32 ELEMENTARY FORESTRY. 



Pounds of Water. 



Birch and Linden 600 to 700 



Ash 500 to 600 



Beech 450 to 500 



Maple 400 to 450 



Oaks 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 sum- 

 mer days 700 to 900 pounds while on other days its exhal- 

 ations were probably not more than 18 to 20 pounds. 



But while trees transpire large amounts of water our 

 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 5000 pounds per acre. The water 

 in fresh cut woods forms a large part of its 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. 



ELEMENTS OF CONSERVATION OF WATER SUPPLIES. 



In discussing the elements of dissipation as to the degree 

 of their effect under forest cover as compared with the same 

 elements at work in the open field, we have seen that the shade, 

 the low temperature, the relative humidity, the absence of 

 strong air currents and the protective and water-holding 

 capacity of the forest floor are all factors in the conservation 

 of the water supplies. We have also seen that the quantity 

 of water lost by evaporation, the greatest sourse of dissipa- 

 tion, may be more than six times as great in the open as in 



