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



conditions of an arid climate. Plants are closely associated in which 

 the vegetative organs are so unlike as to indicate the possession of 

 very dissimilar relations to climate and soil. In some cases the 

 physiological behavior of these plants has been found very different. 

 Survival under conditions that are very favorable for short periods and 

 very unfavorable for longer ones is assured in a great variety of ways. 

 The mostcommon examples are reduction of leaf size, assumption of 

 leaf functions by the stem, storage of water, and development in the 

 annuals of rapid growth and early maturity. 



The principal types of vegetation are more clearly marked in 

 Arizona than in the States that were originally heavily forested or in 

 those covered by different types of grassland. The dominant plants 

 of the desert, the grassy areas, the open woodlands of juniper and 

 pinyon, and the forests of pine, spruce, and fir, all differ greatly in 

 stature, density, foliage, and seasonal habits. In any place where it is 

 possible to see two of these types of vegetation in the landscape it is 

 easy to distinguish them and to be certain of their identity at a distance 

 of several miles. Almost all of the characteristics distinguishing the 

 great communities of plants are related, in ultimate analysis, to the 

 amount and seasonal distribution of their water supply. 



In considering plant life as ' Vegetation," attention is focused on the 

 anatomy and physiology of the plants, and the relation of their 

 structure and life processes to the environmental conditions. It is 

 also possible to view the plants from the standpoint of their phylo- 

 genetic relationships, and to investigate the distribution of the "flora." 

 Maps of the vegetational areas of Arizona and of the floristic areas 

 would not be identical. Between the two maps, however, there would 

 be a strong general resemblance, for the differences in vegetation are 

 very commonly accompanied by differences in flora. There are many 

 plants, however, that occur only in part of each vegetational area, 

 and many others that occur in at least two of them. 



In order to describe the plant areas of Arizona from a vegetational 

 standpoint, without reference to floristic differences or the controlling 

 climatic conditions, a modification of the classification and nomen- 

 clature adopted in describing the vegetation of the United States 

 from the same standpoint will be used. 4 



Nine major types of vegetation are found in Arizona. Three of 

 these are desert, one an arid grassland, and one an arid chaparral. 

 There are small areas of true grassland, two types of forest, and one 

 small area of alpine vegetation above timber line. These nine types 

 are as follows: California microphyll desert, Arizona succulent desert, 

 Great Basin microphyll desert, Desert-grassland transition, Grassland, 

 Arizona chaparral, Western xeric evergreen forest, Northern mesic 

 evergreen forest, Alpine summits. 



These types of vegetation are described briefly in the following 

 pages. 



Types of Vegetation 



California Microphyll Desert. — There is much in common be- 

 tween the vegetation of the parts of Arizona and California that lie 

 within 50 to 100 miles of the Colorado River south of the confluence 

 of the Virgin River. On the Arizona side there is a somewhat greater 



4 Shreve, Forrest, a map of the vegetation of the united states. Geog. Rev. 3:119-125, map. 

 1917. 



