CHOICE OF SPECIES IN ARTIFICIAL REGENERATION 27 



tent of the soil and the relative humidity of the air. The fluctua- 

 tions in water content, in particular, are directly traceable to 

 variations in precipitation. However, aside from the beneficial 

 influences of precipitation in adding to the supply of water avail- 

 able for absorption by forest growth and through its effect on 

 humidity checking the loss from transpiration, it directly influences 

 the phenomena of woodland growth through its mechanical action on 

 the trees and the soil. 



Regions of scanty rainfall suffer most from the mechanical 

 action of rain on both the vegetation and the soil, because a 

 large part of the annual precipitation occurs during a few storms 

 of large and rapid downpour. East of the Mississippi River the pre- 

 cipitation is fairly evenly distributed throughout the year, and vio- 

 lent storms are the exception rather than the rule. In most parts 

 of the West, however, the opposite is the rule, and attention should 

 be given to overcoming its harmful effects. So far as the direct 

 effect upon the tree itself is concerned this can be done by select- 

 ing species having small, thick leaves better adapted for resisting 

 mechanical injury. Emphasis should be placed upon the neces- 

 sity of maintaining forest growth upon all unprotected soils liable 

 to excessive erosion. Forest growth checks the velocity with 

 which the rain strikes the soil and extends the period of time over 

 which it falls. The wealth of lesser vegetation on the forest floor 

 and the layer of litter, moss and humus hinder the speedy run -off 

 of the water and consequently lessen erosion. In general, forest 

 growth should be removed gradually from steep hillsides and the 

 young growth started some years before all the old timber is 

 taken away. 



When forests are started by seeding or planting on naked 

 slopes or on sites subject to excessive erosion, special means must 

 be devised to keep the soil in place and prevent damage by the 

 washing out of seeds and young plants. When seeding is done 

 on unprotected slopes, the seed should be sown along contour 

 lines, never in lines running up and down. When necessary 

 to make seedbeds on sloping ground, the beds should be small 

 and made level by terracing. When planting is done in furrows, 

 they should follow contour lines. It is sometimes advisable 

 to check erosion by a series of parallel ditches which follow con- 

 tours and are more or less broken to prevent water from flow- 

 ing in them. These ditches should be from 6 inches to 1 foot in 



