172 CLIMAX FORMATIONS OF WESTERN NORTH AMERICA. 



Structure.— The general structure of the desert scrub is clearly revealed 

 by the color-tones, as seen in a bird's-eye view of one of the great intermoun- 

 tain valleys. Such a view of the Santa Cruz Valley from the Tucson Moun- 

 tains shows three clear differentiations of the scrub mass. The long dissected 

 bajada slopes are olive-green with Parkinsonia, Cereus, Fouquiera, and their 

 associates. The flood-plains of the valley are vivid green with Prosopis and 

 Acacia, and the central mass on the general level of the plain is bronze with 

 Larrea. The courses of the streams are marked chiefly by Populus, Sapindus, 

 Fraxinus, Juglans, and Celtis, while a nearer view reveals serai communities 

 of Hymenoclea, Baccharis, and Chilopsis in the sandy washes and of Atriplex 

 and Dondia in saline areas. Finally, there is a striking inversion of the species 

 of the valleys, by which Prosopis, Acacia greggii, Parkinsonia torreyana, 

 Olneya tesota, and others are carried up the bajada slopes or up broad canyons 

 into the zones above. 



As a consequence, the general zonation of the regions is as follows: (1) sub- 

 climaxes of the river-bed and salt-spots; (2) the valley community of Prosopis, 

 Acacia greggii, and Parkinsonia torreyana; (3) the central mass of the midland 

 plain, consisting chiefly of Larrea, often with much Acacia constricta; (4) the 

 community of bajadas and foothills, characterized by Parkinsonia micro- 

 phylla, Cereus, and Fouquiera; (5) the Prosopis-Acacia areas of the canyons 

 and savannahs of the mountain ranges. The same zonal relations are seen 

 in miniature throughout the area wherever changes of soil, drainage, or eleva- 

 tion occur to modify the control of the dominants. As these features recur 

 constantly, owing to the more or less torrential nature of the rainfall, each 

 zone is much modified by the mingling or alternation of dominants which 

 normally grow above or below it. Such alternation is a regular feature of 

 the association and explains the variety and frequence of the groupings as 

 shown below (plate 38). 



Groupings.— A special study has been made of the grouping of the major 

 dominants over the Tucson plain and the bajadas and foothills of the sur- 

 rounding ranges. The primary purpose was to throw light upon the degree 

 of equivalence of the various dominants, based upon Larrea as the final 

 dominant. It serves also to give a clear idea of the relative importance of the 

 different species, and the frequence and complexity of the various groupings. 

 The area covered is about 40 miles in length from the Santa Catalina Moun- 



