28 SEEDING AND PLANTING 



depth and at intervals of from 15 to 40 feet. They not only 

 serve to check the flow of water but are of great additional use in 

 dry regions because they retain the surface flow and permit it 

 to sink gradually into the soil. 



14. THE MECHANICAL EFFECT OF SNOW IN ITS RELATION TO 

 THE CHOICE OF SPECIES. In most mountain regions the mechan- 

 ical effect of snow upon the tree itself is far greater than that of 

 rain. The effect upon the soil, however, is much less and strik- 

 ingly different in that it serves an important function as a soil 

 cover, thus protecting the young growth from adverse climatic 

 conditions and the soil from excessive freezing and drying by 

 winter winds. Thus, on the Laramie plains in Wyoming and on \ 

 other similar situations in the West, the chief obstacle to the | 

 establishment of forests by seeding or planting is the absence of a 

 protective snow blanket in winter. As a result, the great cold / 

 and high winds dry the surface soil so completely that young/ 

 trees are killed outright or more or less severely injured. In v 

 northern regions, all species come through the winter in much 

 better condition when the ground is covered with snow than when 

 it is unprotected. Both single trees and forest stands exhibit wide 

 variation in the degree of resistance which they offer to the me- 

 chanical effect of snow. The degree of injury depends upon the 

 species, the age of the trees, and the density and character of the 

 canopy. In many regions, particularly if mountainous, this matter 

 must be given careful consideration in the selection of species for 

 seeding and planting. 



Sno^vbreak results when the weight of the snow exceeds the re- 

 sistance offered by the tree stems and branches. Evergreens offer 

 a much larger surface for the accumulation of snow and are much 

 more subject to injury from this cause than deciduous species, 

 although the latter are by no means entirely immune. Pure forests 

 and those of even age are much less resistant to snow injury 

 than mixed and uneven stands are; consequently, most natural 

 woodlands exhibit less injury than stands established by seeding 

 and planting. The more irregular and varying the level of the 

 canopy, the less the amount of snow held back, and consequently 

 the less the damage. Engler l has shown that the damage from 



1 Engler, Arnold: Einfluss der Provenienz des Samens auf die Eigen- 

 schaften der forstlichen Holzgewachse. (Mitteilungen der schweizerischen. 

 Centralanstalt f. d. forstliche Versuchswesen, X. Bd., S. 189-214. 1913.) 



