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 rdéle 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 réle 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, 
