266 



PLANT SOCIOLOGY 



ties at a depth of 10 cm. and altitudes of 2,130, 2,440, and 2,740 m. in 

 the Santa CataUna Mountains of Arizona. The results of these 

 observations, extending from May to September, give quantitative 

 proof of the fact that the difference in soil temperature between south 

 and north sides increases with rising altitudes. In accord with this, 

 there are sharp contrasts in vegetation between north and south slopes 

 in the higher parts of the mountains, 



A representative microclimatic cross section through a valley run- 

 ning east-west in the Rocky Mountains is given by Bates (1923) (Fig. 

 134). As a forester Bates was chiefly concerned with the changes of 



North slope 



South 



OX. 



Fig. 134. — Section of the "micro-climate" of an east-west valley in the Rocky 

 Mountains, Colorado: showing the effect of exposure on surface temperature (A); 

 soil temperature at 1 ft. depth {B)\ and evaporation (C). July-August mean. {After 

 Bates.) 



surface and soil temperatures and with evaporation. The inclination 

 of the slopes was about 25 deg. The increase in temperature on the 

 south sides corresponds, as might be expected, with the rise in evapora- 

 tion and the decrease of soil moisture. However, the high maximum 

 temperatures on the soil surface in south exposures must be held 

 responsible for the composition of the tree layer, as also for the 

 destruction of many seedlings. Seedlings of Pinus ponderosa, which 

 germinate quickly and establish a rather deep root system, have been 

 found the most resistant. 



When it happens that the side shielded from rain lies on the south 

 slope of the mountains, the differences due to exposure are extreme, as 

 seen on south slopes of the Bernese Alps in Switzerland, the Otztal 



