or iiiorc. DifYcreiiccs between tlaily maxima atul 

 minima are greater than in any other biome. Frosts 

 are limited to mid-winter ; the frost-free period aver- 

 ages more than 280 days. Frost and snow are com- 

 mon, liowever. in tiie Great Basin (Kincer 1941); 

 consequently, the Great Basin area is sometimes 

 called the cold desert in contrast to the hot desert 

 adjacent to the south. 



The Great Basin is not a single large basin : 

 rather, it is broken up into a number of small ones 

 separated by low mountain ranges running in a 

 north-south direction and seldom exceeding heights 

 of 1800 m (6000 ft). These ranges support pifion- 

 juniper woodland, ^^'ater drains from the surround- 

 ing hills into these small basins from w-hich there are 

 usually no outlets. High rates of evaporation make 

 the basin lakes very salty or may dry them up to 

 produce alkaline flats. 



A characteristic topographic feature of the desert 

 is the alluvial fan of erosion products washed down 

 mountain slopes by the torrential rains. Broad flat 

 basins occur between adjacent mountains. Extensive 

 sand dunes occur in some portions of the desert, and 

 sand and dust storms are spectacular features. 



VEGETATION 



Desert bushes and shrubs in North Amer- 

 ica are seldom more than 1-2 meters high and are 

 spaced 3-10 meters apart (Fig. 26-2). Joshua-trees, 

 paloverdes, and saguaros are, however, more con- 

 spicuous. The shrubs seldom form a canopy except 

 along washes. The intervening ground between the 

 shrubs is usually a wind-swept desert pavement of 

 either fine texture soil, gravel, or rock, and always 

 with very little humus. The shrubs and bushes have 

 shallow, wide-spreading, many-branched root sys- 

 tems adapted quickly to absorb any surface moisture. 

 There is very little moisture in the subsoil. Stems 

 and branches often bear prickles or thorns and inter- 

 twine to form a dense tangle. A rich variety of 

 thorny, succulent cacti occurs in the Western Hemis- 

 phere only, and is divisible into tree, cholla, and 

 barrel types. Between the shrubs, a few short annual 

 grasses may grow, but after rains, the ground often 

 becomes thickly covered with a carpet of flowers and 

 grasses that quickly mature, seed, and then disappear 

 in the dry weather that follows. 



These desert plants have sclerophyllous adapta- 

 tions to retard transpiration and survive long periods 

 of drought. Foliage becomes greatly reduced, even 

 absent altogether, during long periods ; stems con- 

 tain the chlorophyll necessary to carry on photo- 

 synthesis. Cell walls are thick, highly ligneous, and 

 have thick cuticles. The cell sap increases in osmotic 

 pressure and hydrophilous colloids. Cacti store con- 



sidcrahle water in llieir stems as a reserve for use 

 when there is no water in the soil. Many other kinds 

 of adaptations occur in these xerophytes (Weaver 

 and Clements 1938, Zohary, in Cloudsley-Thompson 

 1954). 



Biotic succession is not conspicuous in the desert 

 i)ecause of the low rate of reactions by organisms 

 ujwn tiie habitat (Shreve 1942). When the vegeta- 

 tion is disturbed it is usually replaced directly by 

 the same type without intervening serai stages 

 (Muller 1940). Physiographic succession is evident, 

 however, depending upon distances from water chan- 

 nels, differences in elevation, leaching of salts out 

 of the alkali flats, and in sand dune areas. There is 

 considerable variation in the distribution of different 

 species because of local difTerences in the chemical 

 and physical nature of the soil, soil moisture, and so 

 forth (Shantzand Piemeisel 1924). 



PLANT ASSOCIATIONS IN 

 NORTH AMERICA 



Covillea-Franseria association (desert 

 scrub) : Creosote bush, Covillea glutinosa, is generally 

 distributed and with bur sage, Franseria, sometimes 

 forms a monotonous, uniform growth on the flat 

 inter-montane plains, relieved only by the larger aca- 

 cias, saguaros, paloverdes, and mesquites along the 

 washes. The richest variety of vegetation is on the 

 outwash plains and lower mountain slopes, where 

 there is greater penetration and retention of soil 

 moisture. The desert scrub has three faciations 

 (Shreve 1942, Axelrod 1950). The Mohave desert 

 to the west is a rolling plain with a monotonous uni- 

 form cover of low shrubs, interspersed conspicuously 

 with the curious Joshua-tree. The Sonoran desert. 

 sometimes subdivided into Colorado and Arizona 

 deserts (Benson and Darrow 19-14), or as many as 

 six sub-units (Jaeger 1957), is much more diversi- 

 fied, with tall shrubs, trees, and succulent cacti of 

 many forms, especially along the washes, and a few 

 grasses. The Chihuahuan desert to the east is almost 

 completely separated by mountain ranges from the 

 rest of the desert, and some of the western species are 

 replaced by new ones that come in. The yucca-like 

 sotol is conspicuous (Jaeger 1957). 



Atriplex-Artemisia spinescens association 

 (shadscale association) ; Artemisia tridentata- 

 Agropyron association (sagebrush association) : 

 Shadscale, Atriplex conjertijolia, and bud sage, Ar- 

 temisia spinescens, as well as other small (less than 

 1 m high), widely scattered, more or less spinescent, 

 microphyll shrubs are dominant in the southern part 

 of the Great Basin and have contact with desert 

 .scrub. Greasewood and a few grasses are important 



Desert biome 333 



