trees in the more 

 favorable situations form patches of embryonic woodland, and, 

 as these spread and unite with one another, a more or less con- 

 tinuous forest may be evolved. Not infrequently trees come in 

 so rapidly and in such force at the outset that the heath stage 

 in the succession is virtually eliminated. The succession does not 

 proceed with equal rapidity everywhere, even within a given 

 physiographic unit area. For, owing to locally unfavorable 

 edaphic conditions, succession in some situations lags behind that 

 in others, with the result that there commonly arises a complex 

 of associations, in which various stages in the developmental 

 series are represented. This promiscuous intermingling of primi- 

 tive and advanced associations becomes less pronounced as time 

 goes on, but even in the midst of a climax forest there may be 

 situations in which succession has never progressed beyond the 

 rock face-crevice stage. 



During the early phases of forest development, the white 

 spruce commonly stands out as the predominant tree : the balsam 

 fir, as a rule, is second in importance. Common associates in the 

 rising forest are the paper birch, conspicuous by reason of its 

 light color and large size ; the black spruce, red maple, and 

 mountain ash ; and, less commonly, the white pine. As the forest 

 matures, the relative importance of the two dominant trees 

 undergoes certain changes, due very largely to the differing 

 degree to which the two are tolerant of shade. The white spruce 

 is a relatively intolerant species. Its seedlings thrive only in 

 situations where there is abundant light. While it reproduces 

 prolifically in the open, young trees are rarely encountered in the 

 forest. The balsam fir, on the other hand, is relatively tolerant 

 of shade. Like the white spruce, it reproduces best in well 

 lighted situations, but unlike the white spruce its seedlings are 

 also capable of thriving in moderate shade. The result is 

 obvious. With the diminished illumination of the forest floor 

 which accompanies the growth of the forest, there is a marked 

 decrease in the rate of reproduction of the white spruce, while 

 the balsam fir is much less affected. It follows that, as the 



