44 



MISC. PUBLICATION 257, U. S. DEPT. OF AGRICULTURE 



above the ground. In aspen woods where the atmometers were fairly 

 close together he obtained the following results: 



Evaporation rate 

 compared with 

 that at ground 



Height (meters): (=?) 



3.7 1. 48 



1.8 1. 39 



.9 1. 16 



Huber (109) found that in an oak crown in July, if Piche atmometers 

 at 4 m from the ground show an evaporation of 100, then at 12 m the 

 evaporation rate is 131. Likewise if the evaporation rate in May was 

 100 at 2 m from the ground, at 10 cm it was 2.9. The increase in rate 

 between 10 cm and 2 m is seen to be, therefore, much more rapid near 

 the ground than higher up in the canopy. Likewise Geiger (67) 

 showed that in a pine stand there were differences of 25 percent in the 

 relative humidity between the air above the forest floor and that above 

 the crowns. 



Thone (212) set up five atmometer stations on a transect through a 

 hardwood hammock (in northern Florida) and two additional stations 

 at heights of 4 and 18 m. His results are not very well summed up 

 but, in general, he found that evaporation rates at the ground level 

 were to those at 4 and 8 m about as 12:15:18. 



Similarly Piitt and Pessin (171) found that the evaporation from a 

 white spherical atmometer near an oak tree in a rather dense woods 

 near Mount Desert, Maine, increased from about 431 cc at 1.5 m 

 from the ground to about 487 cc at 6 m and to 738 cc at 9.5 m during 

 43 days in midsummer. Also there was much more variation with the 

 exposure at the greater heights. 



Allee (3) noted that the evaporation power of the air on the forest 

 floor in the tropical rain forest of Panama was one-half that of the lower 

 forest canopy and one-sixth that from the surface of a neighboring 

 lake. Here conditions were very constant and the results are inter- 

 esting from that point of view, although a discussion of conditions in 

 tropical rain forests lies somewhat outside the scope of this review. 



Within the crowns themselves there is a variation in the transpiring 

 power. Thus Huber (109) found that if the evaporation rate inside 

 an oak crown 4 m from the ground is 100, then at the edge of the crown 

 it is 104; and Hanson (83) found that the evaporation at the south 

 edge of crowns of Llmus, Acer, Tilia, Quercus, and Fraxinus was 

 one and one-half to two and one-third times that within the crowns, 

 while the humidity within the crown was 1 to 6 percent higher than 

 at the edge. Hanson is one of the few workers in this field who 

 measured both the evaporating power and the humidity. 



Since the temperature and rainfall show seasonal variations, it is 

 not surprising to find that the humidity in forests also varies with the 

 season. On the basis of Muttrieh's observations (160) the average 

 for the years 1886-90 in Prussia is shown in table 19. 



Table 19. — Relative humidity of air in the forest above that in the open, 1886-90 



Species and number of stands 



Age or type of stands 



Season 



Spring 



Summer 



Pall 



Winter 



Beech, 6 . . 



70-80 j'ears 



Percent 

 1.0 

 3.4 

 4.4 

 1.5 



Percent 

 7.9 

 5.4 

 8.2 

 5.5 



Percent 

 4.6 

 4.8 

 5.6 

 2.8 



Percent 

 1.9 



Spruce, 5. ... 



60-70 years 



1.4 



Pine, 4. 



2.5 



Pine, 1. 



Plantation 



.9 







