162 CLIMAX FORMATIONS OF WESTERN NORTH AMERICA. 



fasdcukUum has a wider range of adjustment, from climax conditions with 15 

 to 20 inches of rainfall to desert ones at 5 to 10 inches, where it is represented 

 by Eriogonwn f. polifolium. Salvia apiana seems the least mesophytic and 

 consequently is more or less subclimax, growing on rocky slopes or in other 

 more or less disturbed areas. 



THE DESERT SCRUB CLIMAX. 

 LARREA-PROSOPIS FORMATION. 



Nature. The desert scrub, or mesquite, resembles sagebrush and chaparral 

 in both appearance and character. As the name indicates, it is distinctly the 

 most xerophytic of the three, reaching its best development in a rainfall of 

 5 to 12 inches. The dominants are bushy shrubs, 3 to 6 feet high for the most 

 part. The chief exceptions are Prosopis and Acacia, which often form trunks 

 and become small trees on flood-plains and in other favorable situations. 

 Most of the dominants possess the abihty to produce root-sprouts, though to a 

 smaller degree than the chaparral. To this they doubtless owe the many- 

 stenmied habit as well as their dominance. With the exception of the typical 

 dominant, Larrea, most of the species are deciduous, though many are imper- 

 fectly so and a number have evergreen stems or branches, as in Opuntia, 

 Parkinsonia, and Koeberlinia. The characteristic feature which distinguishes 

 the desert scrub most readily from chaparral and sagebrush is its very open 

 structure. The bushes usually stand 10 to 30 feet apart in t3T)ical situations, 

 and it is altogether exceptional that the crowns touch each other, even in the 

 case of the less xerophytic Prosopis and Acadia. The spacing is evidently a 

 consequence of low rainfall and resultant low water-content, necessitating a 

 large area for adequate absorption by the roots. As would be expected, this 

 seems to be correlated with the root habits of the various dominants. The 

 individuals of Larrea are more widely separated than those of Prosopis, by 

 reason of a shallow root system as well as a lower chresard. This produces 

 three results generally typical of the desert scrub, all due to the large intervals 

 in which more or less water is available superficially. The first is the presence 

 of tall undershrubs which occupy the intervals in greater or less abundance, 

 such as Franseria, Isocoma, Parthenium, Gutierrezia, Hilaria, etc. A second 

 consequence is the development of a characteristic population in the intervals 

 during the winter rains in February and March. A third result, which has 

 an important bearing upon the relation of desert scrub to contiguous forma- 

 tions, especially the grassland, is the readiness with which it forms parks or 

 savannahs. Such parks are an especial feature of the Southwest, where they 

 mark the broad transition between the desert scrub and the grassland. 



With respect to systematic relationship, the desert scrub is less homogeneous 

 than chaparral or sagebrush. The three chief dominants belong to as many 

 different families, Larrea to the Zygophyllaceae, Prosopis to the Mimosaceae, 

 and Flourensia to the Asteracea^. The Aster aceae and Leguminosae are most 

 important, and the Rhamnaceae next, while the Liliaceae are represented by 

 Yucca, the Gnetaceae by Ephedra, the Chenopodiaceae by Atriplex, and the 

 Poa/xae by a shrubby grass, Hilaria rigida. 



This particular type of scrub is known by various names in the different 

 sections. In Texas it is called chaparral or mesquite, the latter being the 

 usual name where Prosopis is prominent or predominant. From New Mexico 



