THE RELATION BETWEEN EVAPORATION AND PLANT 
SUCCESSION IN A GIVEN AREA^ 
Frank C. Gates^ 
As a result of an investigation into the relative amounts of evapora- 
tion from the chamaephytic or ground layer of certain genetically 
connected, adjoining plant associations at Havana, Illinois, during 
the summer of 1910, Gleason and Gates (i) concluded: "that succes- 
sions between associations are not caused by any conditions of evapor- 
ation." In conclusion to a much more extensive series of investiga- 
tions, bearing on the same subject. Fuller (2) concludes: "the decreased 
rate of evaporation ... is the direct cause of successions between 
different associations." Weaver (3) concludes: "A study of the dif- 
ferences of the rates of evaporation in the various plant formations 
and associations shows that these differences are sufficient to be im- 
portant factors in causing succession, at least through the earlier stages, 
where light values are usually high." 
Each investigation dealt with neighboring associations in a limited 
area, thereby accentuating the action of local factors and minimizing 
the obscuring interference of climatic factors. An inspection of the 
pertinent data obtained in each of these investigations shows that they 
are similar; yet diametrically opposite conclusions are drawn. 
To obtain new data on the relationship between evaporation and 
plant succession, three series of experiments were carried on during the 
summers of 1915 and 1916, at the University of Michigan Biological 
Station at Douglas Lake, Michigan. During 1915, twenty-six stan- 
dardized Livingston atmometers were employed for a period of 40 
days, inclusive of the time of maximum evaporation during the year. 
In 1 9 16, sixteen newly standardized instruments were employed during 
varying periods inclusive of the severest summer evaporation in years. 
Each instrument was set up in close proximity to certain plants. The 
1 Contribution from the University of Michigan Biological Station at Douglas 
Lake, Michigan, No. 41. 
2 Owing to the press of duties attendant upon the establishment of the University 
of Michigan Botanical Garden, Dr. H. A. Gleason was unable to collaborate, as 
planned. 
161 
