6 Miscellaneous Circular 15, U. 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 bal- 
ance 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 reservoir and ditches with silt are avoided. Forests 
provide homes for harmless wild life and furnish to humanity 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 New 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 year’s 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 reach the light. The age of a tree that has been 
