292 George E. Nichols, 



the nature of the regional cHmax forests is essentially similar, 

 in so far as their ecological aspect is concerned ; but, just as 

 in Cape Breton, there is considerable local variation in their 

 composition — in the presence or absence of certain species and 

 in their relative abundance when present. Leaving out of con- 

 sideration the species of the fifth group, which occupy a rela- 

 tively insignificant position here, it is possible to distinguish, 

 with reference to the presence or absence in the forest of mem- 

 bers of the first four groups outlined above, eleven different 

 group-combinations of trees which may comprise an equal 

 number of floristically different types of climax forest. Indicat- 

 ing the respective groups by letter, these various group-combina- 

 tions are as follows: (i) A-B ; (2) A-B-C; (3) A-B-C-D; 

 (4) A-B-D; (5) A-C; (6) A-C-D; (7) A-D; (8) B-C; (9) 

 B-C-D; (10) B-D; (11) C-D. 



In sections of the country where all four groups of climax 

 trees (A, B, C, D) are well represented, forests comprising any 

 and each of these group-combinations may be encountered. As 

 might be expected, however, while the trees of groups B and C 

 are about equally well represented in forests throughout the 

 transition region, those of group A are most generally repre- 

 sented southward, those of group D northward. It is along the 

 southern borders of the transition region, in that part of 

 the area where climatic conditions presumably are most favorable 

 to the trees of group A (and least so for those of group D), 

 and which these have occupied for the longest time that the first 

 group-combination {A-B: the "northern hardwood" type of 

 forest) is most extensively developed. Here the trees of group 

 D tend to be localized in situations which are edaphically 

 favorable: they develop best in areas which are somewhat 

 swampy. Conversely, along the northern borders of the transi- 

 tion region, in that part of the area where climatic conditions 

 presumably are less favorable for the deciduous species of 

 group A (but more so for those of group D), or which, it may 

 be, these species in their post-glacial migration have reached only 

 in comparatively recent time, the trees of group D are commonly 

 a conspicuous and even the predominant element in forests of 

 the regional climax type. Here the trees of group A tend to be 

 restricted to the better drained soils. It is important to note 

 in this connection, however, that even along the northern 



