WATER UTILIZATION BY TREES 43 



Geiger (67) recorded the following wind velocities in a 65-year-old 

 pine forest, 15 to 16 m high, at different heights above the surface within 

 the stand: 



Wind velocity 

 Height (meters): {meters per second) 



16.85 1. 61 



13.70 . 90 



10.55 . 69 



7.40 . 67 



4.25 . 69 



1.10 . 60 



According to these figures, there is very little decrease in the wind 

 velocity between the bottom of the crowns at 11 m and the ground; 

 only within a few centimeters of the ground is there any marked decrease 

 in the strength of the wind. In the canopy (13.70 m) the wind increases 

 about 40 percent, and a few meters above the crowns (16.85 m) it is 

 two and one-half times as strong as within the stand (10.55 m). 



Since the wind movements affect the transpiration and the humidity, 

 it is not surprising that shaking of the leaves also affects transpiration. 

 Stahl (204) found by weighing that the movement of the leaves of 

 Populus tremula increases the transpiration. The water loss of leafy 

 twigs that had been in steady movement was greater than that from 

 twigs with fixed leaves, and in every case there was a significant 

 decrease in water loss in the branches in which movement of the leaves 

 was suppressed. 



HUMIDITY IN FORESTS 



Many papers have been written on the humidity within forests and 

 the factors influencing it, with special reference to the problems associ- 

 ated with forests rather than to those that affect humidity everywhere, 

 such as air movements and temperature. First may be mentioned 

 changes of humidity with distance above the ground in forests. The 

 humidity may be measured directly or, as is frequently done in ecologi- 

 cal studies, the evaporation rate is measured by the use of Livingston 

 atmometers. The humidity alone, as has been seen, does not determine 

 the transpiration rate but must be considered along with other factors, 

 such as light and temperature. A measurement of the evaporation 

 rate is, therefore, of greater ecological significance, since in it these 

 other factors are included. 



The factors which vary with the evaporation rate have been studied 

 especially by Burger (25), who found a very close correlation between 

 evaporation from atmometers and the temperature, humidity, and 

 hours of sunlight. There was, however, little correlation between 

 wind and evaporation because of the fact that in times of wind there 

 was likely to be an excess of humidity, a lower temperature, or an 

 absence of light. This does not mean, therefore, that wind does not 

 increase the evaporation but merely that over periods of time the 

 amount of evaporation cannot be computed from the wind data with 

 the same accuracy as from the light, temperature, and humidity 

 figures. 



Gates (66) studied the evaporation rate at various levels in different 

 types of woods and under various ecological environments in Michigan. 

 Although he did not place the atmometers one above the other in the 

 same spot but scattered them over considerable territory, he found in 

 pine woods, in general, that the evaporation rate increased with height 



