WATER UTILIZATION BY TREES 35 



maximum evaporation rate before noon, 89 percent before 11 a. m., 

 and 52 percent before 10 a. m., the greatest number of maxima 

 occurring about 9 a. m., whether the habitats were wet or dry. 



The seasonal temperature cycle, while also affected somewhat by 

 light, is easier to detect than the daily cycle, because the seasonal 

 light variations are not so great as the daily ones, while the seasonal 

 temperature variations are much greater. Clark (36) measured the 

 transpiration rate from May 2 to October 17 of cut twigs of Fagus 

 grandtfolia Ehrh. and Quercus alba L. At various intervals twigs were 

 removed from the trees to the laboratory and their daily losses in 

 percentage of dry weight were recorded. Between 8 a. m. and 5 p. m. 

 Fagus lost about 1.21 percent of the dry weight per hour, while 

 Quercus transpired only 0.43 percent in the same period. Transpira- 

 tion was highest in the spring and decreased fairly regularly until the 

 fall, reaching a minimum at the time of leaf fall. The water content 

 of the leaves, however, reached a minimum 1 to 2 months before leaf 

 fall, leading Clark to conclude that there was no connection between 

 the leaf water content and the transpiration, temperature, or relative 

 humidity. 



Weaver and Mogensen (229) reported similar decreases in tran- 

 spiration in the late summer with Ulmus, Acer, and Quercus, and found 

 that at this time of year in Nebraska the transpiration rate per unit 

 area was no greater from broad-leaved trees (2 to 4 years old in pots) 

 than from conifers. Their figures are as follows: 



Loss per square decimeter, July 20 to Aug. 15: Grams 



Pinus ponderosa 2. 24 



Pseudotsuga taxifolia 1. 95 



Picea engelmanni 2. 24 



Acer glabrum 4. 33 



Acer saccharinum 5. 36 



For the last week of September and the first week of October: 



Acer saccharinum, 9 trees 2. 66 



Ulmus americana, 9 trees i 3. 56 



Quercus macrocarpa, 7 trees 5. 18 



Picea engelmanni, 3 trees 4. 18 



Abies grandis, 5 trees 5. 17 



Pinus ponderosa, 3 trees 4. 20 



Pinus banksiana, 3 trees 4. 80 



This decrease in transpiration at the end of the summer in broad- 

 leaved trees is undoubtedly associated with the fall of the leaves. 

 According to most observers, the leaves at this time contain a smaller 

 percentage of water. Furthermore, the conducting tissues between 

 the leaf and the stem are being disconnected preparatory to abscis- 

 sion, and also the stomata are tending to close. All three of these 

 factors doubtless play a role in the decreased transpiration at this 

 period. 



Molisch (154), who considered this problem from a general point 

 of view, determined that when leaves of woody plants are placed in a 

 saturated atmosphere so that the transpiration is decidedly checked, 

 most of the leaves fall off iu a few days; likewise, leaves accustomed to 

 a moderate degree of humidity, when placed in a dry atmosphere, fall 

 as soon as the abscission layer is formed. Thus, the fall of leaves 

 both in the autumn and during drought periods in the middle of the 

 summer, common in the case of pines, is probably associated more 

 with the humidity and soil moisture than with temperature; but this 

 question will be discussed further in the section devoted to humidity. 



