56 THE TREE AND ITS CHARACTERISTICS 



5. Dead those which have been so shaded in the struggle for light 

 and competition for soil moisture that they have failed to 

 receive sufficient sustenance to continue living. Overtopped 

 trees may gradually die in the struggle for existence. Sup- 

 pressed and dead trees are sometimes placed in the same 

 class. 



Trees may also be classified as to age, generally in 20-year periods. 

 If all the trees in a forest are within an age class of 20 years, the 

 forest is said to be even-aged. Trees should be cut when financially 

 rather than physiologically mature if forestry is being conducted as a 

 business. Some trees, such as poplars, aspens, and cottonwoods, are 

 relatively short lived and mature, physiologically, at 50 to 80 years. 

 Oaks may grow to be 500 to 1500 years of age; the giant sequoias 

 may be from 2000 to 3000 years old. 



Trees are generally managed on rotations of 20 to 100 years or 

 more, but rotations of 20 to 40 years are applied to pulpwood, fuel- 

 wood, chemical or distillation wood, posts, grape stakes, etc.; from 

 40 to 60 years for cross ties, poles, piling, etc.; and 50 to 80 years 

 or more for saw logs, cooperage bolts, and other special purposes. 

 Willows are grown on one-year rotations for basket-ware. 



4. VARIETIES OF TREES 



According to the Forest Service, there are 862 separate native tree 

 species in the United States. Including all the varieties and hybrids, 

 1177 are recognized by authorities. Many genera are indigenous only 

 to the United States, such as hickory, redwood, persimmon, and some 

 others. Some genera include many species; for example, there are 

 50 different kinds of oaks, 34 pines, 10 firs, and 7 spruces. Probably 

 the greatest variety of useful trees to be found anywhere in the world 

 occurs natively in the American forests. 



Only about 60 species or groups, such as southern pine, reach com- 

 mercial importance, and only 5 comprise about 90% of all the lumber 

 cut annually. 



Trees may be grouped into two broad divisions on the basis of 

 both botanic and use factors. This classification, however, is gener- 

 ally used by botanists as well as foresters. 



1. Broadleaf, deciduous, or hardwood trees, such as the oaks, 



birches, maples, gums, hickories, ashes, poplars, cottonwoods, 

 elms, etc. 



2. Evergreen, needle-leaf, or softwood trees, as the pines, spruces, 



firs, cypress, redwood, cedars, hemlock, etc. 



