Vegetation of Northern Cape Breton. 395 



of the balsam. In this connection attention may be called to 

 earlier remarks (p. 285) regarding tlie ecological relations of 

 the balsam. 



The white spruce and paper birch are much more dependent on 

 an adequate lig-ht supply for successful reproduction than is the 

 balsam fir. This is demonstrated by the relative abundance of 

 the young growth of these two species in a windfall area or clear- 

 ing, as contrasted with the great scarcity of any but large trees 

 in the forest. In the case of the paper birch, to quote Cooper 

 ('13, p. 22), "low birth-rate is compensated by a very low 

 mortality and it is thus able to maintain itself in making a good 

 proportion of the mature stand." The same observation may 

 apply equally well to the white spruce, which apparently is 

 relatively more abundant here than on Isle Royale. The black 

 spruce grows best in well lighted situations, but, like the balsam, 

 it is capable of maintaining itself for years in moderate shade. 



With reference to their tolerance of shade in this climate, the 

 writer would arrange the climax trees of the northeastern 

 coniferous forest climatic formation in northern Cape Breton in 

 about the following order: paper birch and aspen (very 

 intolerant), white spruce (intolerant), balsam fir, black spruce, 

 and mountain ash (tolerant), and yellow birch (very tolerant). 



III. THE EDAPHIC FORMATION-COMPLEX OF THE REGION 



A. Preliminary Observations 



One of the most perplexing features of this region, when it 

 comes to the exact analysis and delimitation of the various 

 formation-types, is the manner in which these overlap and inter- 

 grade. This condition is attributable primarily to the abundance 

 of atmospheric moisture. In less humid climates soil moisture 

 plays an all important role in determining the character and dis- 

 tribution of vegetation, and as a result differences in soil and 

 topography are associated with corresponding differences in plant 

 cover. In general it can be stated that the influence of soil and 

 topography on the chai'acter and distribution of plant associations 

 is least pronounced in humid climates ; most pronounced in arid 

 climates : or, in other words, that this influence is inversely pro- 

 portional to the dryness of the climate. This tendency toward 

 uniformity in a humid region is of course due in part to the fact 



