264 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1915. 



In a forest there is altogether a different climate, a different soil, 

 and a different ground cover than outside of it. A forest cover does 

 not allow all the precipitation that falls over it to reach the ground. 

 Part of the precipitation remains on the crowns and is later evap- 

 orated back into the air. Another part, through openings in the 

 cover, reaches the ground, while a third part runs down along the 

 trunks to the base of the tree. Many and exact measurements have 

 demonstrated that a forest cover intercepts from 15 to 80 per cent of 

 13recipitation, according to the species of trees, density of the stand, 

 age of the forest, and other factors. Thus pine forests of the north 

 intercept only about 20 per cent, spruce about 40 per cent, and fir 

 nearly 60 per cent of the total precipitation that falls in the open. 

 The amount that runs off along the trunks in some species is very 

 small — less than 1 per cent. In others, for instance beech, it is 5 per 

 cent. Thus if a certain locality receives 50 inches of rain, the ground 

 under the forest will receive only 40, 30, or 20 inches. Thus 10, 20, 

 und 30 inches will be withdrawn from the total circulation of mois- 

 ture over the area occupied by the forest. The forest cover, besides 

 preventing all of the precipitation from reaching the ground, simi- 

 larly keeps out light, heat, and wind. Under a forest cover, therefore, 

 there is altogether a different heat and light climate and a different 

 relative humidity than in the open. 



The foliage that falls year after year upon the ground creates deep 

 modification in the forest soil. The changes which the accumulation 

 of leaf litter and the roots of the trees produce in the soil and sub- 

 soil are so fundamental that it is often possible to determine cen- 

 turies after a forest has been destroyed whether the ground was 

 ever occupied by one. 



The effect which trees in a stand have upon each other is not con- 

 fined merely to changes in their external form and growth ; it extends 

 also to theii* internal structure. The specific gravity of the wood, 

 its composition, and the anatomical structure which determines its 

 specific gravity differ in the same species, and on the same soil, and 

 in the same climate, according to the position which the tree occu- 

 pies in the stand; Thus in a 100-year-old stand of spruce and fir the 

 specific gi'avity of wood is greatest in trees of the third crown class 

 (intermediate trees). The ratio of the thick wall portion of the 

 annual ring to the thin wall of the springwood is also different in 

 trees of different crown classes. The difference in the size of the 

 tracheids in trees of different crown classes may be so great that in 

 one tracheid of a dominant tree there may be placed three tracheid 

 cells of a suppressed tree. The amount of lignin per unit of weight 

 is greater in dominant trees than in suppressed trees. 



Forest trees in a stand are thus influenced not only by the ex- 

 ternal physical geographical environment, but also by the new social 



