1914] 



FULLER— EVAPORATION AND SOIL MOISTURE 



215 



for the critical one, especially in forest associations. Within this 



stratum 



depends the character of the succeeding vegetation. Therefore, if 

 the vegetation of an association so affects the evaporation rate of 

 this stratum that it permits the survival of seedlings of more meso- 



me 



accom 



from 



due to the evergreen character of the association, and is seen in the 

 low rates in May and October, which are always decidedly below 

 those of the succeeding oak dune association, and are often even 

 lower than those of the oak-hickory and beech-maple forests. 

 That this association is very close to the succeeding one may be 

 seen by referring to table I. Here it will be seen that the mean 



TABLE I 



Mean weekly evaporation rates in cc. from a standard atmometer for three 



seasons under investigation 



Association 



Cottonwood dune 



Pine dune 



Oak dune 



Oak-hickory forest 

 Beech-maple forest 

 Edaphic prairie . . . 



IQIO 



21. I 



10.3 



8.1 



ign 



24.6 

 10.3 



11. 8 



9-8 



74 

 12.5 



1912 



21.3 



9 1 



10.9 



7.8 



5-6 



12-5 



Average 



22.3 



IO.4 



II .O 



8.8 



7.0 



12-5 



— 



I compara- 

 tive rat. 



319 

 149 



iS7 

 126 



100 

 179 



evaporation rates were during two of the three years slightly less 

 than those of the succeeding association. The same is true for the 

 average of the rates, but if the comparison be made for the 10 weeks 

 of summer, from the last week of June to the first of September, as 

 shown in table II, the comparative rates are reversed, and the 

 pine dune association has a slightly higher average. This is believed 

 to represent the actual condition during the weeks of greatest 

 stress of moisture conditions. 



forests 



witho 



meso 



of the edaphic prairie express the condition of an association 



