May 24,1924 Significance of the Southwestern Desert Vegetation 787 
northern edge of the great southern desert area, or they have a higher elevation. 
Of these five stations, all but one have absolute maxima above 115 degrees and 
the average monthly maxima exceed 100 degrees for three months in the year. 
The remaining stations have an annual average maximum ranging from 74 to 
80 degrees. In only one of these stations, Barstow, does the average maximum 
for any month reach 100 degrees, in fact only rarely does it reach 95 degrees. 
The highest absolute maximum in this region, 114 degrees, is at Barstow. All 
of these stations have recorded maximum temperatures of 100 degrees or more. 
Considering the minimum temperature we find only six stations in the whole 
series fall to zero or below. No station, however, is frost free. An average 
minimum temperature of between 50 and 60 degrees is recorded for about half 
of the series, while the other half ranges from 39 to 50 degrees. 
It is thus seen that the temperature conditions in much of the type of desert 
considered in this paper are relatively extreme. The plants growing in this 
desert are able to withstand excessively high temperatures but apparently can 
not push into any of the regions where the temperature falls far below zero. 
They appear to be able to withstand freezing temperatures, but even these 
low temperatures undoubtedly do much to change the character of the growth, 
since many of the more tender succulent plants can not well resist freezing. 
Creosote bush and mesquite are able to withstand temperatures of zero or below, 
provided, of course, these temperatures do not occur too often. Creosote bush 
and mesquite require relatively high temperatures during the growing period. 
We may summarize briefly the extreme record for this desert type of vegeta¬ 
tion. Desert-sage occurs in regions having a range of rainfall of from 2 to 12 
inches and doubtless would flourish with a much higher rainfall. The average 
temperature ranges from 56 to 77 degrees, with average maxima ranging from 73 
to 90.7 degrees and with absolute maxima ranging from 109 to 134 degrees. 
The minimum temperature for this plant ranges practically to zero. Creosote 
bush has a wider range than the desert-sage, since it occupies somewhat higher 
elevations and extends into regions of rather lower temperatures. Its range is 
the same as the total range for the southern desert shrub. 
The areas in which the Yuccas, cacti, and other succulents are prominent do 
not include the hotter and drier stations of the lower land. These are also more 
limited by temperature so that the range of the succulents expressed roughly 
is from 4 to 12 inches in rainfall, from 63 to 73 degrees in average annual tem¬ 
perature, and, for the most part, below 85 degrees in average maximum tem¬ 
perature. Their absence from the hotter desert regions is probably more the 
result of the distribution of rainfall and the excessively long drought periods than 
the actual lower temperatures, since it is doubtful if the higher temperatures are 
effective in eliminating the succulents, if the distribution of rainfall is favorable 
A selected series of stations may now be considered in relation to vegetation 
and temperature and rainfall conditions (fig. 17). In passing eastward across 
the southern desert shrub, the conditions for growth become more favorable. 
Barstow is in the Mohave desert, where the vegetation is very sparse, and mostly 
of the creosote bush type. Casa Grande is in the Gila Valley and represents a 
much more luxuriant type of vegetation. Creosote bush, mesquite, and desert- 
sage are much more luxuriant than at Barstow. Tucson lies farther east and in a 
much more favorable region. It is located at the upper edge of the creosote 
bush zone and this plant is associated with a great profusion of desert forms, 
such as mesquite, giant cactus, paloverde and the Opuntias. It represents a 
luxuriant type of pure desert shrub. As one passes from Barstow to Tuscon, the 
rainfall increases and the vegetation becomes more luxuriant. Barstow has a 
dry summer, from April to October, while bgth Casa Grande and Tucson have a 
drought period of only three months, April to June, inclusive. 
