80 



(Phragmites) at yy inches is taken as lOO per cent, or the standard. 

 Lower down, at 42 inches, the rate is 70 per cent., at 10 inches, 53 per 

 cent, and at the surface, 33 per cent. Among the cattails {Typha), in 

 the upper part of the vegetation, at 69 inches evaporation was 85 per 

 cent. ; at 42 inches it was 36 per cent. ; at 10 inches, 20 per cent. ; and 

 at the surface, 8.5 per cent. These results show that at successively 

 lower levels in the vegetation the rate of evaporation is greatly .re- 

 duced. They tend also to confirm the results of Yapp and Dachnow- 

 ski. It seems, then, fair to conclude that the rate of evaporation above 

 the swamp vegetation increases rapidly with downward progression, 

 and probably with upward progression also. A vegetable layer, com- 

 parable to the mulching of straw used by gardeners, thus acts as a pow- 

 erful conserver of moisture. There are great differences within a few 

 vertical feet in the open ; what is the condition within the forest ? 



Intensity of evaporation. 



Sta. A. Maple-beech forest. 6 feet (2m.) 

 above soil 



Sta. B. Maple-beech forest. 10 inches 



(25 cm.) above soil 



Sta. C. Maple-beech forest. On slope of 



ravine 30 feet deep (10 m. ) 

 13.3 feet (4 m.) below general 

 surface. 



Fig. 10. Diag:ram showing the relative evaporation in a beech-maple woods, six 

 feet above the soil (A), near the surface of the soil (B), and in a ravine (C). 

 [Adapted from Fuller ('12).] 



The character of vertical differences in evaporation within the for- 

 est has not been given as much attention as the similar changes in the 

 open; but attention has already been called to the moisture-conserving 

 effect of a forest litter, the evaporating rate in one instance being only 

 13 per cent, when compared with that from a water surface in the open. 

 McNutt and Fuller ('12) have shown that grazing in an oak-hickory 

 forest changed the average daily rate of evaporation for 189 days 

 from 9.89 c.c, in the ungrazed forest, to 12.74 c.c, in the grazed for- 

 est, at Palos Park, 111. There are thus, within the forest, changes in 

 evaporation with differences both in the ground cover and in the litter 

 on the forest floor which correspond to the change in the vegetation in 

 open places. 



Vertical differences in evaporation have been tested in a maple- 

 beech-forest in northern Indiana by Fuller ('12b), who used the po- 

 rous-cup method. His results have been summarized in Figure 10. 

 This diagram shows that the evaporation at six feet above the surface 

 is nearly twice as much as that at 10 inches above the surface, and 



