CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 285 



tures are not as high as farther south, and frosts are common. The 

 combination of lower temperatures, lower evaporation rates, and 

 better distribution of rainfall explains the use of the term, "semi- 

 desert;' for the area. Likewise distinctive is the growth form of the 

 dominants, made up largely of shrubby chenopods and composites, 

 which further supports the desirability of its recognition as a for- 

 mation distinct from the scrub of the southern desert. 



-"I 



'V-S4^^^^*^-.-^- : i^ / >^&.^Zte./.l»f*.A*:~^><-x~*-S,J* f ~M.^*«r*X 



Fig. 150. Typical dry desert expanse with shadscale (Atriplex conferti- 

 folia) dominance. Mineral County, Nev. Characteristic gravelly desert pave- 

 ment shows here .— Photo by W. D. Billings. 



The two major communities are simple, with few dominants in 

 each, and often extend uninterrupted for miles. The sagebrush as- 

 sociation is dominated by Artemisia tridentata (common sage- 

 brush), which is climax in the northern portion of the Great Basin 

 or at relatively high altitudes. The shadscale association, with shad- 

 scale (Atriplex confertifolia) and bud sage ( Artemisia spines c ens) 

 as its important species, ranges through the south and at low alti- 

 tudes. In its northern and eastern distribution, shadscale is found 

 on heavy lowland soils containing some alkali, but, to the south, it 

 is climax on gray desert soils with a shallow carbonate layer and 

 regardless of salts. Sagebrush tends to occur on brown soil, either 



