FLOWERING PLANTS AND FERNS OF ARIZONA J 5 



microcarpa, Echinocereus engelmannii, and Opuntia leptocaul'is . Even 

 in such a relatively small area as the one under consideration several 

 species of cacti are conspicuous elements of the vegetation in certain 

 parts of the area and wholly absent from the rest of it. This is true of 

 Echinocactus lecontei, Opuntia echinocarpa, 0. acanthocarpa, and 0. 

 stanlyi. 



The habitats in which cacti are least abundant, and sometimes 

 absent over large areas, are the flood plains and the level or nearly 

 level areas of fine soil subject to sheet floods and consequent deposition. 



The flood plains of the succulent desert area were originally clothed 

 with heavy stands of large trees of mesquite or else with thickets of 

 Atriplex canescens (saltbush), and Sporobolus wrightii (sacaton). 

 The texture of the soil, the depth to ground water, and the relative 

 quantity of soluble salts seem to have determined this difference. 

 The largest tree of the desert lowlands is Populus jremontii (cotton- 

 wood), which usually occurs singly or in small groups but forms a 

 veritable forest for several miles along the Gila River in the vicinity of 

 Hayden and at several places along the Verde River. Other common 

 trees of streams and streamways are Salix gooddingii (willow) , Plata n us 

 occidentalis (buttonwood), Sambucus mexicana (elder or tapiro), and 

 several species of Fraxinus (ash). Among the shrubs common in 

 these situations are Baccharis glutinosa, Condalia lycioides, Chilopsis 

 linearis, and Hymenoclea spp. 



In both the larger and smaller valleys there are nearly level central 

 areas through which in many cases there is a very poorly defined 

 drainage system, frequently resulting in flooding during periods of 

 heavy rainfall. Such valleys are occupied by saltbush in some cases 

 and by Larrea in others, the only common associates of these plants 

 being mesquite, as a bush or small tree, Acacia greggii (catclaw), and 

 several species of Lycium. In soils of high salt content Suaeda 

 torreyana and Sarcobatus vermiculatus (greasewood) are common. 



The lower bajadas, as already stated, are the optimum habitat for 

 Larrea, which in some places covers them in nearly pure stands for 

 many square miles. The plants which most commonly break the 

 uniformity of these areas are Acacia constricta, A. greggiij Opuntia 

 fulgida, 0. phaeacantha, Koeberlinia spinosa (crucifixion- thorn), and 

 Lycium andersonii. 



A striking feature of the vegetation in the microphyll and succulent 

 deserts is the large number of short-lived herbaceous plants that 

 appear in the early spring and late summer, following the principal 

 rainy seasons. In favorable years these plants carpet the desert. 

 often completing their life cycle in 6 to 9 weeks. Brilliant displays 

 of color often result from the simultaneous flowering of large pure ov 

 mixed stands, especially on sandy soil. There are several hundred 

 species of ephemeral herbaceous plants, each of which is confined to 

 one of the two growing seasons. 



Great Basin Microphyll Desert. — Lines drawn north and east 

 from Flagstaff would define the northeastern corner of Arizona, in which 

 a high percentage of the surface is occupied by microphyll desert of the 

 type prevailing throughout the Great Basin. The range of elevations 

 is greater here than in the southern desert areas of Arizona (from 3,000 

 to 6,000 feet), the rainfall is less but somewhat more evenly distributed 

 through the year than in the south, and the vegetation is more desertic 



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