8 MISC. PUBLICATION 162, U. S. DEPT. OF AGRICULTURE 
digest the food necessary for growth. Their crowns may fill the space 
overhead. Their lower branches, thus shut away from the sun- 
light, die and drop off, and in this way is developed the typical forest 
tree with long clean trunk, or great upward-stretching branches, and 
narrow crown high above the ground. Such trees make the best lum- 
ber. Trees grown in the open develop widespreading branches, and 
their lower limbs branch out from the trunk nearer to the ground. 
Like their elders, the younger generations of trees have to fight 
for their existence. Openings in the forest are usually thickly filled 
with young growth shooting up from the ground or sprouting from 
the stumps of old trees which have died or have been cut. Some of 
the seedlings have outstripped their companions in growth and have 
full benefit of the sunlight which filters down to them. These the 
forester calls dominant, while those coming on, still in the thick of 
the fight, he calls intermediate. Other seedlings, not being able to 
keep pace with the vigorous ones, lag behind in the race. These the 
forester calls suppressed trees and, unless a fortunate chance gives 
them light and growing space, they will develop into unhealthy and 
crooked trees, or perhaps die out altogether. Thus from beginning 
to end the life of a tree is a struggle for a place in the sun. 
FoREST SoiL 
Next to sunlight, the forest soil is perhaps the most important influ- 
ence in the life of young trees. If they are to develop into a thrifty 
and upstanding older generation, the soil must furnish them plenty 
of water and the various food elements that tree growth demands 
from it; the soil must be porous, and not hard packed, so that it may 
be easily penetrated by water. 
Although trees make demands upon the soil, they also help.to enrich 
it and increase its power to absorb and store water. The litter on the 
forest floor is made up of fallen leaves and needles, of dead branches, 
down trunks, and other vegetable remains. By the gradual processes 
of decay and chemical change and through the agency of water, micro- 
organisms, and various animals that trample and otherwise disturb 
the surface of the ground, this forest litter disintegrates into humus. 
Largely with the help of percolating water, fine particles of humus 
work down into the mineral soil beneath. Here they are further 
broken down by bacteria and other organisms into various nitrogenous 
products. 
Humus also improves the physical condition of soil. It makes 
a compact soil looser and a sandy or light soil more stable by causing 
the particles to form into crumbs. A crumb structure allows the most 
space for the two elements vital to plant growth—air and water. It 
takes the addition of only a small percent of humus to the soil to 
increase greatly its ability to absorb water. The combination of 
porous mineral soil with the interlacing roots of the trees and other 
forest plants, overlaid by a spongy mass of humus, makes the forest 
a prime factor in the control of stream flow. 
