8 MISC. PUBLICATIOX 162, U. S. DEPT. OF AGRICULTURE 



the food necessary for growth. Their crowns may fill the space 

 overliead. 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 elean trunk, or great upward-stretching 

 branches, and narrow crown high above the ground. Such trees 

 make the best lmnber. Trees grown in the open develop wide- 

 spreading branches, and their lower limbs branch out from the trunk 

 nearer to the ground. 



Like their elders, the younger generations of trees nave 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 oíd trees which ha ve died or ha ve been cut. Some of 

 the seedlings ha ve outstripped their companions in growth and have 

 full benefit of the sunlight which filters down to them. These the 

 forester calis dominant, while those coming on, still in the thick of 

 the fight, he calis intermedíate. Other seedlings, not being able to 

 keep pace with the vigorous ones, lag behind in the race. These the 

 forester calis suppressed trees and, unless a fortúnate 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. 



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 ageney of 

 water, micro-organisms, and various animáis that trample and other- 

 wise disturb the surf ace 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 producís. 



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. 



