THE FOREST AS A CONDITION. 67 



that a dense old growth, such as one may find 

 here and there on the Pacific coast, with trees 

 towering 250 to 300 feet above ground and so 

 close together that no ray of light reaches the soil, 

 must have a different effect from the low and 

 scanty growth of cedar and pinon which we find 

 on the slopes of the Rocky Mountains and else- 

 where, or the young coppice growth of New Eng- 

 land, interposing but little shade. Whether the 

 forest lies to the leeward or in the direction of the 

 prevailing wind, whether it be coniferous and ever- 

 green through the year, or only summer-green, will 

 also have to be considered in estimating its pro- 

 tective value. 



While the single tree undoubtedly acts in the 

 same manner as a collection of trees, its influence 

 cannot reach very far beyond its surroundings, nor 

 can it be very appreciable. It is also quite evi- 

 dent that neither a few scattered trees and bushes, 

 nor a belt of trees, like a wind-break, nor a small 

 clump of trees in a large open field, nor even an 

 extensive orchard, can act singly as practically 

 appreciable climatic factors, although all these 

 aggregations of trees must have their influence 

 upon their surroundings. 



It is the effectiveness with which sun and wind 

 are excluded from the soil, and thereby air tem- 

 peratures and air humidity are modified, that de- 

 termines also the degree and distance beyond the 

 limits of the cause to which the modification is felt. 



