The Origin of the Desert Climax and Climate 99 



Prosopis and Acacia are postclimax, being found typically in 

 washes or valleys along with other treelike species, while Bac- 

 charis, Hymenoclea, and Pluchea are essentially serai. The ma- 

 jor dominants of the Basin sagebrush are those listed in the 

 first column; Sarcobatus constitutes the subclimax of the halo- 

 sere, and the remaining species belong to the Coastal sagebrush. 

 As noted earlier, most of the chaparral dominants are north- 

 eastern, but Arctostaphylus and Ceanothus are derived from the 

 Pacific Coast. 



Tree Transads 



Because of their higher water requirements and more direct 

 exposure to the influence of a changing climate, trees present few 

 simple transads. But three species of this group occur in the 

 desert at this time, namely, Salix lasiolepis, exigtia, and Populus 

 fre?nonti; the first two are usually shrubs, and all are confined 

 to valleys or washes with higher water-content than that of the 

 desert proper. For similar reasons, the same genera furnish most 

 of the transads no longer present in the desert, such as Salix lev- 

 igata, nigra, fttwiatilis, and Populus trichocarpa. The other 

 member of this group is Celtis mississippiensis in the variety 

 reticulata. 



Phylads and Genera 



TREES 



In addition to single species that cross the desert, there is a much 

 larger number of phylads, that is, phyletic lines of closely related 

 species, that stretch from the Atlantic or the Mississippi Valley 

 to the Pacific. In some of these the relationship is so intimate 

 that the ecologist would regard the forms concerned as sub- 

 species or varieties of a single linneon. This is true of the phylad 

 Quercus minor-gambeli-garryana, the last two members of 



