174 CLIMAX FORMATIONS OF WESTERN NORTH AMERICA. 



and the flood-plain are especially instructive. The former are regularly- 

 higher, due partly to the fact that they were taken on the north slope, but the 

 two sets are sufficiently alike to furnish a ready explanation of the charac- 

 teristic inversion by which Prosopis and Acacia greggii in particular occur in 

 the foothills (plate 39). 



Successional relations. — No quantitative studies have been made of the 

 actual or potential succession in the desert scrub climax. While the movement 

 is necessarily slow in a region so arid, there can be no question of the general 

 occurrence of succession in flood-plains, especially in salt-spots, dunes, and 

 washes, and in secondary areas. Even in what seem stable areas, the move- 

 ment toward the Larrea consociation as the final climax dominant is clear. 

 Spalding (1909 : 16, 30) has noted this tendency on Tumamoc Hill as well as 

 in the wash, and it can be discovered wherever topographic or biotic factors 

 produce bare areas or otherwise modify existing ones. 



The type of succession is peculiar to desert. Succession is regularly meso- • 

 tropic, i. e., developing in hydrophytic or xerophytic habitats which con- 

 stantly become more mesophytic to the final climax (Clements, 1916 : 182). 

 In the desert scrub the seres are all xerotropic, in that the earlier stages are 

 less xerophytic than the climax. This results in the sere being exceptional in 

 having a small number of stages and a slow rate of movement. Moreover, 

 while the topographic relations of erosion and deposition to the climax plain 

 are essentially as in ordinary succession, the climax itself is xerophytic. Con- 

 sequently the Larrea plain is to be regarded as the threefold baseline for 

 topography, climate, and succession, toward which all the others are tending 

 slowly but nevertheless surely. In a region where permanent streams and 

 lakes are all but impossible, the hydrosere is absent or is quickly converted 

 into a halosere (MacDougal, 1914). The latter begins in a soil more arid than 

 that of the climax, but its position seems normally to result in a habitat which 

 is less xerophytic, since Prosopis usually enters before Larrea. Apart from 

 this, the hill and valley communities are both less xerophytic than the 

 Larrea plains and hence show similar sequences. 



Of the valley dominants, Prosopis is the least xerophytic. This is shown 

 by its form, which is often that of a tree, capable of forming continuous wood- 

 lands, and by its association with such mesophytic river-bank species as 

 Fraxinus, Sapindus, etc. Acacia greggii follows Prosopis closely in its water 

 requirements, and is followed in turn by Condalia lycioides and Acacia con- 

 stricta. While their association is much less frequent, Olneya and Parkin- 

 sonia torreyana appear but little more xerophytic than Prosopis, and this is 

 likewise true of Dalea spinosa. The three lowland chollas, Opuntia fulgida, 

 0. f. mamillata, and 0. spinosior have a wide range of equivalence. In general 

 they are most abundant in the ecotone between Prosopis and Larrea, but 

 they extend well into both consociations, especially the latter. The three 

 cacti are often the dominants in Larrea plains on which subaerial processes are 

 active. Yucca, Ephedra, and Koeberlinia resemble Prosopis more nearly in 

 their demands, but the latter is often found in the lower Larrea levels also. 

 Atriplex canescens and A. polycarpa are regularly associated with Prosopis, 

 perhaps largely because of the ability of the latter to withstand salt. 



The dominants of the foothills and the upper bajadas approach Larrea 

 in requirements, as indicated by the frequence of their association. Here the 



