[Chap. LIU THE VEGETATION OF NORTH AMERICA 747 



occurs throughout the forest. Black spruce and tamarack (a deciduous 

 conifer) also occupy large areas, especially the poorly drained ones, as 

 well as wet bogs, and "muskeg." Both species sometimes occur on the 

 upland, and the black spruce is frequent on rock outcrops, cliffs, and 

 mountain tops. Other trees of importance are balsam fir, which is a 

 common associate of the white spruce east of the Rockies; jack pine, on 



Fig. 376. Black spruce forest on a bog near Big Falls, Minnesota. Photo from U. S. 



Forest Service. 



sand plains and on sand and gravel terraces; balsam poplar, on flood 

 plains; and trembling aspen and canoe birch, now occupying extensive 

 burned-over areas. The most characteristic shrubs are thickets of alder 

 and birch, low junipers, with blueberries, huckleberries, and other 

 heaths, and sweet gale on acid soils. This formation is also characterized 

 by an abundance of mosses, liverworts, and lichens. 



The northern boundary is an intricate pattern of interlacing sites. 

 Those with deeper soils and a longer growing season are occupied by 

 forest peninsulas and isolated groves; the tundra occupies the shallow 

 soils and short-season areas. The southern boundary in the west merges 



