788 
Journal of Agricultural Research 
Vol. XXVIII, No. 8 
As a result of this difference, Casa Grande and Tucson show a relatively great 
development of summer annuals. It seems probable that if the rainfall during 
August and September were increased somewhat, both places would develop a 
grass cover. Lordsburg is in a grass country with creosote bush a prominent 
plant. The grasses of the mesquite-grass type grow during the summer rainy 
period. The distribution of rainfall from July to December is similar to the 
Great Plains type, in a region intermediate between grassland and desert shrub. 
Lordsburg is somewhat cooler than the other stations considered, but moisture 
conditions chiefly are important in explaining this change in vegetation. Big 
Springs has the temperature required for the southern desert shrub and the rainfall 
sufficient to develop a grass cover. As a result, we have mesquite scattered over 
a mesquite-grass type. The range of temperature is a little greater at Big 
Springs and Lordsburg than at the other stations. 
A similar series, but one which introduces temperature changes of significance, 
is shown in figure 18. Mohawk is located low in the Gila Valley. The tem¬ 
perature is very high, the mean annual being 74.5 degrees and the mean being 
above 90 degrees for three months of the year. During June, July, and August 
the mean maximum ranges from 105 degrees to 110 degrees and the absolute 
maximum reaches more than 100 degrees for seven of the twelve months 
of the year. The lowest temperature recorded is 22° F. Passing northeast¬ 
ward to Phoenix the rainfall becomes twice as heavy. Except for the more 
luxuriant growth, there is comparatively little change in vegetation. Creosote 
bush and desert-sage are the prominent types. The temperature is somewhat 
lower at Phoenix, but not low enough to eliminate any important element from 
the vegetation. At Socorro a marked change is noted in the vegetation. Creo¬ 
sote bush is near its limit, while the mesquite grassland and Yucca are promi¬ 
nent. Temperature falls to —16 degrees and the average is approximately 18 
degrees lower than at Mohawk. The great change of temperature does not 
prevent the occurrence of mesquite and creosote bush. The rainfall is similar to 
Lordsburg, that is, of the Great Plains type, from July to November. Amarillo 
is in pure short-grass country. The average temperature is only a little lower 
than at Socorro. The summer rainfall is typical of the Great Plains and suffi¬ 
cient to develop a pure short-grass cover. 
In this series, temperatures at Socorro and Amarillo are probably a little too 
low for the desert-shrub type, while the distribution of rainfall favors a grass 
cover. The short-grass cover would probably give way to mesquite grass if 
temperatures were as high at Amarillo as at Lordsburg or Big Springs, while 
Socorro would probably have the short-grass type instead of mesquite grass if 
the distribution of rainfall were similar to Amarillo. 
A similar series selected in Arizona (fig. 19) shows a change from the luxuriant 
creosote bush type at Maricopa to the short grassland at Congress and to yellow 
pine at Williams. These changes are accompanied by temperature and pre¬ 
cipitation differences. Maricopa and Congress do not show marked difference 
in temperature means, but Maricopa has a much higher summer maximum. 
Rainfall is twice as heavy at Congress as at Maricopa. This high rainfall favors 
the development of short-grass at Congress, while at Maricopa the grasses do 
not develop. At Williams, where the yellow pine is the dominant type, the 
rainfall is three times as heavy as at Maritfopa, and the mean temperature about 
.20 degrees lower. 
Minimum temperatures below zero are recorded only at Williams, and these 
low temperatures—the summer maximum seldom reaches 100 degrees—are in 
sharp contrast to Maricopa, where during four months of the year the average 
maximum exceeds 100 degrees and inhere zero temperatures are never recorded. 
