THE EASTERN DESERT SCRUB. 169 



of the rainfall is peculiar to this general region in that less than a third of the 

 total usually falls in the first six months, while July, August, and September 

 receive more than half. On the higher levels near the mountains, May and 

 June are marked by more rain and the climate there becomes adapted to 

 grassland. Since relatively high ranges occur at intervals of 50 to 100 miles 

 from the Davis and Guadalupe Mountains on the east to the Santa Catalina 

 and Whetstone chains on the west, it is evident why the desert scrub con- 

 stantly mixes and alternates with grassland throughout the region. 



While factor studies of the dominants are lacking, their successional rela- 

 tions are brought into evidence repeatedly by changes in altitude, topography, 

 and soil. These have to do chiefly with water-content, but salinity must 

 frequently be taken into account as well. The basic sequence of the associa- 

 tion is shown by Prosopis, Flourensia, and Larrea, wherever ridges and valleys 

 occur. This is especially marked in the valley of the Pecos River from Fort 

 Stockton and Grandfall to the foothills of the Davis Mountains. The primary 

 sequence is Prosopis in the middle of the valley, a mixture of Prosopis and 

 Flourensia, in which Flourensia becomes more and more abundant until 

 Larrea appears as the slope begins, followed by mixed Flourensia-Larrea, 

 which becomes pure Larrea on the ridges or Larrea with sparse Flourensia 

 and Prosopis. More frequently, the valleys are shallower and poorly drained, 

 with Flourensia in the center, followed by a zone of Flourensia-Larrea on the 

 slope and of nearly pure Larrea on the ridge. In the case of valley washes, 

 where the soil is more or less sandy, Prosopis and Larrea exhibit a similar 

 relation from valley to ridge. This typical relation to water-content is also 

 found in sandy soils where Prosopis forms hummocks and dunes. As the soil 

 becomes more stable and the available water decreases, Flourensia enters and 

 finally Larrea, or where Flourensia is absent, Larrea enters directly. Of the 

 other dominants, Yucca radiosa and Acacia greggii most nearly resemble 

 Prosopis in their water use. The former has a wider margin of adjustment to 

 more xerophytic conditions, and the latter a narrower one. Yucca macrocarpa 

 is more xerophytic than Y. radiosa and is more often associated with Larrea 

 as a consequence. Ephedra torreyana makes much the same demands as 

 Prosopis and Yucca radiosa, often occurring in sandy soils with them, as well 

 as in gumbo valleys with Flourensia. Condalia and Koeberlinia usually occur 

 sparsely though generally in the harder soils. They have been seen but rarely 

 in dominant or pure communities, and such cases were in small closed valleys 

 or "swags, " often with a gypsum soil. Acacia constricta has in general some- 

 what higher water requirements than Larrea, while Fouquiera approaches 

 the latter closely in many places. Its preference, however, is for rocky slopes 

 in which the available water should be higher. 



The undershrub dominants are all more xerophytic than the shrubs with 

 which they are associated. This is indicated by their lower stature and the 

 location of their root-systems at a higher level. Gutierrezia and Isocoma are 

 nearly equivalent and are regularly associated with Prosopis, or a mixture of 

 it with Yucca, Atriplex, or Acacia, usually in sandy soil. They are essentially 

 corresponding species, occasionally occurring together, but found for the most 

 part in their respective associations. Zinnia and Krameria are typical asso- 

 ciates of Larrea. Parthenium is found more frequently with Larrea, but 

 occurs in various mixtures of the three primary dominants. 



