142 



PLANT SOCIOLOGY 



can Phragmitetum found the daily evaporation at the surface of the 

 soil, at 25 cm., at 107 cm., and at 198 cm. above the surface to be 

 respectively 2.5, 4, 5.3, and 7.5 cc, while Fuller (1912) in the beech- 

 maple forest of Indiana reported the evaporation at 2 m. above the soil 

 to be double that at 25 cm. This leads to the conclusion that plants 

 may grow in close proximity to each other and yet be subject to widely 

 different growth conditions. 



Thone (1922) working in Illinois made comparative measurements 

 with the ordinary spherical atmometer and the blackened radio- 

 atmometer. On the forest floor the maximum difference in readings of 

 the two kinds of atmometer was in the first 10 days of May under 

 conditions of high illumination. The rest of the time the readings 

 were about parallel. 



June 



July 



August 



September 



16. 23. 



t¥. 21 28. V. ;;. 18. 25. 



Fig. 78.— Evaporation graphs (a) for the Andropogon prairie; (6) for the Corylus 

 scrub; and (c) for the Tilia americana-Ostrya virginiana forest near Peru, Nebraska. 

 {After Weaver.) 



The relative dryness (relative xerophytism) of the habitat may be 

 deduced from the relation between water used by the plant and evapor- 

 ation. The higher the relative dryness the more open is the ground 

 cover, the fewer the tree seedlings, and the fewer the annuals that per- 

 sist through summer. 



The evaporating power of the air, although greatly underrated by 

 practical foresters (c/. Blihler, 1918, p. 150), is undoubtedly an 

 extremely important factor in forest ecology. It is often decisive in 

 reforestation of grassy slopes, in reclaiming alluvial sands, and in the 

 conflict between forest and steppe. Even in the forest areas of north- 

 ern Switzerland, southern Germany, and eastern France, in the beech- 

 wood climax regions, one may search in vain for seedUngs of beech in 

 dry grassland (Xerobrometum), and oak seedlings are one and all 

 destroyed in dry summers. Only the drought-resistant Scotch pine, 



