168 CLIMAX FORMATIONS OF WESTERN NORTH AMERICA. 



THE EASTERN DESERT SCRUB. 



LARREA-FLOURENSIA ASSOCIATION. 



This community consists primarily of Larrea mexicana, Prosopis juliflora, 

 and Flourensia cernua, though other species' often play a dominant part in it, 

 as shown by the following table : 



While any of the shrub dominants may occur alone, this is rarely the case, 

 even with the three chief species. In the great majority of cases, two of the 

 latter occur mixed in varying proportions, usually with a smaller quantity 

 of one of the lesser dominants. This is shown by the occurrence of the four 

 principal groupings, as follows: 



Other important groupings are Acacia with Prosopis and Larrea, Atriplex 

 with Prosopis, and Yucca-Ephedra or either alone with Prosopis, or with 

 various mixtures of the primary dominants. A layer of undershrubs is more 

 or less constantly present. Usually this consists of Gutierrezia, less frequently 

 of Isocoma, Krameria, or Zinnia, or two or three of these may be mixed in 

 varying degree (plate 36). 



This association occupies the levels above the saline valleys and playas to 

 altitudes of 3,500 to 4,000 feet, where it passes into grassy parks in which the 

 shrubs are secondary. It occupies trans-Pecos Texas, as well as a considerable 

 area northeast of the great bend of the Pecos River, adjacent Mexico, southern 

 New Mexico, and eastern Arizona. Two of the dominants, Prosopis and 

 Acacia, form an extensive community on the plain of the lower Rio Grande 

 and extend over much of the Panhandle region as a low open scrub in the 

 grassy plains. 



Correlations and sequence. — The general correlation of the Larrea-Flourensia 

 scrub is with an annual rainfall varying from 16 inches along the Pecos to 

 8 inches in south-central and southwestern New Mexico. Over the same 

 area, the annual evaporation ranges from 40 to 60 inches. The distribution 



