THE DESERT SCRUB CLIMAX. 165 



appearance. Prosopis and Acacia usually constitute exceptions, particularly 

 when found on flood-plains and in washes. However, on uplands and on 

 dunes, these too are regularly many-stemmed. As would be expected, the 

 taller dominants are uniformly deep-rooted, the depth of the roots being 

 determined largely by that of the soil. Naturally all are intense xerophytes, 

 in which the chief adaptations are the reduction and loss of leaves and twigs, 

 the development of evergreen leaves or branches, or of a thick or glutinous 

 cuticle. The great majority are deciduous, Larrea and Yucca furnishing the 

 most important exceptions. Nearly all agree likewise in being somewhat 

 resistant to alkali. This is a direct outcome of the prevalence of halophytic 

 areas in the valleys and bolsons. Many of the most striking playa regions 

 occur within the area of the formation. The succession on saline soils is 

 essentially identical from one end of the area to the other. This is true also 

 of the sand-dune and hummock succession, which is probably the most wide- 

 spread of all. The third important succession is that of rocky ridges and 

 slopes. While this shows the same stages in both associations, it is character- 

 ized by Agave, Yucca, Opuntia, and Dasylirium in the east, and by Fouquiera, 

 Parkinsonia, Cereus, and Encelia in the west. The close equivalence of these 

 two subclimaxes is shown by the presence of Fouquiera, Dasylirium, and Yucca 

 radiosa in both. 



With respect to its origin, this formation is one of the most homogeneous. 

 The dominants are all subtropic or Mexican in distribution, with the exception 

 of Atriplex canescens, while this and Gutierrezia are the only ones that range 

 far beyond the formational limits to the north. This conforms to its occur- 

 rence as a broad belt on both sides of the Mexican boundary. The gradual 

 differentiation of this formational mass has reached a point where it seems 

 desirable to recognize two associations, the one centering about the Rio 

 Grande and the other around the Colorado. The reasons for this are dis- 

 cussed in the following section. 



Structure of the formation. — The general abundance of Larrea throughout 

 the formation gives the impression that the latter contains a single associa- 

 tion. A scrutiny of the various groupings discloses a number of constant 

 differences between the eastern and western portions, which warrant the 

 recognition of two corresponding associations. The statistical evidence from 

 more than 250 localities is supported by the comparative studies made in 

 1918, when the formation was examined in its entirety from Texas to Cali- 

 fornia. In the table on the following page the occurrence of the dominants 

 and their major groupings are shown for the two associations. The line which 

 separates them is in general that of the Galiuro and Dragoon ranges in south- 

 eastern Arizona. 



It is evident that Larrea, Prosopis, and Flourensia far outrank all of the 

 others in importance, and that Franseria is three times as frequent as Acacia 

 in the association where they meet, in addition to being much more abundant. 

 Acacia, Atriplex, Yucca, Ephedra, Fouquiera, and Condalia are all more or less 

 regular associates of the primary dominants in both associations. While 

 typically less abundant, they sometimes equal or exceed them in number. 

 The division into two associations rests chiefly upon the complete absence of 

 Flourensia in the one, as a dominant at least, and of Franseria in the other, 



