16 VEGETATION OF A DESERT MOUNTAIN RANGE. 



dropuntias), Opuntia toumeyi, Opuntia blakeana (procumbent plato- 

 pimtias), and Acacia paucispina. 



The shorter and steeper Upper Bajadas which fringe the southern 

 and southwestern edge of the Santa CataUnas are clothed with a much 

 more diversified vegetation, in all respects similar to that of other 

 Upper Bajadas which lie below 3,500 feet (1,067 m.) in other localities 

 in southwestern Arizona. The freedom of the soil from caliche is here, 

 as elsewhere, responsible for the existence of a diversified vegetation 

 rather than a pure stand of Covillea. 



The Upper Bajadas, as exemplified along the south face of the Santa 

 Catalinas at about 3,000 feet elevation (915 m.), bear what may be 

 regarded in many respects as the most highly developed type of desert 

 vegetation to be found in southern Arizona or northern Sonora. In the 

 Upper Bajadas may be found a greater number of species of perennial 

 plants than in any other distinctly desert situations. In them also 

 the number of individual perennial plants per unit area is greater than 

 in any areas outside the flood plains of such rivers as the Santa Cruz 

 and Gila. The only areas that compare with the High Bajadas in 

 these respects are the volcanic hills in which basaltic rock has weathered 

 to a fine clay which is very retentive of soil moisture, as is well exempli- 

 fied in Tumamoc Hill, the site of the Desert Laboratory. The andesitic 

 and rhyolitic hills in the vicinity of Tumamoc are much poorer than 

 it is in the number of individual plants per unit area, although perhaps 

 nearly as rich in their flora. 



On the Upper Bajadas there often occur, in almost equal admixture, 

 from 15 to 25 perennial species of plants of such size as to dominate the 

 physiognomy of the vegetation. These same species may be found on 

 the more nearly level Lower Bajadas, but any one of them may often 

 be absent for many miles, may be sporadically represented by a few 

 individuals, or may occur in dense but local colonies (particularly in 

 the case of the cacti). Occasionally as many as 5 to 10 of the species 

 may be within sight at the same time. 



The flora which characterizes the Upper Bajadas of the Santa 

 Catalinas ranges without substantial loss down to sea-level on the gulf 

 of Cahfornia,* and the vegetation formed by their commingling may 

 be found as a belt covering the high bajadas which encircle all of the 

 mountain ranges and clothing all of the low basaltic hiUs. A climb of 

 2 hours from the base of the Santa Catalinas will discover greater 

 changes of vegetation and flora than can be encountered in the 150 

 miles (242 km.) between Tucson and Adair Bay. 



The Upper Bajadas present the desert characteristics of openness 

 of stand, lowness of stature, and commingling of diverse vegetation 

 types. The first of these features is common to the vegetation of all 



*SeeHornaday,W.T. Camp Fires on Desert and Lava. New York, Scribner, 1909. MaoDougal, 

 D. T. Across Papagueria. The Plant World, 11: 93-99, 123-131, 1908. 



