

LOWER SONORAN ZONE. 21 



The most widely and generally diffused shrub of the desert is the 

 creosote bush, Larrea tridentata. It occurs from the bottom of Death 

 Valley, at an altitude below sea-level, to about 1,500 meters on the 

 mountains, at most points falling short of the lower limit of timber. 

 A shrub almost as widely distributed, and with practically the same 

 altitudinal limits, is Franseria dumosa. The upper limit of the belt of 

 these two shrubs is only a little below the lower limit of timber, and 

 has therefore been adopted as a convenient unit in the geographic 

 botany of the region. The area outlined by these species has been 

 correlated by Dr. Merriam with simitar areas in adjacent regions, and 

 the name Lower Sonoran applied to the zone thus constituted. In other 

 words, the Lower Sonoran zone, which is a broad transcontinental belt, 

 coincides in the desert region of southern California approximately with 

 the distribution of Larrea tridentata and Franseria dumosa. 



In the remarks that follow concerning this zone, reference is made 

 solely to that section of it which lies in ultramontane California ami in 

 adjacent Nevada, Utah, and Arizona; that is, within the limits of the 

 desert. 



The varying physical features of the region make it necessary to 

 note the different plant habitats which the Lower Sonoran zone affords. 

 The wide mesas, which constitute the gentler valley slopes and the 

 greater portion of the broad desert basins, make up the principal area 

 of the region. The soil of the mesas is composed of gravel, or even 

 bowlders in the more inclined parts, while in portions more nearly hori- 

 zontal and more remote from the mountains it is of a fine and even 

 quality. In various sections it is prevailingly calcareous, argillaceous, 

 or siliceous, according to the nature of the rocks from which it was de- 

 rived. The vegetation of the mesas is affected neither by the sheltering 

 influence of rocks nor by springs or other immediate sources of water 

 but is exposed to characteristic desert conditions. Constituting, there- 

 fore, the greater part of the whole flora geographically, and being fairly 

 typical of the region, it requires the first consideration. 



Among the shrubs of the mesa, Larrea tridentata and Franseria 

 dumosa are, as already stated, the commonest; but at various points 

 some one or more of several other species named in the following list 

 are often abundant: 



Acamptopappus splimroceplialuB. JTymenoclca salsola. 



Amphiachyris fremontii. Krameria parvifolia. 



Aster mohavensis. Lytium andersonii. 



Atriplex confertifolia. Opuntia basilaris. 



Atrip! ex hymenelytra. Otwntia echinocarpa 



Bebbiajuncca aspera., Salazaria mexicuna. 



Cassia armata. Telradymia comosa. 



Cerem en-gelmanni. Yucca arborescens. 



MeMnocactm polyrphalus. Yucca macrocarpa. 

 Ephedra californiea. 



