6 Miscellaneous Circular 15, V. S. Dept. of Agriculture. 



essential to our scheme of civilization. Forests, however, yield other 

 benefits as well, intangible values that can not be reckoned on a balance 

 sheet. They form. fix. and improve the soil. They bind the earth 

 on steep slopes and prevent erosion. Old leaves cover the ground, 

 and through decay make a humus that acts like a sponge. Water 

 from rain and melting snow, instead of rushing away in roaring 

 floods, is held back and fed out to springs and streams in a constant 

 supply. By stabilizing the streamflow. the tearing away or bury- 

 ing under worthless mud of fertile valley fields and the filling of 

 irrigation reservoirs and ditches with silt are avoided. Forests 

 provide homes for harmless wild life and furnish to humanit}- de- 

 lightful retreats from the scorching heat of summer. They are the 

 Nation's playgrounds. 



Tree distribution. — Approximately 500 species of trees are native 

 to the United States and a great many others have become estab- 

 lished through introduction from other countries. Climate, mois- 



Harvesting the crop. 



ture. soil, and certain other influences determine the distribution of 

 trees. Hardwood trees demand rich soil, while as a general rule 

 the softwoods — coniferous trees — though they exceed the hardwoods 

 in commercial importance, are found on poorer soils. These coni- 

 fers, the pines, the firs, and the spruces, with the help of junipers and 

 pinions, make up the Rocky Mountain forests. They are the native 

 trees of the Southwest. Practically no saw-timber forests are found 

 in Arizona and Xew Mexico below an altitude of 6.000 feet. First 

 above the treeless plains and valleys come the junipers and pinions, 

 then the pines, and finally the firs and spruces. 



How trees grow. — Trees grow in height by the formation of new 

 wood cells on the top of the previous years growth and in diameter 

 by putting on a new layer of wood each year under the bark. The 

 rate of growth depends upon soil, moisture, light, and heat. Dif- 

 ferent kinds of trees grow at different rates. Trees in a dense forest 

 grow in height more rapidly than trees in the open on account of a 

 constant struggle to rearh the light. The age of a tree that has been 



