CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 241 



in pure stands. Characteristic tall shrubs are Viburnum alnifolium 

 and V. cassinoides. Typical lesser plants on the shady forest floor 

 are Aster acuminatum, Dryopteris dilatata, Oxalis montana, Clin- 

 tonia borealis, Cornus canadensis, Maianthemum canadense, Aralia 

 nudicaidis, Coptis trifolia, and Chiogenes hispidula. 



With increasing distance from the St. Lawrence center, both 

 westward and northward, the number of species declines. Balsam 



FlG. 113. Interior of boreal white spruce-balsam fir forest as it appears in 

 northern Michigan.— U. S. Forest Service. 



fir is completely absent along the northern boundary and in most 

 of the western range of the type. Beyond the range of fir, the 

 subclimax species, otherwise found in bogs or on burned areas, 

 often appear with white spruce as climax. Along the northern 

 transition tamarack (Larix laricina) may take an essentially climax 

 position as does the black spruce (Pice a mariana), especially on 

 high rocky ground. Both are bog species farther south. To the 

 west, paper birch and jack pine (Finns banksiana) have climax 

 characteristics although both are definitely subclimax nearer the 

 center. 



Successions.— Primary succession occurs mainly on bare rock 

 or ir lakes. 70 The former is initiated by xerophytic mosses and 



