34 PRIKCIPLES OF AMERICAN FORESTRY. 



In some European experiments carried on during the 

 period of vegetation, the amount of water transpired by the 

 different species per pound of dry matter in the . leaves 

 was as follows: 



Pounds of Water. 



Birch and Linden 600 to 700 



Ash 500 to 600 



Beech 450 to 500 



Maple 400 to 450 



Oak 200 to 300 



Spruce and Scotch Pine 50 to 70 



Fir 30 to 40 



Black Pine 30 to 40 



Average, deciduous trees 470 



Average, evergreen trees 43 



This shows that there is a great difference in the amounts 

 of water transpired from deciduous trees and evergreen 

 trees. In this case the deciduous trees transpired about 

 eleven times as much as the evergreens. 



The variability of transpiration from day to day is of 

 wide range; a birch standing in the open and found to 

 have 200,000 leaves was calculated to have transpired on 

 hot summer daj'S 700 to 900 pounds, while on other days 

 its exhalations were probably not more than 18 to 20 

 pounds. 



But while trees transpire large amounts of water, otir 

 agricultural crops and other low vegetation transpire 

 much larger amounts to the same areas. A small factor 

 in the dissipation of water supplies is the amount of water 

 that is retained in the plant itself. As before mentioned, 

 this may amount annually to about 5,000 pounds per acre. 

 The water in fresh-cut woods forms a large part of their 

 weight. In hard woods, such as Ash, Oak, Elm, and Birch, 

 it forms 38 to 45 per cent., and in soft woods 45 to 55 per 

 cent, or more. 



