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 1,872 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 life 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 cafions of the Santa Catalinas 
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 cafions and arroyos t present distinctive 
features of vegetation, departing more and more from the large cafions 
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 cylin- 
* The word “region” is not here used in any of the technical senses in which it has been em- 
ployed in phytogeography. 
+ The Spanish word arroyo is in common use in the southwestern United States to designate 
streamways which are usually without water. 
