240 THE STUDY OF PLANT COMMUNITIES • Chapter X 



where tundra could survive as relicts. The relict vegetation obvi- 

 ously belongs to the Tundra Formation because of the growth 

 form and the duplication of characteristic genera as well as many 

 species. The greater importance of grasses and the presence of 

 numerous endemics in the western mountains suggest that both 

 the Sierran and Petran tundras might be classed as associations of 

 the Tundra Formation. 



Boreal Forest Formation.— This great forest, often called "taiga" 

 in its northern extent, spans the continent in a broad band to the 

 south of the tundra. Along the Atlantic coast it extends from 

 Newfoundland on the north to the New England states on the 

 south. Westward, the southern boundary touches the Great Lakes 

 region, trends northwestward across Saskatchewan and along the 

 Rocky Mountains, and then to the Pacific coast in Alaska. The 

 band is, therefore, narrowed abruptly in the far west although it 

 extends much farther to the north there than it does over much of 

 the continent. 



Climate is scarcely less severe than that of the tundra. The short 

 growing season from June through August is cool, and winters 

 are very cold. Precipitation is moderate, averaging perhaps twenty 

 inches, except on the east coast where it may be forty inches. The 

 precipitation : evaporation ratio is, however, favorable because of 

 the low temperatures. The topography is almost entirely that pro- 

 duced by glaciation. Lakes are scattered everywhere, and many of 

 them have filled to form extensive bogs or muskegs. The mineral 

 soils are either thin and residual, overlying the rock masses ex- 

 posed by glaciation or, along the southern boundary, deep moraine 

 and outwash. All are immature and often poorly drained. Sub- 

 soils, in the bogs especially, may not be frost-free even in mid- 

 summer. 



Climax— The climax forest of white spruce and balsam fir is 

 best developed in and about the St. Lawrence river valley where 

 the trees reach maximum size and grow in close stands under a 

 variety of conditions. Here, and over much of the range, Picea 

 glanca and Abies balsam e a form dense stands under whose canopy 

 there are relatively few dependent or secondary species. Paper 

 birch (Be tula papyrifera) is a constant associate although it is 

 successional after fire or disturbance and often occurs as subclimax 



