VEGETATION OF THE SANTA CATALINA MOUNTAINS. 15 



THE DESERT REGION.* 



Under the designation of ''Desert" are comprised all those portions 

 of the Santa Catalina Mountains in which the vegetation is open, low, 

 and diversified in the assemblage of growth-forms, with a predominance 

 of microphyllous trees and shrubs and an abundance of cacti. Such 

 a vegetation is to be found covering the upper bajadas and extending 

 up the slopes of the mountain to elevations of 4,000 to 4,500 feet 

 (1,220 to 1,372 m.), according to slope exposure. The vegetation of the 

 Upper Bajadas will be described for the sake of the contrast which it 

 affords with the vegetation of the upper portions of the mountain, as 

 well as to give a picture of the plant Ufe by which the mountain is 

 surrounded and from which it has derived many of its characteristic 

 species. The desert slopes of the mountain itself exhibit at first a close 

 resemblance to the bajada, and then lose most of the larger bajada 

 plants before the entry of the dominant plants of the Encinal region. 

 This circumstance admits of a subdivision of the Desert region of the 

 mountain into Lower Desert Slopes and Upper Desert Slopes. The 

 latter region is much poorer than the former in cacti and much richer 

 in grasses, both from the standpoints of the number of species and the 

 number of individuals. The Upper Desert is similar in vegetation to 

 many of the Upper Bajadas, such as those to the northwest of the Santa 

 Catalinas and to the east and west of the Santa Rita Mountains, and 

 might well be designated as ''semi-desert" or "desert-grassland transi- 

 tion." It is, however, essentially similar to the desert plains in its 

 vegetational make-up, and in no part of Arizona does it serve as a 

 transition to true grassland. The largest canons of the Santa CataUnas 

 possess some plant communities that are radically unlike the vegeta- 

 tion of the desert itself, but not unlike the communities which surround 

 the springs and wells of the desert plains. These are the groups of 

 aquatic and palustrine plants which accompany the streams and pools 

 of the cafions. The smaller caiions and arroyos t present distinctive 

 features of vegetation, departing more and more from the large canons 

 and approaching more nearly the character of the desert areas away 

 from water. All of these areas have been treated as a part of the Desert 

 Region. 



THE UPPER BAJADAS. 



The Lower Bajadas of the Tucson region are covered by a vegeta- 

 tion in which Covillea tridentata (jediondia, creosote bush) is always the 

 predominant plant and is often almost the sole plant of more than 

 2 feet in height over areas many square miles in extent. The plants 

 which most commonly enter this community are Prosopis velutina 

 (mesquite), Opuntia fulgida, Opuntia spinosior (both arborescent cyiin- 



* The word "region" is not here used in any of the technical senses in which it has been em- 

 ployed in phytogeography. 



t The Spanish word arroyo is in common use in the southwestern United States to designate 

 Btreamways which are usually without water. 



