WATER UTILIZATION BY TREES 



47 



The ecological relations of trees centering around humidity have 

 been hinted at above in the discussion of the work of Cribbs (40), but 

 experimental studies were carried out by Weaver and Thiel (230), who 

 studied the absolute and relative transpiration of seedlings of Acer 

 saccharinum and Quercus macrocarpa which had been planted in pots 

 and then placed in three different associations: (1) Prairie, (2) scrub, 

 (3) woodland. Every 4 days the plants were changed from one site 

 to another. In another series of researches, branches were cut from 

 various types of trees (Acer, Prunus, Ulmus) and their transpiration 

 studied by means of potometers. Kesults showed that evaporation 

 and transpiration were greater in the prairie than in the scrub, but 

 greater in the scrub than in the high forest. 



Similar results are reported by Weaver. In one of his studies (226) 

 he compared the evaporating power of the air in a Douglas fir and in 

 an Engelmann spruce forest with that in more open communities. In 

 the forests the water-holding capacity of the soil is increased by the 

 duff and litter, which aids in conserving both winter and summer 

 precipitation. 



The shade of the trees and various forest layers reduces the evaporating power 

 of the air, while the water loss from the soil is further retarded by the layer of 

 duff (226, p. 108). 



Table 23 shows the average daily evaporating power of the air in 

 the communities studied from June 10 to August 23. Likewise 

 Weaver found that the evaporating power of the air in a chaparral 

 thicket in Nebraska was only half of that 75 feet outside. 



Table 23. — Average 



daily evaporating power of air in communities of Douglas fir 

 and Engelmann spruce, by weekly periods 



Period 



Douglas 



fir 



Engelmann 

 spruce 



Period 



Douglas 

 fir 



Engelmann 

 spruce 



June 10-17 



Cc 

 28.8 

 11.1 

 20.4 

 15.2 

 5.7 

 6.1 



Cc 

 21.6 

 7.1 

 15.0 

 11.1 

 4.4 

 5.2 



July 22-29 



Cc 

 11.3 

 16.0 

 8.6 

 8.5 

 11.6 



Cc 



8.5 



June 17-24 



Julv 29 to Aug. 5. 



11.4 



June 24 to July 1 



Aug. 5-12. 



6.2 



July 1-8 



Aug. 12-17 



6.3 



July 8-15... 



Aug. 17-23.. 



8.8 



July 15-22.. 











As has been frequently pointed out, the forest and its environment 

 react reciprocally upon each other; the environment influences the 

 forest and the forest in turn influences the environment. Similarly 

 the excess humidity in forests may result in distinct structural changes 

 in the leaves. Leaves growing in a very damp atmosphere are 

 usually broader and thinner than those growing in dry surroundings, 

 which tend to be more xeromorphic. It should be kept in mind, 

 however, that if available water is present in the soil a decrease in the 

 relative humidity of the atmosphere will not result in structures as 

 xeromorphic as when available soil moisture also is lacking. 



The relative humidity may affect not only the morphology of the 

 leaf but also its physiology. Molisch (154) found that when leaves 

 of plants growing in moist air were placed in air relatively dry, all 

 or part of the leaves soon formed abscission layers and dropped off. 

 This fail of the leaf may occur also in moist air when the soil moisture 

 is insufficient. When air and soil dryness are united, the fall of the 



