CLIMATE OF THE SANTA CATALINA MOUNTAINS. 



61 



dryness of the three months preceding the taking of the first set of 

 samples, together with the 25 days of rainless weather just preceding 

 the taking of the samples, had reduced the moisture of the superficial 

 soil to an amount which was near the minimum for the year, as repre- 

 sented by the percentages for June. The percentages for June were 

 all slightly lower than those for April. The reading of 5.2 per cent for 

 the south slope at 5,000 feet in April is undoubtedly too high. The 

 fall in moisture on the north slope at 7,000 feet from 9.2 per cent in 

 April to 3.2 per cent in June is doubtless significant of the long reten- 



TABLE 7. Soil moisture in the arid fore-summer at a depth of 15 cm. on north and south 

 exposures at seven altitudes on the Santa Catalina Mountains. 



The percentages are based on dry weight. 



tion of moisture derived from whiter rains, characteristic of the forested 

 elevations. In June the north slope at 8,000 feet had fallen to a slightly 

 lower percentage of moisture than the north slope at 7,000 feet in April. 



Between May 15 and 20, 1914, another series of moisture samples 

 was secured at the same localities and extended to the 9,000-foot 

 station. The preceding winter had been slightly below the average in 

 precipitation, but the rainfall for March had been above the average. 

 At the time of the taking of the samples there had been no rains for 

 six weeks. This series of percentages is similar to those secured in 

 1911, and the three may be taken together as indicating the average 

 soil moisture conditions of the arid fore-summer. 



A significant feature of all three of the series of moisture determina- 

 tions is the fact that there is no appreciable increase of soil moisture 

 up to an elevation of 7,000 feet, beyond which elevation there is a 

 sharp rise in the percentages, particularly those for the north slopes. 

 In other words, so far as the superficial soil moisture conditions are 

 concerned, the arid fore-summer carries the desert up to the lower 

 limit of the Pine Forest. 



One of the underlying causes of the importance of slope exposure 

 for vegetation is revealed in a comparison of the percentages of soil 



