THE WESTERN DESERT SCRUB. 173 



tains on the north to the Santa Rita on the south, and about 30 miles wide 

 from the Rincons to the Tucson Range at the west. It was traversed in all 

 directions and the results are thought to be representative. The total number 

 of localities considered is 110. 



The number of dominants in each grouping, irrespective of those of second- 

 ary importance, varies from 2 to 6, with the following frequency: two domi- 

 nants, 24; three, 28; four, 38; five, 14; six, 5. 



Factor relations. — The wide adaptability of the desert scrub climax is shown 

 by its occurrence in a region where the rainfall varies from 2 to 16 inches. In 

 this respect it excels even the sagebrush formation. The desert scrub of Texas 

 and New Mexico is found in a rainfall of 7 to 16 inches. The western type has 

 a much wider range. In southeastern Arizona it occurs in a rainfall as high 

 as 12 to 14 inches, while in the Colorado Desert it is still dominant under a 

 rainfall of 2 to 3 inches. The evaporation is also much higher in the western 

 association. From April to September it ranges from 54 inches at Tucson to 

 71 inches at Calexico, in comparison with 40 to 54 inches in Texas and New 

 Mexico. The contrast would be even greater if the total annual evaporation 

 were considered, as should be the case with a community containing so many 

 evergreen or nearly evergreen dominants. The more xerophytic nature of the 

 climate is clearly reflected in the greater number of cacti in the Larrea-Fran- 

 seria scrub, though this probably has a direct relation to winter temperatures 

 as well (Shreve, 1914 : 194). In so far as the leafy shrubs are concerned, the 

 lower rainfall and higher evaporation are compensated in large measure by 

 the distribution of rain during the year. In southeastern Arizona nearly 

 60 per cent of the rain falls during the period from April 1 to September 30, 

 and the dominant vegetation is grassland savannah, as already pointed out 

 for much of the area in southern New Mexico. The percentage of spring and 

 summer rain decreases rapidly across southern Arizona to become 30 per cent at 

 Yuma and 20 per cent at Calexico. The latter represents the typical winter 

 rainfall of California, which finds expression at the lower levels in the charac- 

 teristic sclerophyll chaparral. Thornber (1910; cf. Spalding, 1909 : 96) has 

 pointed out the relation of this difference of rainfall in eastern and western 

 Arizona to the abundance of winter annuals as well as that of grasses. He 

 also shows that there is a constant difference throughout the year of one-half 

 to one inch of rainfall between the desert scrub and the desert plains grassland 

 (1. c, 256). 



While a number of quantitative studies have been made of the desert scrub 

 in the region of Tucson, those of Spalding (1909 : 91) are the only ones which 

 bear directly upon the comparative factors for different dominants or com- 

 munities. The curves showing the march of water-content from October 1907 

 to April 1908 are of the most significance. These are given for the Parkin- 

 sonia-Fouquiera community of Tumamoc Hill, the Larrea consociation of the 

 slope, the community of Parkinsonia torreyana and Acacia greggii in a wash, 

 and the Prosopis consociation of the flood-plain. These are in general agree- 

 ment with the topographic and successional evidence in that the hill and 

 flood-plains show the highest water-content and the slope and wash the 

 lowest. However, it seems certain that the Larrea slope or plain is typically 

 drier than the wash containing Parkinsonia, Acacia, and Prosopis, in spite of 

 the figures. This is clearly indicated by the author himself in his discussion 

 of conditions in the wash (1. c, 14). A comparison of the curves for the hill 



