CHAPTER III. i 



THE ARBOREAL VEGETATION OF THE MONSOON DESERT. 



A detailed description of the vegetation of Ai-izona and New Mexico would be out of 

 place in the present volume. Not only does it lie beyond the writer's field of knowledge, 

 but it has been ably done in AlacDougal's volume on the Deserts of North America,* and 

 in other publications of the Desert Laboratory. The purpose of this chapter is merely to 

 call attention to one of the pecuUar results of the twofold rainy season of the southwest. 

 The result is much more obvious in the warm southern parts of Arizona than in the regions 

 of greater altitude. Hence I shall confine myself chiefly to that region. 



To one famiUar with the deserts of the Eastern Hemisphere or of other parts of North 

 America, the vegetation of the less elevated portions of southern Arizona, northern Sonora, 

 and, to a less extent, southern New Mexico is surprising. The annual rainfall at Tucson 

 at an elevation of 2,300 feet amounts, it will be remembered, to about 12 inches. The 

 amount elsewhere may be seen by referring to figure 1 on page 10. Few parts of southern 

 Arizona and New Mexico have more than 12 inches of rain, most of the country has less, 

 and Yuma, as is well known, has only about 3 inches. So far as habitabiUty is concerned 

 the country is genuinely a desert. There are several places in Arizona, especially in the 

 southwestern part of the State, where the whole of Massachusetts with its 3,500,000 

 people could be set down without disturbing a single farm, or cattle ranch, or any other 

 place where people are making a living from the soil as distinguished from mines and 

 railroad enterprises; the same is true of Sonora. Nevertheless the general aspect of the 

 country is green, even in the dry seasons. Most regions having a rainfall of only 10 or 12 

 inches are bare and treeless, as may be seen in Utah or Nevada, or in Syria and Persia. 

 There arboreal vegetation, away from the water-courses, is almost entkely restricted to 

 insignificant grayish-green forms Uke sage-brush. In a measure this is due to unrestricted 

 grazing, but by no means entirely; for in places where flocks never graze the bushes are 

 small and rare and trees are unknown. In the southern part of Arizona, on the contrary, 

 bushes are found almost everywhere except on the mountain sides, and the aspect of the 

 desert is distinctly arboreal and verdant. Thousands of square miles are covered with the 

 useless creosote bush, a shrub which grows to a height of from 4 to 6 feet or more, and is 

 thickly studded with small guimny leaves. The individual bushes are commonly 10 or 15 

 feet apart, and the ground between them is often bare or covered only with a short-hved 

 growth of grass. Nevertheless, the bushes are close enough to one another to give a pro- 

 nounced green color to the landscape as a whole. In addition to the creosote bush there 

 are numerous larger species of bushes and trees. The most prominent is the mesquite, 

 which sometimes grows to a height of 40 or 50 feet in relatively damp bottom lands. Com- 

 monly it attains a height of 15 or 20 feet, and grows in loose groves resembling extensive 

 peach orchards. Among the creosote bushes and mesquites numerous other trees are 

 found, such as the ironwood and several other species of acacia, and the palo verde, whose 

 green bark, tiny round leaves, and dainty yellow blossoms constantly attract attention. 

 Everywhere, too, a profusion of healthy green cacti add to the verdure, some being re- 

 cumbent, Uke the smaller forms of the flat-leaved prickly pear, some bushy like the spiny, 

 many-branched cholla, and some assuming the dimensions of large trees, like the saguaro 

 or giant cactus, whose fluted columns are often 40 or 50 feet high. (See Plate 1, p. 34.) 



* D. T. MacDougal: Botanical Features of North American Deserts. Cam. Inst. Wash. Pub. 99, 1908. 



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