ANGIOSPERM TREES 283 



leaves set with coarse teeth, growing to a slightly larger size, and 

 having some minor differences in bark color. Both of these trees 

 are of more silvicultural than commercial valud, since they 

 rehabilitate burned over areas in the north, forming a temporary 

 forest cover and allowing the more valuable forest trees — spruces, 

 pines, and yellow birch — to become established under their 

 protective influence. 



The BALSAM POPLAR grows to be a larger tree than the pre- 

 ceding two species, commonly reaching a size of eighty by two 

 feet. Its northern range is much like that of trembling aspen, but 

 its southern limits are reached in northeastern United States, the 

 Lake States, and the northern Rocky Mountains. The leaves are 

 longer and more taper-pointed than the preceding, and the leaf 

 stalks are round in cross section. Balsam poplar wood is similar to 

 that of the aspens. 



The Eastern Cottonwood has broadly triangular and 

 coarsely toothed leaves. It becomes a large tree, often well over 

 one hundred feet in height, and is the most important of the 

 eastern poplars. It ranges from the south Atlantic coast north- 

 westward across southern Canada to the Rocky Mountains, 

 southward to the Rio Grande and Gulf of Mexico. A fast growing, 

 short lived tree, it matures in less than seventy five years and 

 thereafter deteriorates rapidly. The wood is light, soft, and weak, 

 and warps badly in drying. The drought resistance of cotton- 

 wood when once established led to its being widely used as a 

 tree for shade and ornament in the arid regions of the west. 



The Northern black cottonwood is the largest of Ameri- 

 can poplars and is the largest broad-leaved tree in the Pacific 

 Northwest, where it sometimes reaches heights of over two hun- 

 dred feet. Its range is limited to the Pacific Coast area from Alaska 

 to California, extending eastward into Montana and Idaho. The 

 leaves are long-triangular in shape, with fine, rounded teeth. The 

 wood is soft and fine grained. 



The Birches 



The BIRCHES (Betula), numbering about ten tree species, have 

 iternate leaves that are egg-shaped to triangular in outline, and 

 toothed (fig. 201). There are no terminal buds, and lateral 



