20 FORESTRY 



water which falls upon them. The hardening influences 

 are overcome by the protection of the forest. This capac- 

 ity to absorb water is increased by the fact that dead 

 leaves, fallen branches and even the stumps of standing 

 trees tend to obstruct the flow of water and give it a chance 

 to soak into the soil. This increased absorption and water- 

 holding capacity of the soil improves its condition and re- 

 sults in a better growth of the trees. It is one of the chief 

 ways in which the forest creates favorable conditions for 

 its own existence. As the litter increases it begins to de- 

 cay, and finally disintegrates into a dark material known as 

 humus in which the structure of the original leaves and 

 twigs is no longer noticeable. This humus has a high value 

 both as a sponge to hold water, and for its chemical and 

 physiological effects on the soil. The mineral food con- 

 tained in the vegetation is made available for use again and 

 the presence of humus promotes both the chemical disinte- 

 gration of the soil by which new portions of plant food are 

 released and the bacteriological activity which plays an 

 important part in the activity of roots. 



The stand may be said to reach its maximum vigor 

 about the time that the trees have attained nearly their 

 full height. Height growth in some species continues slow- 

 ly until death, in others it ceases almost altogether long 

 before the tree has passed its prime. But in all species the 

 greatest vigor in height growth comes in the first half or 

 third of its existence. When about completed, the stand 

 is composed of a much reduced number of trees, whose 

 crowns are well developed and throw a dense shade. There 

 is almost no underbrush and the litter on the forest floor 

 has accumulated and decayed until it forms a heavy layer 

 of humus partly blended with the soil below. 



The Differentiation of Crown Classes.— During this 

 early struggle, culminating with the completion of rapid 

 height growth, the trees have not grown at the same rate 

 in either height or diameter. A slight advantage at the 



