170 CLIMAX FORMATIONS OF WESTERN NORTH AMERICA. 



Prosopis is the most tolerant of salinity, though practically all the dominants 

 posssess this ability in a large measure. Prosopis occurs regularly with 

 Sporobolus airoides over great saline flats, especially in the valley of the Pecos. 

 It is constantly associated with Atriplex canescens on sandy dunes and hum- 

 mocks throughout. Typical areas of great extent occur from Sierra Blanca 

 to El Paso along the Rio Grande, and northward through the Jornada del 

 Muerto and the Tularosa Desert. Prosopis also associates with Atriplex 

 canescens and A. polycarpa on alkaline plains and occasionaly thrives in saline 

 meadows of Distichlis spicata. Its ability to withstand high concentrations 

 is most conclusively shown by its intimate association with Spirostachys 

 occidentalis and Dondia moquini in the Pecos Valley. Flourensia and Ephedra 

 are both more halophytic than Larrea as a rule. In the pure gypsum soils of 

 the Pecos Valley, Larrea is the first shrub to enter in the drier areas, and 

 Condalia the first in the swales. Both of these are followed by Prosopis, and 

 this by either Flourensia or Acacia. 



The serai sequence of Prosopis, Flourensia, and Larrea is confirmed by their 

 climatic relations in regions of greater rainfall, such as Texas. In the form of 

 savannah or forest-like thicket, Prosopis occurs generally in the western half 

 of Texas under a rainfall of 20 to 30 inches. Flourensia first appears at about 

 20 inches, but does not become dominant until a rainfall of 16 inches is 

 reached. Larrea appears last at a rainfall of about 16 inches, where it quickly 

 takes rank as a dominant. 



Prosopis also differs from Larrea and Flourensia is being a characteristic 

 dune-former, a habit doubtless related to its more mesophytic nature. Mes- 

 quite dunes and hummocks are a typical feature of the formation from the 

 Panhandle of Texas to the Salton Sea. They are due to the ability of Prosopis 

 to grow faster than the sand accumulates, a property almost wholly lacking 

 in both Larrea and Flourensia. It is shared in greater or less degree by 

 Atriplex canescens, Ephedra torreyana, Yucca radiosa, Artemisia filifolia, and 

 Dalea scoparia, with the result that one or more of these are usually asso- 

 ciated with Prosopis in such areas. The much wider range of the mesquite 

 is due to the fact that its sugary pods are eagerly eaten by animals, which 

 scatter the well-protected seeds. It suffers some disadvantage in that it is 

 often browsed by stock, while Flourensia is rarely eaten, and Larrea practically 

 never. This becomes a marked handicap where the kangaroo rats are abun- 

 dant, as they make their mounds in mesquite bushes almost exclusively and 

 feed upon both branches and roots. 



SOCIETIES. 



No study has yet been made of the seasonal aspects of the eastern desert 

 scrub and an adequate treatment of its societies is impossible. Since the 

 majority are alike for both associations, a fair understanding of them may be 

 obtained from the list on page 176, 



THE WESTERN DESERT SCRUB 



LARREA-FRANSERIA ASSOCIATION. 



Nature. — This association is regarded as the more typical one of the forma- 

 tion. The evidence of this is found chiefly in the larger number of dominants 

 and in the more extensive continuous areas occupied by them. The western 

 scrub also exhibits much more of the traditional appearance of desert, due 



