80 



FOREST INFLUENCES. 



acre would amount to 1,972,000 pounds, while the precipitation during the same 

 period would be 2,700,000 pounds. 



The transpiration of a thirty-hve-year-old beech with thinner leaves, pf which 

 there were 3,000, with a dry weight of 0.79 pounds, would under the same conditions 

 transpire 470 pounds per 1 pound of foliage, or 373 pounds per tree (about 2^ pounds 

 per day from .June to November) ; and since about 1,600 such might be found on an 

 acre, the total transpiration might amount to 593,800 pounds per acre, or consider- 

 ably less than the amount of rain-fall. 



Calculated for summer months during .Tune, July, and August alone, the require- 

 ment of the two beech growths was 20,000 and .5,000 pounds per day an acre respec- 

 tively. Conifers, as was stated, transpire one-sixth to one-tenth of the amount 

 which is needed by deciduous trees. 



The amounts transpired by agricultural crops and other low vegetation, weeds, 

 etc., have been found to be consideraljly larger, as will be seen from the results of 

 the latest investigations by Wollny, which I have calculated per acre to make them 

 comparable with the foregoing results : 



Agricultural crops. 



Winter rye 



Barley 



Peas 



Red clover (first season) ... 



Summer rye 



Oats 



Beaus 



Red clover (second season) 



Time of vegetation. 



Apr. 20- Aug. 

 do 



do ....:. 



Apr. 20-Oct. 

 Apr. 20- Aug. 

 Apr. 20^Sept. 

 Apr. 2U-Sept. 

 Apr. 20-Oct. 



3, 1879 



1,1879 



14,1880 



14, 1880 



10, 188J 



1, 1880 



Water con- 

 sumption 

 per acre. 



Pounds. 

 2, 590, 186 



2. 720, 2.38 

 .3,144,128 



3, 070, 012 

 3, 000, 486 

 3, 422, 584 



3, 1.30, 2.33 



4, 109, 198 



I repeat again that these figures can only be very rough approximations denoting 

 maxima of transpiration, and that the amounts transpired per acre depend largely 

 on the amf)Unts furnished by precipitation. Therefore our forest areas within the 

 arid region of the country- probably transpire a minimum of water, their scattered 

 growth and their coniferous composition, with the scanty rain-fall, reducing the 

 amounts to lowest limit. 



Taking a rain-fall of 20 inches, which represents say 4,-500,000 pounds of water per 

 acre, a coniferous forest, assumed to trausitire one-sixth of the amount found for the 

 older beech-forest under most faA'orable conditions of precipitation, would require 

 hardly more than 330,000 pounds (presuming the same weight of foliage), or not 8 

 per cent of the total precipitation. To be sure, this amount must be available dur- 

 ing the period of vegetation. 



THEORETICAL CALCULATION OF HEAT ABSORPTION.* 



There is another way in which the average amount of transpiration 

 can be ai)proximated, perhaps more closely than by the method of direct 

 measurement. The water transpired comes mostly, if not entirely, from 

 the soil. On evaporating it leaves behind it the matters held in solution, 

 a portion of which is inorganic and is that which constitutes the ash of 

 the plant. Assuming that the absorption by the plant is not sensibly 

 selective in the average from a large number of individuals, then a 

 knowledge of the annual addition of inorganic material, i. e., ash, to the 



* The computations in this section are made on the decimal system because of its 

 greater simiilicity. 



