294 JOHN ERNST WEAVER 



and the various rates of evaporation, while determined for a 

 relatively local region, are probably representative of a much 

 larger area — ^perhaps several thousand square miles in Eastern 

 Washington. 



Following the valuable suggestion of Fuller^ ''that a plant 

 association of wide distribution (such as the beech-maple forest 

 of the Eastern United States) be used as a basis for comparison 

 (of evaporation), and that the conditions in other associations 

 be expressed in the same units," we find that evaporation in the 

 cedar association from June 14 to September 21 is only .5 cc. per 

 day greater than in the beech-maple forest. In the fir-tamarack 

 association from May 10 to September 5 atmospheric conditions 

 in the lower stratum are 120% as severe; in the average prairie 

 formation of the plains 250%; and in the bunchgrass-rimrock 

 association 345% as unfavorable for plant life, as regards the 

 evaporating power of the air. While these data represent the 

 evaporation in the lower strata only, yet this is the critical one 

 for the establishment of seedlings and therefore the most important 

 ecologically. 



Evaporation at different stations in the same plant association 

 exhibits variations similar both in character and degree, and these 

 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 suc- 

 cession 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 formations and associations shows that these 

 differences are sufficient to be important factors in causing 

 succession, at least through the earlier stages, where light values 

 are usually high. 



The writer hopes to continue and extend these investigations 

 during coming seasons. 



'' Fuller, George Damon, Evaporation and Plant Succession. Bot. Gaz., 

 52: 193-208, 1911. 



