CHOICE OF SPECIES IN ARTIFICIAL REGENERATION 49 



over the surface as run-off. By shading the soil and providing a 

 loose mulch of leaves and other litter, the forest also checks the 

 loss of soil moisture through evaporation. The obstructions offered 

 by the crown cover extend the time over which the precipitation 

 reaches the soil, and the logs and other obstructions on the forest 

 floor check the flow of water over the surface. The result is a 

 larger proportion of the precipitation enters the soil. 



Although forest vegetation causes the surface layers of the soil 

 to have a higher and more uniform moisture content, this is not 

 necessarily true of the lower layers. Where the total amount of 

 precipitation falls below a certain minimum, the lower layers of 

 the soil are rendered drier by forest growth because the trees draw 

 the large amount of water used for transpiration and growth 

 from several feet below the surface. 



The forest prevents erosion by protecting the soil from the 

 mechanical action of wind and water. This increases the volume 

 of soil at high levels in hilly and mountainous regions and pre- 

 vents the covering up of fertile land below by silt and other debris 

 from above. The selection of species for use on sites subject to 

 erosion should be restricted to those that are most effective in 

 holding the soil, i.e., to species which produce a dense and uniform 

 crown cover and a maximum amount of forest litter. 



12. The Character of the Forest in Reference to its 

 Beneficial Effect on the Site 



The degree of effect which forest vegetation has upon the site 

 depends upon the form and structure of the stand and upon the 

 species composing it. Thus, an even-aged stand in which the 

 trees are approximately of the same size affords the greatest pro- 

 tection from the sun. A stand in which all age classes are repre- 

 sented affords maximum protection from the wind. 



In order that forest vegetation may react upon the soil to its 

 greatest benefit the stand must provide the following: 



a. A dense and uninterrupted leaf canopy. 



6. A large yield of litter for the production of humus. 



Our different species of forest trees provide these two requisites 

 in varying degrees. Most conifers, although sufficiently dense in 

 foliage to provide ample protection, are deficient in yield of 

 leaves. Thus, hemlock sheds but one-twelfth of its foliage yearly 

 and spruce but one-seventh. On the other hand, a large number 



