94 VEGETATION OF A DESERT MOUNTAIN RANGE. 



periods of the Desert are rarely broken on the mountain by rainfall 

 of significant amount. 



The records of rainfall for 8 years show that the summer rain at 

 8,000 feet may be from 1.9 to 3.5 times as great as that at 3,000 feet 

 (see table 4). The average of the 8 years shows the summer rain at 

 8,000 feet to be about 2.4 times that on the Desert. The average of 

 a longer series of years will probably approximate this amount and 

 the securing of the winter precipitation would probably make little 

 difference in the proportion. 



The very conditions of low evaporation which favor the water rela- 

 tions of the plants in the Forest region are also favorable to the preser- 

 vation of the moisture of the soil. The effect of the winter rains upon 

 the soil moisture of the Forest is accordingly carried forward many 

 weeks (see table 7 for soil moisture at 9,000 feet after 6 weeks without 

 rain) . The slow melting of snow on north slopes still further prolongs 

 the effect of winter precipitation. These causes underlie the vernal 

 activity of herbaceous plants in the Forest region (see p. 29) and the 

 growth by trees of the Forest region during the arid fore-summer. 



TEMPERATURE FACTORS. 



The role played by temperature in differentiating the vegetation of 

 the various altitudes of the Santa Catalinas is by no means so simple 

 a matter as that played by moisture conditions, and is far from being 

 capable of expression in a concise mathematical form. It has already 

 been shown that the frostless season decreases from a length of 40 

 weeks on the Desert to a length of 19 weeks in the Forest at 7,600 feet 

 (see tables 10 and 11, and fig. 2), and that the temperature has an 

 average apartness of 26 between Desert and Forest (see table 13). 

 No instrumentation has been carried on which would establish the 

 quantitative nature of the difference between other phases of the tem- 

 perature conditions. The shortening of the growing season with in- 

 crease of altitude, and the concomitant lowering of the temperatures 

 of this season, are factors of great moment to the vegetation, but no 

 work has been done to establish the precise temperature and tempera- 

 ture-duration requirements of any species of plants. The shortness of 

 the growing season in the Forest and the coldness of the nights of 

 mid-summer (40 to 50, see table 14) are both hostile to growth 

 activity and may well be limiting factors in the upward distribution 

 of many species of the Encinal. 



The temperature conditions of winter are equally important with 

 those of summer in underlying the limitation of species and vegetations, 

 and their role may be played independently from that of the summer 

 temperature conditions, or the two may play conjointly upon the 

 same species at the same altitude. Among the various phases of winter 

 temperature conditions are the length of the period subject to frost, 



