50 GEAVES :, PLACE OF FORESTRY AMONG SCIENCES 



although the age of all these trees may be practically the same. 

 The same struggle for existence, therefore, which produced the 

 dominant and suppressed trees works toward a natural selection, 

 since only those which have conquered in the struggle for exist- 

 ence, and are endowed with the greatest individual energy of 

 growth, reproduce themselves. 



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 evaporated 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 precipitation, 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 Thus 10, 20, and 30 inches will be withdrawn from the total 

 circulation of moisture over the area occupied by the forest. 

 The forest cover, besides preventing all of the precipitation from 

 reaching the ground, similarly 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 subsoil are so fundamental that it is often possible 

 to determine centuries 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 



