76 



MISC. PUBLICATION 257, U. S. DEPT. OF AGRICULTURE 



to 7-year-old plants to lose a given amount of water. In table 27 are 

 shown the average results of seven trials. 



Table 27. — Amount of time required for cut and uncut shoots of young trees to lose 



}i gram of water 



Species 



Pinus sylvestris 



Picea ezcelsa 



Quercus pedunculata 



Before cutting 



Minutes 



Seconds 

 42 



After cutting 



Minutes 

 3 

 1 

 3 



Seconds 



This use of cut twigs to determine the transpiration of whole trees 

 and of stands may even lead at times to ridiculous results. Pfaff 

 (170) set himself the task of determining the transpiration of an oak 

 tree from May 18 to October 24 by determining the water losses for 

 3 minutes from branches removed at regular intervals during this 

 period. The branches were removed from the north side of the tree 

 four different times a day (6 a. m., 11 a. m., 4 p. m., and 9 p. m.) for 

 each of the 160 days. He found that the night transpiration was 

 one-half to one-third that of the daytime and transpiration in the 

 sun three and one-half times that in the shade. Calculating the total 

 number of leaves as about 700,000, he determined that from the 

 unfolding of the leaves until leaf fall the tree transpired 112,100 

 kg of water. Using the area covered by the crown and that pene- 

 trated by the roots as a basis of calculation, in order to give this 

 amount of water 539 cm of precipitation on the area would be neces- 

 sary. As a matter of fact, the annual precipitation in this locality 

 (Wiirzburg) is only 65 cm. The oak, therefore, used up in 6 months, 

 according to these theoretical figures, eight and one-half times as 

 much water as fell on the area during the entire year. 



To make the matter still more ridiculous, as pointed out by Burger- 

 stein, this figure of 112,100 kg is to be considered only a minimum 

 value because it is calculated entirely on the basis of shade transpira- 

 tion. If one calculated on a sun basis, the water needs of a tree would 

 be so great that the precipitation during a decade would not suffice to 

 provide the water losses for one growing period. 



In the light of these disadvantages and the various problems 

 involved, some of the results obtained by various workers on the 

 actual water needs of economic forest species may be examined. In 

 general all trees seem to thrive better on moist soils; but some are 

 able to tolerate wet or dry soils better than others. Among those 

 that can stand drier soils are certain species of Betula, Populm, and 

 Pinus. Rohinia can stand much more drought than the nature of 

 its leaves would lead one to expect, and Fraxinus also will tolerate 

 dry soils, provided a moderate amount of lime is present. Some 

 species of Betula and Pinus will also tolerate considerable excess 

 moisture; while most species of Populus, Alnus, and Salix have more 

 than average water requirements. 



Turning to specific figures obtained by European workers, among 

 the earliest researchers in this field was Klauprecht, as reported by 

 Morosov (159, p. 110), who determined the loss in weight of leaves in 

 the course of a day as measured in percent of their original weight, 



