164 CLIMAX FORMATIONS OF WESTERN NORTH AMERICA. 



Madre Mountains, the other to the Gulf of California. Its greatest extension 

 northward is found in Nevada, while it ranges southward in Mexico through 

 Chihuahua and Durango. In altitude it reaches its major expression between 

 1,000 and 3,000 feet, but it occurs from sea-level to 4,000 feet or higher. 



With respect to climate, the desert scrub is especially distinct. This is 

 true of both temperature and rainfall. It has the lowest rainfall, the greatest 

 evaporation, and the highest mean temperature of all the western climaxes, 

 though the sagebrush approaches it closely in the matter of rainfall. The 

 precipitation ranges from 2 to 12 inches, with the major portion of the forma- 

 tion lying between 5 and 10 inches. The highest rainfall occurs in trans- 

 Pecos Texas, with 15 inches, and the lowest in the Colorado Desert with 2 

 inches. There is a general but irregular decrease from east to west, correlated 

 with changes in the structure of the formation itself (fig. 6). 



Las Vecraa, Nevada 



4 in. 



I I I I I , I I I I I I 



Parker, Arizona 

 Sin. 



Il...-lll.ll 



Bagdad, CaliComia. 

 4 in. 



I.lll 



Fio. 6. Monthly and total rainfall for representative localities in the associations of 



the desert scrub climax. 



The floristic unity of the desert scrub is even greater than that of the other 

 two scrub climaxes. The two most important dominants, Larrea mexicana 

 and Prosojyis juliflora, occur throughout. This is likewise true of Atriplex 

 canescens and Fouquiera splendens, though these are probably subcUmax in 

 character. Acacia, Yucca, and Ephedra extend throughout the area, but the 

 species change to some degree. Acacia greggii is found from western Texas to 

 Lower California, while A. constricta ranges nearly as widely. Yucca radiosa 

 and Y. macrocarpa are present from western Texas through Arizona, but are 

 largely replaced in California and Lower California by species ecologically 

 equivalent. While four or five species of Ephedra occur in the formation, 

 these are essentially similar if not identical in ecological character. Among 

 the undershrubs, Gutierrezia, Isocoma, Krameria, and Zinnia are distributed 

 over most of the fonnational area. 



The ecological unity of the desert scrub is indicated by the fact that prac- 

 tically all of the dominants are many-stemmed, bushy shrubs, usually with the 

 habit of root-sprouting well developed. This is essentially true of Yucca, 

 especially the most important species, Y. radiosa, in spite of its very different 



