The Vegetation of North America 317 



chian Mountains southward through Pennsylvania to northern 

 Alabama, where it is confined to the mountain summits. 



The northern evergreen forest region is characterized by long 

 winters with deep snows and a short, warm growing period of 

 about 3 to 4 months' duration. The total rainfall varies from 

 40 inches in the east to 1 5 inches in the west, and the evaporation 

 from a free water surface is equivalent to about one half to two 

 thirds of the rainfall. The humidity is high, varying from 70 

 to 80 per cent of saturation. The snowfall begins before the 

 ground is frozen, and where the snow is heaviest the ground 

 remains unfrozen throughout the winter and the slow melting 

 of the snow keeps the soil moist far into the summer. This is 

 important, for it insures the trees an adequate water supply at 

 all times. On the tundra to the north the soil is permanently 

 frozen at a comparatively slight depth. South of the evergreen 

 forest the ground freezes from 2 to 4 feet every winter, and this 

 is possibly one of the factors which limits the southern extension 

 of some of the evergreen trees. Another important factor is the 

 competition of the deciduous hardwood trees. On good soil the 

 hardwoods soon shade out the evergreens, with the exception of 

 hemlock. On poor soils, sand plains, sand dunes, and sandstone 

 cliffs the evergreens are more successful in maintaining a foothold. 



In general, the soils of the evergreen forest region are shallow, 

 and the drainage is poor except in the highlands. The soils are 

 shallow because of the glaciers that once covered all of this part 

 of North America to a thickness of i to 2 miles. The ice of these 

 glaciers flowed toward the south, smoothing off the land surface 

 at the north, and carrying away whatever soil there was in pre- 

 glacial times. As the glaciers disappeared only 20 to 30 thousand 

 years ago, there has been comparatively little time for soil to 

 accumulate. From these facts we^may infer that during glacial 

 times the northern evergreen forest occurred farther south, from 

 southern New Jersey to Kentucky and Nebraska, and has moved 

 into its present region in geologically recent times (Fig. 311). 



