WATER UTILIZATION BY TREES 



41 



humidity, temperature and air movements are among the most im- 

 portant. The effect of temperature lias been repeatedly mentioned: 

 The higher the temperature with a given amount of moisture in the 

 air, the lower the relative humidity, i. e., the higher the temperature, 

 the more water vapor the air can contain before the saturation point 

 is reached. The absolute humidity or total water content varies 

 directly with the temperature, given an unlimited water supply, as 

 does also the total amount that it is possible for air to contain before 

 becoming saturated; but the relative humidity varies inversely with 

 the temperature, the total water being constant. 



Rubner {178, p. 72) has pointed out that the absolute humidity in a 

 forest is not essentially different from that in the open, while the 

 relative humidity, on the other hand, may be very different. Since 

 the air temperature in a forest is generally considerably less than that 

 in the open, especially in the summer, it follows that at different 

 seasons the relative humidity must be considerably greater than in 

 the open. For example, if the forest temperature is 15° C. while 

 that in the open is 20°, when the relative humidity in the open is 70 

 percent, in the forest (even with the same absolute humidity) the 

 relative humidity will be 94 percent. 



SOIL TEMPERATURE 



While Cribbs {40) showed a very close relation between soil tem- 

 perature and transpiration in Tilia, the correlation between soil 

 temperature and relative humidity is probably not so close, since the 

 latter is affected more by the air temperature, which in turn is in- 

 fluenced only indirectly by the soil temperature. 



Baldwin (7) found no direct relation between soil temperature at 

 1 foot and the evaporation as measured by atmometers in a 50-year- 

 old stand of Betula papyrvfera and Pinus strobus in New York. His 

 data for this station are given in table 16. 



Table 16. — Weekly averages of temperature and evaporation in the open and in a 

 50-year-old stand of paper birch and white pine, 1982 







Open 





Woods 



Period 



Eainfall 



Air tem- 

 perature, 

 7 a. m. 



Soil tem- 

 perature 



Evapora- 

 tion 



Soil tem- 

 perature 



Evapora- 

 tion 



June 27- July 3. .. ... 



Inches 

 

 3.12 



.41 

 2.67 

 2.62 

 1.21 

 1.46 

 1.18 



.78 



o F 



F. 

 59.1 

 59.2 

 62.0 

 63.0 

 63.3 

 62.7 

 62.4 

 62.3 

 62.0 



Cc 



93 



70 

 115 

 96 

 75 

 80 

 72 

 76 

 72 



o F _ 

 54.3 

 53.0 

 65.1 

 55.6 

 56.4 

 56.4 

 57.5 

 56.5 

 56.5 



Cc 

 53 



July 4-10 



57.9 

 56.8 

 56.5 

 58.8 

 57.9 

 56.3 

 57.7 

 56.3 



28 



July 11-17. 



58 



July 18-24 



45 



July 25-31 



17 



Aug. 1-7 



18 



Aug. 8-14 



Aug. 15-21. 



33 



22 



Aug. 22-28 



37 







Total 



Average 



13.45 

 1.49 



458. 20 

 57.2 



556.0 

 61.8 



749 

 83.2 



501.3 

 55.7 



311.0 

 34.5 



AIR MOVEMENTS 



Air movements, as is to be expected, affect transpiration because 

 they replace saturated air with nonsaturated air in the vicinity of the 

 transpiring leaves. Wiesner {239) found in Populus tremula that 



