EDAPHIC OR SOIL FACTORS: PHYSICAL 



229 



Maianthemum bifoUiim., Paris quadrifoUa, and Oxalis aceiosella, was 

 accompanied by an increase of the air capacity of the soil from 8 to 

 23 per cent. 



The studies of Braun-Blanquet and Pawlowski (1931) show that 

 the air capacity of a given soil is a relatively constant factor. In 

 soils v/here the annual variations of air content are very great (Des- 

 champsietum mediae), the air capacity increases a little whenever the 

 air content is at its annual maximum. 



20 W 60 8 W0 % 20 W 60 80 100 % 



5^ 



cm Depth 



5 



Deciduous forest Dry meadow 



^^ Solids \^n Wafer ^mA'ir 



Fig. 116. — Solids, water, and air in the soils of Zofingen. The left-hand diagrams 

 from 100-year-old deciduous upland forest, the right-hand from a permanent dry grass- 

 land. Upper diagrams from soil as taken in the field, the lower with maximum capillary 

 water capacity. {After Burger.) 



Moor vegetation requires only a low air capacity of the soil (see 

 Kopecky, 1914), If, however, the air content falls below a certain 

 minimum for a considerable length of time, numerous plants die. 

 This phenomenon is most apparent in floods or in a long-continued 

 high level of ground water. 



Actual Air Content. — While the air capacity of the soil is rather 

 constant, the actual air content at any given time may vary widely. 

 After a rain, the air in the soil is always less than during a period of 

 drought. A relatively impermeable soil shows an annual curve very 

 different from that derived from a permeable soil. The seasonal 

 variations of the air content of a soil often influence vegetation more 

 than the air capacity itself. The first investigations of the seasonal 

 fluctuations of associations were carried on by Braun-Blanquet and 



