14 MISC. PUBLICATION 42 3, U. S. DEPT. OF AGRICULTURE 



in diameter, in the midst of which the soil is usually built up several 

 feet above the surrounding level. All of the visible branches of such a 

 clump are small, but underground they are connected by large limbs, 

 and in fact the whole is a single large, nearly buried, tree. The 

 inhabitants of the region often dig for wood. 



Arizona Succulent Desert. — This type of vegetation extends 

 south into Mexico, and its rich display of succulent plants is scarcely 

 equaled elsewhere in the United States. In the open plains and low 

 mountains of southwestern Arizona there is a very gradual transition 

 from the mierophyll desert to the succulent desert. The broad plains, 

 or lower bajadas, are in fact often poor in succulents in regions where 

 the upper bajadas and low hills are covered with heavy stands of them. 

 From its poorly defined western edge the Arizona succulent desert 

 extends eastward and northward to a very irregular boundary, which 

 lies between 3,000 and 3,500 feet elevation in southern Arizona, but 

 somewhat lower in Yavapai and Mohave Counties (pi. 2). 



The matrix of the vegetation is Larrea, usually accompanied on 

 coarse soils by heavy stands of Franseria deltoidea. Over extensive 

 plains Larrea retains its dominance in communities where the number 

 of succulents is large. It is only on upper bajadas, and particularly 

 those of granitic mountains, that Larrea is replaced by Cercidium 

 microphyllum and C.floridum, Acacia constricta, Olneya, and Prosopis, 

 plants which in turn form the matrix for the still heavier stands of 

 cacti clothing the upper bajadas and the lower hills. Only rarely and 

 very locally do the cacti form even as much as 75 percent of the plant 

 population. The heaviest stands of either the arborescent opuntias, 

 O.fulgida, O. spinosior, O. versicolor, or the pricklypears, O. engelmannii 

 and O. phaeacantha, are never without their accompanying non- 

 succulent shrubs. The local distribution of shrubs is relatively uni- 

 form and bears a more or less obvious relation to the topographic and 

 soil features. The occurrence of the heaviest stands of cacti appears, 

 however, to be random and fortuitous. This circumstance undoubt- 

 edly has its basis in the vegetative multiplication to which so many 

 individuals of Opuntia owe their existence, as well as to the partial 

 independence of soil-moisture conditions, which is given them by their 

 water-storing tissue. 



The Arizona succulent desert is given its most distinctive character- 

 istic by the abundance and variety of the cacti, but it is also marked 

 by a large number of plants that differ greatly from one another in 

 form, mode of branching, character of foliage, and seasonal habits. It 

 is on the upper bajadas, the hills, and the lower slopes of the mountains 

 that this type of vegetation is seen in its best development (pi. 3). 

 The commonest perennials found throughout the area, in the approxi- 

 mate order of their abundance, are: Larrea tridentata, Franseria 

 deltoidea, Cercidium micro phyllum, Acacia constricta, Opuntia fulgida, 

 O. spinosior, Fouquieria splendens, Prosopis juliflora, Acacia greggii, 

 Opuntia engelmannii, Cereus giganteus, Opuntia phaeacantha, Celtis 

 palUd-a, Lyewm andersonii, Simmondsia chinensis, Opuntia versicolor, 

 iCercidium floridum, Olneya tesota, Jatropha cardiophylla, Krameria 

 parrifolia, and Echinocereus engelmannii. 



The smaller perennials persist from season to season by survival of 

 roots or larger branches. They form a conspicuous element of the 

 vegetation in favorable localities that have not been heavily grazed. 

 Amono" the abundant and characteristic smaller cacti are Mammilla ria 



