166 CLIMAX FORMATIONS OF WESTERN NORTH AMERICA. 



and upon the more uniform distribution of Prosopis in the Larrea-Flourensia 

 type. These differences are reflected in the figures showing the occurrence 

 of the four groupings. Of the other dominants, Atriplex is more important 

 in the west, and Yucca and Ephedra in the east, while Fouquiera varies but 

 little between the two. Parkinsonia, Cereus, and Dalea are confined to the 

 western, and Rhus to the eastern association. Of the undershrubs, Gutier- 

 rezia is much more important in one, and the corresponding genus, Isocoma, 

 in the other. Hilaria and Encelia are confined to the Larrea-Franseria asso- 

 ciation, Microrhamnus and Parthenium practically to the Larrea-Flourensia, 

 while Krameria and Zinnia show little difference. All in all, the evidence 

 supports the recognition of two recently differentiated but fairly distinct 



associations. 



Summary of dominants. 



Associations. — The desert scrub formation consists of two associations, 

 the Larrea-Flourensia and the Larrea-Franseria. In addition there is the 

 closely related community of Prosopis and Acacia, typical of the lower valley 

 of the Rio Grande and probably to be regarded as a subclimax. Besides the 

 differences in composition already noted, the two associations differ much in 

 structure. In the Larrea-Flourensia type, the three dominants are of nearly 

 equal importance, as is shown by their respective frequence, viz, Larrea 110, 

 Prosopis 110, and Flourensia 97, as well as by that of the four major groupings. 

 Moreover, the latter show that the dominants are frequently or regularly 

 mixed on more or less equal terms. Gutierrezia is the characteristic under- 

 shrub. In the Larrea-Franseria community the ground-tone is given chiefly 

 by Larrea. The uniform olive-brown color is less broken by Prosopis, except 

 where the two mix along the fine of contact in valleys and draws, or in sandy 

 soils. Flourensia is altogether lacking. Franseria, though an undershrub, 

 often ranks next to Larrea in importance, and gives a distinctive impress to 

 the community. In some places a similar role is taken by Hilaria, and not 

 infrequently Franseria, Hilaria, and Encelia are to be found mixed or alter- 

 nating with each other. A further distinction between the two associations 

 is found in the much greater frequency of Yucca and Ephedra in the Larrea- 



