WATER UTILIZATION BY TREES 



45 



The observations of Ebermayer, as recorder! by Rubner (178, 

 p. 73), for 1868-6*9 (table 20) give somewhat higher results. 



Table 20. — Relative humidity of air in the forest above that in the open, 1868-69 



Species and number of st 



inds 



Age of stands 



Season 



Spring 



Summer 



Fall 



Winter 



Beech, 2 



60. 



40_ 

 36, 



Years 



Percent 

 5.3 

 7.9 

 3.6 



Percent 

 12. 9 



JO. 7 

 1.2 



Percent 

 3.7 

 9.1 

 3.5 



Percent 

 2.4 





6.8 



Pine, 1 



4.2 







A similar table compiled by Toumey from Swiss and German ob- 

 servations (215, p. 21 4) gives the following figures (table 21): 



Table 21. — Excess of relative humidity in the forest over that in the open country, 



in percent of saturation 



Season 



Beech 



Spruce 



Scotch 

 pine 



Larch 





Percent 

 1.91 

 9.35 

 4.07 

 1.73 



Percent 

 6.92 

 8.56 

 7.01 

 4.76 



Percent 

 2.93 

 3.87 

 4.24 



2. 70 



Percent 

 2.83 





7.85 



Autumn ' ' . . __ .... "' ... .. 



5.45 



Winter.. 



.34 







Mean of year 



4.27 



6.81 



3.44 



4.12 



It seems certain from these data that during the growing season 

 there is an increase in the relative humidity in the forest. This can- 

 not be considered as necessarily favorable, since too high relative 

 humidity may check transpiration sufficiently to hinder growth. In 

 high altitudes, high humidity may also lead to an excessive growth of 

 lichens, but in the lower regions and in dry periods an increase in the 

 relative humidity in the forest is, of course, favorable for the trees. 

 This is especially important in spring (May) when the air is relatively 

 the driest, as shown by the following figures from Bavaria: The rela- 

 tive humidity from March to August in the open was 81, 77, 67, 71, 

 72, and 73 percent for these respective months, while in the forest the 

 humidity for these same months was 85, 82, 76, 80, 82, and 81 percent, 

 respectively (178, p. 73). 



In the forest during May, however, the relative humidity is nearly 

 10 percent higher than in the open. In Switzerland, April is the 

 driest month, according to Buhler (23), who attributes the failure in 

 many cases of natural reproduction in open old stands to the increased 

 evaporation at this season. Spruce stands, since they are green all 

 the year, show on the average a higher relative humidity throughout 

 the year than deciduous beech stands. In the growing period, how- 

 ever, beech stands seem to have a higher relative humidity than 

 spruce. 



For various parts of Europe the differences in relative humidity 

 within and outside of forests have been tabulated by Zon, as recorded 

 by Toumey (215, p. 213). His figures are given in table 22, which 

 shows that the relative humidity inside the forest is usually 3 to 10 

 percent greater than outside. 



