36 A Study of the Vegetation of 



dry weight. The chart shows that at no time did these virgin 

 forests of fir-tamarack or cedar exhaust all of the water in the 

 surface ten inches of soil. Likewise, samples at a depth of 2 

 feet on August i, 1914, gave a water content of 66 per cent, and 

 19 per cent, in the cedar and fir-tamarack forest respectively. 

 Similar determinations at 2 feet in the rocky pine-covered soils 

 gave a water content of about 10 per cent., which is very near the 

 wilting coefficient. The slow growth and dwarfed appearance 

 of these trees reflect the unfavorable habitat conditions. 



These studies on the evaporating power of the air and the 

 amount and range of soil moisture in different plant communities 

 may be briefly summarized. 



Evaporation at different stations within the same plant com- 

 munity exhibits variations similar both in character and degree, 

 and the variations in the rate of evaporation gradually become 

 less and less as the climax type of vegetation is approached. 



The great amount of evaporation in the earlier stages of succes- 

 sion seems a sufficient cause for the xerophytic character of the 

 vegetation. 



A study of the differences of the rates of evaporation in the 

 various plant communities shows that these differences are suffi- 

 cient to be important factors in causing succession, at least 

 through the earlier stages, where light does not play an important 

 role. 1 



If sufficient light is available, there is no question but that 

 humidity of the air and the soil are the most important factors 



1 While reading proof of this manuscript the recent paper by Gates came 

 to the writer's notice. (Gates, F. C. The relation between evaporation 

 and succession in a given area. Am. Journ. Bot, 4: 161-178, 1917.) As a 

 result of an investigation carried on at Douglas Lake, Mich., he concludes 

 that a decrease in evaporation is not a prerequisite to succession ; lowered 

 evaporation being coincident with and not an antecedent to it. Gates 

 worked in a region with summer rainfall, where the average daily rates 

 of evaporation were low. The highest average daily summer evaporation 

 obtained on open ground was only 14.7 cc. Certainly his conclusions are 

 not applicable to a region of practically no summer rainfall and where, in 

 areas covered with bunch-grass, the daily evaporation rate for the whole 

 growing season averaged 28 to 37 cc., with the average daily rate for cer- 



36 



