CLIMATIC FACTORS 141 



niiiiiniu of the si'ipl^^ i^nJ Ihc gonoial course of 1 he curves are in <j;cneral 

 agreement for all the communities studied, but the amounts of evapo- 

 ration and the differences in the evaporating power of the air in the 

 different communities are characteristic. 



The development of vegetation in the Lake Michigan dune area 

 proceeds from xeric to mesic conditions. This is clearly shown 

 by the progressive reduction of evaporation from the pioneer stages to 

 the beech-maple climax. The beech-maple climax forest has the lowest 

 evaporation and the least variation in the evaporating power of the 

 air. The Populus deltoides pioneer association of the dunes has not 

 only the most intense evaporation but also the greatest fluctuations. 



The mean daily atmometer readings throughout the three growing 

 seasons in the lower layers of the forest communities are thus related to 

 the development of vegetation: Populus deltoides dunes 22.3 cc; 

 Pinus hanksiana dune forest 10.4 cc; Quercus velutina dune forest 11.0 

 cc. ; Q. horealis forest 8.8 cc. ; Fagus grandifolia-Acer saccharum forest 

 7.0 cc. 



The evaporation maximum in May, before the trees leaf out, is very 

 marked and indicates the different conditions to which the vernal and 

 aestival herbaceous plants are exposed. 



The investigations of Weaver (1914), in the mesophytic conifer 

 forests of eastern Washington show that throughout the summer the 

 evaporating power of the air is essentially the same as in the beech- 

 maple forests of Indiana investigated by Fuller. In other words, 

 the mesophytism of the two climax types is of the same order; the 

 moisture conditions of their atmospheres, the same. 



Striking results are given by the investigations of Weaver, and of 

 Weaver and Thiel (1917, 1918) in the forest and prairie areas of 

 Washington, Idaho, and Nebraska. Their evaporation curves make a 

 valuable complement to the characteristics of the associations investi- 

 gated, even though these were rather too broadly conceived. The 

 similarity of the course of the evaporation curves in different associa- 

 tions brings out clearly the dominating influence of the general climate 

 (Fig. 78). 



The quantitative demands of individual communities are indicated 

 by the intensities of the evaporation measured in their midst. It 

 should be noted, however, that the aerial parts of different plants in the 

 same community may be exposed to very different evaporating power 

 of the air, depending on the height from the ground at which the 

 foliage is displayed. Yapp (1909) found the evaporation in an English 

 fen at 2 cm. above the surface to be 20 per cent of that at 10 cm. and 

 only 5 per cent of that at 20 cm. Similarly, SherfT (1913) in an Ameri- 



