1g1t] FULLER—EVAPORATION AND SUCCESSION 205 
average rate for the season. At no point does it reach to half the 
height of that from the cottonwood dune, but surpasses that of the 
pine dune in October. 
The data of these observations relate only to the stratum of 
vegetation immediately above the surface of the soil, and would 
be quite different at a height of one or two meters. This lower 
stratum, however, is the critical one for a forest association, for 
the development of tree seedlings occurs within its limits, and 
Cottonwood dune 
Pine dune 
Oak dune | 
Beech-maple fi t 
Fic. 5.—Diagram showing the comparative evaporation rates in different asso- 
ciations on the basis of the average daily amount from May 6 to October 31, 1910. 
Cottonwood dune 
Pine dune 
Oak dune 
Beech-maple f 
Fic. 6.—Diagram es the comparative ev spoention rates in different plant 
auiovintions on the basis 0 4 I day for any week between 
May 6 and October 31, 1910. 
therefore it is the portion of the habitat which determines the 
forest succession and hence the most important ecologically. A 
single example may be cited from the meager data obtained during 
the past season regarding the rates of evaporation in the more 
elevated strata. Very near station no. 13 in the beech-maple 
forest, an instrument was established 2.5 meters above the surface 
of the soil, and showed for the season an average of 12.7 cc. daily, 
as compared with 9.1 cc. daily for no. 13, whose atmometer was 
20 cm. above the surface. 
