PRESENT FORESTS OF NORTH AMERICA 29 



growth of willows and tamarack. The topography and low mean 

 temperatures serve to limit the number of arborescent species, 

 which is also influenced by the character of the soil, and as the 

 more northerly parts of the belt are reached the forests become more 

 and more open and stunted until the black spruce becomes a shrub 

 associated with dwarf willows and birches. 



Floristically this northern region has been divided into the fol- 

 lowing four illy defined and not clearly marked districts: (1) The 

 Labrador District; (2) the Hudson Bay District; (3) the Mackenzie 

 District; and (4) the Alaska District. As regards the tree species 

 these districts are not distinct for of the eight common types found 

 in Labrador the following five range from the Atlantic to the Pacific: 

 the white spruce {Picea canadensis) , the black spruce (Piceamariana) , 

 the canoe birch {Betula papyrifera) , the aspen (Populus tremnloides) , 

 the balsam poplar {Populus balsamifera). The tamarack {Larix 

 laricina) ranges west as far as the Mackenzie River: the jack 

 pine {Piniis divaricata) and the balsam fir {Abies halsamea) range 

 west to the Mackenzie River and are replaced in Alaska by the 

 lodgepole pine {Pimis mnrrayana) and the alpine fir {Abies lasio- 

 carpa) respectively. 



The forests show local variations from place to place dependent 

 upon the temperature, altitude and soil. In general the stand 

 becomes larger and closer southward and more open and stunted 

 northward. For example in Labrador the forests are continuous 

 in the southern part decreasing northward until at latitude 55° 

 the trees are confined to lake margins and river valleys. Here black 

 spruce is the most abundant tree and constitutes fully 90 per cent 

 of the forest on sandy soils, both wet and dry, of the pre-Paleozoic 

 area. The tamarack is always an inhabitant of cold swamps and 

 in the region southwest of Hudson Bay tamarack swamps and wil- 

 low thickets cover most of the country. The aspen is found on 

 ridges of till in the east, it borders the northern extension of the 

 prairie in Athabasca and on the plains along the Mackenzie River. 

 The balsam poplar frequents the heavy clay soils in the wide 

 valleys and river bottoms and tends to be run out in places by the 

 white spruce which also clothes the low divides. The canoe birch 



