100 ILLINOIS ACADEMY OP SCIENCE 



THE STRATIFICATION OF ATMOSPHERIC 

 HUMIDITY IN THE FOREST. 



GEORGE D. FULLER, J. R. LOCKE and WADE McNUTT. 



It has long been know that the air among the tree-tops 

 of a forest has greater evaporating power than that just above 

 the soil beneath, principally because of the fact that the upper 

 air is subject to so much more movement and to more direct 

 insolation. Up to the present time few or no attempts have 

 been made to determine in a quantitative manner how great 

 these differences really are for different forests and for dif- 

 ferent strata in these forests. The Livingston atmometei 

 now provides an efificient means of making such determina- 

 tions and hence during the spring of 1912 a beginning was 

 made by two students of the Department of Botany of the 

 University of Chicago, Mr. Locke working in a flood plain 

 forest along the Vermilion River, near Streator, Illinois, and 

 Mr. McNutt in an upland oak forest at Palos Park, near the 

 city of Chicago. the same in which some former evaporation 

 studies had been made (1). For several reasons, principally 

 on account of the closing of the school year, the records are 

 brief but both continue for some ten days after the trees were 

 in full foliage and hence give some hint of the amount of dif- 

 ference of the evaporating power of the air in the different 

 strata concerned. They are reported principally with the 

 hope of stimulating further research upon this and similar 

 problems. 



The Streator forest was of the usual flood plain type with 

 an abundance of Acer Negundo, Ulmus americana, and Fraxi- 

 nus americana. Two stations, and 1, were placed upon the 

 surface of the soil, the first being at high water mark and the 

 other some 60 meters farther from the river on gently rising 

 ground, both being within the forest. The other instruments, 

 designated as 3. 5, and 8. were placed in conditions of aver- 

 age shade and foliage at 3, 5 and 8 meters respectively above 

 the surface. The period, of experimentation extended from 

 May 2 to June 5. 1912. The readings were standarized in the 

 usual way (2). by means of the coefficients of the atmometer 

 cups, the results reduced to the average loss per day for the 

 intervals between the weekly readings and plotted as graphs 

 with the weekly intervals as abcissae and the losses per day as 

 ordinates (Fig. 1.). It will be seen that in general the re- 

 sults are that the evaporation at 3. 5. and 8 meters is very simi- 

 lar and is practically double of that upon the ground (o). 



The forest at Palos Park is of the usual upland oak type. 

 It has been more fully described in a previous paper (1). 

 Here four stations were established, one upon the 



