62 



VEGETATION OF A DESERT MOUNTAIN RANGE. 



moisture for north and south slopes. For the stations at 4,000, 5,000, 



and 6,000 feet the percentages for the two slopes scarcely differ by 



more than the error which may be attributed to the inequalities of the 



moisture in adjacent bodies of soil. Nevertheless, in all but two cases 



there is a slightly greater moisture 



content on the north slope than on 



the south. At 7,000 feet the difference 



between the two exposures becomes 



greater, and is still greater at 8,000 



and at 9,000 feet. An inspection of 



the averages (see fig. 10) shows that 



the south slope at 7,000 feet has a 



lower soil moisture than the north 



slope at 6,000 feet. The south slope 



at 8,000 feet, however, has a higher 



moisture than the north slope at 



7,000 feet. 



The fact that there is a very slight 

 difference between the soil moisture 

 of north and south slopes at lower 

 elevations and a greater difference 

 with increasing altitude would sug- 

 gest that there might be a more pronounced set of vegetational phe- 

 nomena resulting from slope exposure at higher elevations than at lower 

 ones. This is, indeed, the case, and will be discussed under a later 

 heading (see p. 98). 



TABLE 8. Soil moisture in the arid fore-summer and arid after-summer at a depth of 15 cm. 

 on north and south exposures, in shade and open, at various altitudes on the Santa Catalina 

 Mountains. 



9.000 



FIG. 10. Graph showing vertical increase 

 of soil moisture at 15 cm. in the Santa 

 Catalina Mountains on north slopes 

 (heavy line) and south slopes (light line) . 

 Average of three determinations in arid 

 fore- summer. 



In the summer of 1910 several samples of soil were taken for the 

 determination of the moisture conditions on opposed slopes and in 

 shaded and open soil, as well as on the top and at the base of a slope. 

 These data are shown in table 8. The September readings, when taken 



