May 24 ,1924 Significance of the Southwestern Desert Vegetation 
779 
BARE FLATS 
Scattered throughout the mesquite thickets and occasionally in the lower 
places in the desert-sage areas in the Gila Valley there are curious spots sometimes 
an acre or two in extent called “slick land,” “bare flats,” or “playa flats” (PI. 
14, B). The first name applies to the slippery nature of the heavy soil when wet, 
as in spring. The name “bare flats” refers to the lack of vegetation on these 
spots. The dry lakes of the Mohave Desert and Death Valley region would 
also be included. In spring, or after a heavy rain, water stands on these bare 
flats for some time. The soil is very heavy and silty on the surface, and when 
dried cracks, leaving deep fissures in the soil. On the most typical of these no 
plants grow. Occasionally there are scattered, small plants of Dondia inter¬ 
media (S. Wats.) Heller, Atriplex polycarpa (Torr.) S. Wats., and Lycium parvi - 
jlorum A. Gray. These plants occurred on small hummocks of lighter soil which 
stand a foot or more above the level of the flat. After rains a small annual, 
Atriplex fasciculata S. Wats., springs up on the more favorable portions of the 
flat. A single station was chosen in a bare flat and a boring made in the spring 
and in the fall. Soil moisture is present in the third and fourth feet, probably 
indicating the influence of a high water table. Of the samples collected the first 
two feet had no available water (Table XXX). The salt content is very high, 
ranging over 2 per cent in the first and second feet. More than one-half of the salt 
content is made up of chlorids (Table II, sample No. 11). 
RELATIONS OF VEGETATION TO CLIMATIC CONDITIONS 
The character of the vegetation in any region is an expression of the controlling 
factors of climate. The effect of moisture supply may be overcome or greatly 
altered by irrigation or by drainage. Temperature control is less likely to be 
locally interrupted except when there are abrupt variations in elevation. To 
express the temperature condition in a comparative way is very difficult, since 
summations are not yet possible, due largely to insufficient knowledge of plant 
physiology. We have not attempted the use of summation data, but have used 
the United States Weather Bureau summaries. These data are presented 
graphically in figures 7-9 and 14-21. In all cases the monthly values are plotted, 
beginning with January at the left. Rainfall is expressed in the usual way in 
figure 14. Below each graph is shown the mean .annual rainfall, and, the length 
of the record in years. The mean monthly temperature (fig. 15) is indicated by 
a bar and the lower extension drawn arbitrarily to 30 degrees below the annual 
mean value. This enables the reader to compare the different stations by a glance 
at the bottom of the mean monthly temperature graph. In figure 16 the abso¬ 
lute monthly maximum and the absolute monthly minimum are represented by 
the top and bottom of the light portion of the bars, while the mean monthly 
minimum and mean monthly maximum are represented by the bottom and the 
top of the shaded portion of the bar. On each graph are written the following 
annual values, the mean, the absolute maximum, the absolute minimum, the 
mean maximum and mean minimum, also the length of record in years, the 
type of vegetation, name of the station, and elevation above sea level. 
The 41 stations shown in figure 14 are chosen to be fairly representative of 
the whole area of the southern desert, and of the types of vegetation considered 
in this paper. If we arrange these stations according to increasing rainfall, 
beginning with the lowest at the left, it is evident that the lowest rainfall occurs 
in those sections in which the vegetation is on the whole sparse and is composed 
either of creosote bush or desert-sage. All stations having a rainfall from 2 to 
3J inches are located in the Colorado desert of California. For the most part 
this rainfall occurs in late summer and there is little rain in early summer. 
