OROGRAPHIC FACTORS 



265 



Gail (1921) determined upon the same geological substrata in arid 

 Idaho a mean pH of 6.5 on a northeast exposure and of 6.8 on a south- 

 west exposure. 



The temperature, measured close to the soil surface, shows more 

 pronounced differences than when measured 5 to 10 cm. above the 

 ground, where the equalizing movements of the air become very- 

 noticeable. In rolling country the temperature differences between 

 the tree layers of various exposures are more readily equalized than 

 in the mountains, where the confining effect of steep slopes increases 

 the contrasts even in the upper layers. 



The changes in soil temperature at various exposures are of even 

 more ecological importance than are those of the air. Such changes 

 exert their influence upon root development even at considerable 



'/2.6 12.6 12.7 



Innsbruck' 

 780 m. 



7A 7.8 



/ Gschinfz Valley, 



12.2 



•^J-^ 



55 i^^' 



5.1 S:^^ 



1340 m. 



^^.5 



N NE 



SE S SW W NW 

 E/:posure 



Fig. 133. — Influence of exposure upon the annual mean of soil temperature at a depth 

 of 8b cm. (From data by Kerner.) 



depths. Kerner's observations in the Tirolese Alps illustrate this 

 point (Fig. 133). 



Foresters have also carried out similar investigations. Biihler 

 (1918, p. 238) determined that in Zurich during the entire vegetative 

 period the soil temperature at a depth of 5 cm. in all exposures except 

 north is higher than the air temperature. The differences are greatest 

 in the middle of the summer, especially around noon. For example, 

 the soil temperature reached 38°C. at 1 p.m., Aug. 23, at a depth of 3 to 

 5 cm., with a simultaneous air temperature of 28.2° in the shade. 

 Underneath the dense leaf cover of twenty-year-old beeches the 

 differences in soil temperature in different exposures are practically 

 equalized. 



In North America Shreve, Bates, Gail, a^nd others have investi- 

 gated the problem of the influence of exposure on vegetation. 

 Shreve set up self-recording thermometers in natural plant communi- 



