THE WESTERN DESERT SCRUB. 



171 



largely to the abundance of cacti in it. In this respect it resembles closely 

 the deserts of Mexico, of which it is probably a continuation. At present 

 it likewise differs from the eastern type in general absence of grasses, though 

 this may be largely the work of animals. While Larrea is still the most typical 

 dominant, the community shows extensive differentiations in which it is 

 nearly or entirely lacking. However, it also appears to have a wider range of 

 adaptation and often becomes a shrub 10 to 15 feet tall in washes. This 

 seems to be connected with the greater abundance of tall shrubs or low trees, 

 such as Parkinsonia, Olneya, and Dalea. As already indicated, a characteristic 

 feature is the great development of communities of low winter annuals, the 

 many species of which cover the ground with a brilliant carpet (plate 37) . 



Extent. — The eastern limits of the Larrea-Franseria community are indi- 

 cated by the Galiuro, Whetstone, and Huachuca Mountains in Arizona. It 

 extends northward in the valleys of the San Pedro, Gila, Salt, and Verde 

 Rivers to find its northern limit along the mountains of central Arizona. On 

 the west the desert scrub occupies the Colorado Desert and reaches into south- 

 western Utah and southern Nevada, though greatly reduced in number of 

 dominants. In California it is the climax vegetation of the Mohave Desert 

 and Death Valley, though much of the area is covered with the halophytic 

 sjbclimax of Atriplex and related dominants. West of the Salton Basin 

 several of the dominants reach the lower slopes of the San Jacinto and Laguna 

 Mountains. Desert scrub is the most important association throughout 

 Lower California, while in Mexico proper it occurs as far south as Zacatecas 

 and San Luis Potosi. The occurrence of both Prosopis and Larrea southward 

 to Argentina indicates a still greater range for this or some related associa- 

 tions. 



DOMINANTS. 



Shrubs: 



Larrea mexicana. 

 Prosopis juliflora. 

 Acacia constricta. 

 Parkinsonia microphylla. 

 Acacia greggii. 

 Opuntia fulgida. 

 Opuntia f . mamillata. 

 Opuntia spinosior. 

 Parkinsonia torreyana. 

 Parkinsonia aculeata. 

 Dalea spinosa. 

 Olneya tesota. 

 Cereus giganteus. 

 Fouquiera splendens. 

 Celtis pallida. 

 Opuntia versicolor. 

 Opuntia arbuscula. 

 Condalia lycioides. 

 Condalia spathulata. 

 Simmondsia californica. 

 Atriplex canescens. 



Shrubs — continued. 



Atriplex polycarpa. 

 Prosopis pubescens. 

 Yucca radiosa. 

 Koeberlinia spinosa. 

 Mimosa biuncifera 

 Ephedra trifurca. 

 Ephedra nevadensis. 

 Salazaria mexicana. 

 Dalea emoryi. 

 Dalea schottii. 

 Lycium spp. 

 Cereus thurberi. 

 Adelia phyllarioides. 

 Holacantha emoryi. 

 Canotia holacantha. 



Half shrubs: 



Franseria dumosa. 

 Franseria deltoidea. 

 Isocoma coronopifolia. 

 Isocoma c. hartwegii. 



Half shrubs — continued. 

 Isocoma veneta. 

 Opuntia discata. 

 Opuntia chlorotica. 

 Zinnia pumila. 

 Hymenoclea salsola. 

 Calliandra eriophylla. 

 Chrysoma laricifolia. 

 Lippia wrightii. 

 Baccharis wrightii. 

 Trixis californica. 

 Opuntia phaeacantha. 

 Opuntia engelmannii. 

 Encelia farinosa. 

 Encelia frutescens. 

 Krameria glandulosa. 

 Hilaria rigida. 

 Psilostrophe cooperi. 

 Yucca baccata. 

 Gutierrezia sarothrae. 

 Bebbia juncea. 

 Parthenium incanum. 



In addition to the above, a large number of other shrubs, half shrubs, and 

 succulents occur frequently but sparsely, or in occasional clan-like groups. 

 Others are dominant at higher levels, such as species of Agave, Dasylirium, 

 or at lower ones, Atriplex, Hymenoclea, etc. The relation of all of these is 

 either actually or potentially successional, or they are of minor importance 

 and can not be further considered in a brief account. 



