TRANSPIRATION. 39 



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 days 

 700 to 900 pounds, while on other days its exhalations were prob- 

 ably not more than 18 to 20 pounds. 



But while trees transpire large amounts of water, our agri- 

 cultural 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. 



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 ele- 

 ments 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 source of dissipation, may be more 



