Ame:rican Forest Congress 299 



rate the fly-off is materially less in forested than in 

 unforested tracts. 



In the matter of transpiration, also, it has been shown 

 that the amount transpired from the forest growth is 

 considerably less than it is from cultivated crops. 

 These matters have been carefully gone into in Mr. 

 Rafter's papers, already mentioned, in Dr. Fernow's 

 book, ''The Economics of Forestry," and in Prof. 

 Toumey's discussion of ''The Relation of Forests to 

 Stream Flow," as well as in many other important 

 papers. 



The conclusions reached are, in effect, that as be- 

 tween forested and unforested tracts, the quantity of 

 run-off is materially augmented in the former case in 

 humid regions where rains occur with more or less 

 frequency, but that in arid regions, where precipitation 

 occurs but rarely, that the retention of the moisture 

 by the forests results in some loss in total run-off, 

 which, however, is more than compensated by the 

 greatly increased flow during the periods of minimum 

 discharge. 



Another important result of forestation must also 

 be considered in this connection. It has been stated 

 that the ideal conditions prevail when the total run- 

 off can be controlled at will, the water being stored in 

 great reservoirs. In a great many instances those who 

 are interested in the development of power are endeav- 

 oring to attain these ideal conditions as nearly as possi- 

 ble, through the utilization of natural reservoir sites. 

 Here, too, the forests serve a most useful purpose by 

 preventing erosion. A tract of land that has been 

 denuded of its supply of timber, especially when the 

 denudation is due to fires so fierce as to destroy the 

 humus and leaf mould with the vegetation, imme- 

 diately becomes subject to the action of storms and 

 the torrential run-off resulting in the rapid erosion 



