14 VEGETATION OF A DESERT MOUNTAIN RANGE. 
slopes. A few of the dominant species of the Encinal are found on 
the higher bajadas, above 3,500 feet (1,067 m.) elevation, but in the 
mountains none of its species is to be found so low as this except on 
north slopes or near arroyos of large drainage area. At 4,000 feet, on 
north slopes, several of the larger Encinal plants are encountered, and 
at 4,300 feet (1,310 m.), on north slopes, several additional dominant 
species are found. Within the Encinal Region it is possible to recognize 
a lower and an upper portion, distinguished chiefly by the openness of 
the former and the closed character of the latter. The closed Upper 
Encinal merges gradually into the Forest, losing some of its character- 
istic species at 6,000 to 6,300 feet (1,830 to 1,920 m.), while others range 
to 7,800 feet (2,380 m.) and a few to 8,300 feet (2,530 m.) on south 
slopes. 
The lowest occurrence of Forest is on north slopes at about 5,800 
feet (1,768 m.) and on south slopes at about 6,300 feet (1,920 m.). 
The Forest is at first rather open and is superposed, as it were, upon 
the closed Encinal, but it becomes heavier and the Encinal elements 
within it become more sparse at elevations of from 6,300 to 6,800 feet 
(1,920 to 2,073 m.), according to the slope exposure. The upper limit 
of Forest is not reached in the Santa Catalina Mountains at their 
highest elevation of 9,150 feet (2,790 m.), nor in the adjacent Pinaleno 
Mountains (Mount Graham) at 10,516 feet (3,205 m.). The forest of 
yellow pine occupies all south slopes up to the summit of Mount 
Lemmon. A forest dominated by fir, spruce, and Mexican white pine 
occupies the north slopes above 7,500 feet (2,287 m.), the earliest 
occurrence of these species being about 1,000 feet (305 m.) lower. 
The description of vegetation which is given in the following pages 
applies only to the south face of the Santa Catalinas. The north face 
presents more abrupt slopes than the south, with most of its ridges 
running north from the main ridge. This circumstance obscures the 
influence of slope exposure, since it presents opposed slopes, facing 
east and west, which are identical in their vegetation.* Furthermore 
the north face of the range is mineralogically diversified, presenting 
exposures of shale, sandstone, limestone, diorite, and gneiss, whereas 
the south face presents an exposure of gneiss only, with a resultant 
mineral identity of soils from base to summit. It has thus been possible 
to carry out a study of climatic influences over a vertical gradient of 
6,000 feet (1,830 m.) with uniform soil, and the east and west ridges 
of the south face have furnished opposed north and south slopes at all 
elevations. 
* Differences between the vegetation of east and west slopes have been pointed out by Blumer 
for El] Rincon Mountain, but the differences noted were of another character from those commonly 
existent between north and south slopes. See: Blumer, J.C. A Comparison between two Moun- 
tain Sides. The Plant World, 13: 134-140. 1910. 
