FIG. 26-1 Faclatlons of the desert biome in North America 

 (after Axelrod 1950, Jaeger 1957). 



in some localities. Sagebrush, Artemisia tridentata, 

 often occurs in nearly pure stands, but where grazing 

 is limited, several species of perennial grasses, espe- 

 cially wheat grass, Agropyron spicatum, become in- 

 termixed to form a continuum leading into the bunch 

 grass prairie to the north. Only a few opuntia cacti 

 occur. Sagebrush is widely distributed as a subcli- 

 max, because of overgrazing, in the bunch-grass 

 prairie and short-grass plains. Various species of 

 Artemisia also extend into central and southern Cali- 

 fornia and together with associated species have been 

 called coastal sagebrush. It may be subclimax to 



chaparral. The original vegetation of these two asso- 

 ciations has become greatly modified as result of 

 overgrazing by domestic animals and increased ero- 

 sion with reduction in grasses and edible shrubs and 

 introduction of exotic species (Fautin 1946, Cottam 

 1947, Billings 1949). 



A tropical thorn forest occurs on the west coast 

 of Mexico, in northern Venezuela, and in other scat- 

 tered localities. It is made up of a dense scrubby 

 growth of small, often thorny and leguminous trees. 

 Cacti are common. Some authors distinguish a thorn 

 forest and a short-tree forest, but there is consider- 

 able intergrading between the two (Gentry 1942) as 

 well as with the tropical deciduous forest. 



PALEO-ECOLOGY 



During early Tertiary, the present desert 

 regions in North America were largely dominated 

 by tropical and warm-temperate forests. Following 

 Eocene, rainfall gradually decreased, and forests 

 were replaced first by grassland, then by desert. Des- 

 ert began to appear during middle Tertiary on the 

 lee side of high mountain ranges, but did not become 

 extensive until middle and late Pliocene time. The 

 present deserts in North America are therefore of 

 comparatively recent origin. 



Desert plants have apparently originated through 

 gradual adaptation to arid climates of more hardy 

 species belonging to all three Tertiary floras. Ar- 

 teinisia, Atriplex, Eurotia, and Sitaeda of the two 

 Great Basin associations are thought to have Arcto- 

 tertiary affinities. Related species in the same genera 

 occur in the Eurasian deserts at the present time. 

 Species of the desert scrub appear to be derived from 



FIG. 26-2 Joshua trees Int, 

 spersed through low shrubs 

 of the Mohave desert in 

 California (courtesy U.S. 

 Forest Service). 



'^..V-f 



T, ^ ;ji. ! i^A. 



334 Geographic distribution of communities 



