THE WESTERN DESERT SCRUB. 175 



sequence is nore difficult to determine because of the irregular topography 

 and the confusing effect of temperature. Moreover, the hotter and drier 

 southerly slopes are younger and less stable than the northerly ones, thus 

 further complicating the problem. In general, it may be said that Fouquiera 

 stands nearest Larrea, Cereus comes next, and Parkinsonia microphylla is 

 last. Though Parkinsonia mixes with Larrea in scores of places, this is largely 

 due to the wide adaptation of the latter. 



The occurrence of the halfshrub dominants is correlated more or less closely 

 with that of the shrubs, and they exhibit a similar zonation. Isocoma is 

 habitually associated with Prosopis, but occurs also in the lower Larrea areas. 

 Opuntia discata is an associate of the cylindric Opuntias and hence has a wide 

 range. The most typical halfshrub associates of Larrea are Franseria dumosa, 

 Hilaria rigida, Krameria glandulosa, Zinnia pumila, and Psilostrophe cooperi. 

 Franseria deitoidea characterizes the Larrea-Parkinsonia ecotone, while 

 Encelia farinosa is typical of the Parkinsonia-Fouquiera community. Lippia 

 wrightii, Chrysoma laricifolia, Parthenium incanum, and Bebbia juncea are 

 usually restricted to the latter also. Calliandra eriophylla is typical of the 

 foothill areas of Prosopis and Acacia and reaches its greatest abundance on the 

 Prosopis savannahs. 



Root relations. — Cannon (1911) has made a comprehensive study of the 

 roots of desert plants, in which he recognizes three types of root systems. The 

 generalized type has the tap-root and laterals both well developed, while of 

 the two specialized types, one has emphasized the tap-root and the other the 

 laterals. The dominants with generalized root systems are Larrea, Prosopis, 

 Acacia, Parkinsonia, Fouquiera, Celtis, Lycium, Franseria, and Encelia. 

 Those with a well-developed tap-root are Condalia, Ephedra, and Koeberlinia. 

 Practically all the cacti have superficial roots with prominent laterals, though 

 the arborescent Opuntias approach the generalized type. As would be 

 expected, the roots of annuals are the most superficial, penetrating the soil 

 rarely more than 8 inches, and with the maximum development at about 2 

 nches. 



In general, there is a tendency to form three layers of roots in the soil, the 

 uppermost of annuals, followed closely by the root-layer of the cacti, and a 

 much broader deep-seated layer composed of tap-root systems, with or without 

 prominent laterals. The tendency to place roots at different levels minimizes 

 the direct competition of the dominants, as Cannon has pointed out (1. c, 64). 

 This is especially effective in the case of the superficially rooted annuals and 

 cacti. The necessary compensation for the period of seasonal drouth is 

 secured by drouth evasion in one and drouth resistance in the other. The 

 wide spread of laterals in many of the dominants explains the characteristic 

 open spacing of the desert scrub and the bush-like habit. The effect of a 

 larger water supply is seen especially on the flood-plain, where Prosopis, 

 Acacia, and Parkinsonia often become trees, and even Larrea may grow to a 

 height of 15 feet or more. This is shown even more strikingly along the mar- 

 gins of roads, where the shrubs become remarkably vigorous as a result of the 

 increased runoff and the freedom from competition. This effect is universally 

 exhibited by the halfshrub, Isocoma, in which the plants along the roadside 

 are often twice as tall as those in the midst of the community. Its response 

 was especially graphic in 1918, when the roadside plants leafed out fully while 

 those of the mass were still leafless (plate 40). 



