THE DESERT SCRUB CLIMAX. 167 



Flourensia type. Over much of the area one or both will regularly occur in 

 abundance with Prosopis or Flourensia, though rarely with Larrea where it 

 is most typical. 



_ Their relation to the subclimaxes of rocky slopes and ridges marks another 

 difference between the two associations. In the east the subclimax consists of 

 Yucca, Agave, Dasylirium, and Fouquiera chiefly, and this explains why Yucca 

 radiosa and macrocarpa are such frequent constituents of the scrub below. In 

 the west Yucca is largely confined to the grasslands, and the subclimax con- 

 sists primarily of Parkinsonia, Cereus, and Fouquiera. All of these mix with 

 Larrea to a considerable extent, and are sometimes found in the heart of the 

 association, where they are often to be regarded as relicts. Another dis- 

 tinctive feature of the Larrea-Franseria type is the presence of the cylindric 

 Opuntias, such as 0. fulgida, 0. spinosior, 0. versicolor, etc. While Opuntia 

 occurs sparsely in the eastern association, it has nowhere been found in the 

 abundance which characterizes it in Arizona. Here the species of Opuntia 

 make important communities on the lower bajada slopes with Larrea or in 

 the broad washes with Prosopis. 



Finally, a unique feature of the Larrea-Franseria scrub is the development 

 of a more or less continuous cover of winter annuals from January or February 

 to April. This is the direct consequence of a secondary maximum of rainfall 

 at this time, and is very similar to what occurs in southern California. This 

 transitory community of annuals is little if at all developed in Texas and New 

 Mexico. Here the rainfall from January to April is less than 15 per cent of 

 the annual, while in Arizona and southeastern California it is 30 to 60 per 

 cent of the total. The distribution of the rainfall seems also to explain the 

 change from one association to the other in eastern Arizona. New Mexico and 

 western Texas receive 60 to 75 per cent of their annual rainfall between 

 April 1 and September 30, while the Larrea-Franseria region of Arizona and 

 California receives but 20 to 50 per cent during the same period. Further- 

 more, the greater tendency of the eastern type to form savannahs is explained 

 by the fact that the seasonal distribution of the rainfall is practically the same 

 as that for grassland. 



Relation to other formations.— The chief contact of the desert scrub is with 

 the grassland formation. This is the case in trans-Pecos Texas, New Mexico, 

 and Arizona, where the contact is with the desert plains association. This is 

 especially true of elevated plains and of bajadas with northerly slopes. On 

 slopes with southerly and westerly exposure or rocky surface the scrub is 

 usually separated from chaparral or woodland by a broad band of the Yucca- 

 Agave community in the east or one of Parkinsonia-Cereus-Fouquiera in the 

 west. The lines of contact are often broad ecotones and, in the case of the 

 grassland, they regularly develop into parks of scrub and grass several miles 

 wide along the mountain ranges and hundreds of miles in length over the 

 southern Great Plains. In Nevada and adjacent Arizona and Utah, the 

 Larrea scrub yields to the sagebrush formation, and in California it lies in 

 touch with the sagebrush or chaparral, or less frequently with woodland. At 

 the western edge of the Edwards Plateau in Texas, desert scrub meets the 

 chaparral of oak, and Prosopis becomes the typical shrub of the level valleys 

 and washes throughout the region. 



