8 MISC. PUBLICATION 162, U. S. DEPT. OF AGRICULTURE 
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 lumber. Trees grown in the open develop wide- 
spreading branches, and their lower hmbs 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 sprounting 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. 
ForEstT 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. 
Althought 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 tr ‘ample and other- 
wise disturb the surface of the ground, this forest litter disintegrates 
into humus. Largely with the help of percolating water, fine par- 
ticles of humus work down into the mineral soil beneath. Here by 
bacteria and other organisms they are further broken down into 
various nitrogenous products. 
Humus also improves the physical condition of a 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 
greatly increase 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. 
