CORRELATION OF VEGETATION AND CLIMATE. 89 
down into the Encinal. The physical factors which underlie the effects 
of topography are, then, to be considered simply as special cases of the 
same influences that are grouped in the effects of altitude itself. It 
is desirable, nevertheless, in studying the correlation of climate and 
vegetation to consider separately the normal gradient of vegetation 
and the departures from the normal gradient. 
THE VERTICAL DISTRIBUTION OF INDIVIDUAL SPECIES. 
The student of vegetation too often loses sight of the fact that vege- 
tation is composed of individual species of plants and that the behavior 
of the vegetation is a function of the behaviors of these species. After 
our review of the vegetation of the Santa Catalinas, and in connection 
with the discussion of its control by climatic factors, it is necessary 
to consider the vertical distribution of the individual species in relation 
to the physical conditions of the mountain. 
There are no species of plants which grow spontaneously both at 
the base and the summit of the Santa Catalina Mountains, except a 
few palustrine forms of Carex and Juncus. The total range of physical 
conditions through the 6,000 feet of elevation here involved is so great 
that no native plant possesses the power of accommodation to the 
complete gamut of Desert, Encinal, and Forest. Indeed, very few 
plants range through half of the entire gradient of conditions, in any 
portion of it. The species which exhibit the widest belts of vertical 
distribution are to be found in the most dissimilar habitats at the lower 
and upper edges of their ranges, which indicates that these species are 
not really capable of existence through 2,000 or 3,000 vertical feet of 
the climatic gradient under the same conditions of topographic loca- 
tion, slope exposure, and insolation. In fact, a close analysis of the 
habitats occupied by characteristic plants, in connection with their 
vertical ranges, indicates that, below 6,000 or 7,000 feet, no plants 
outside the desert succulents and semi-succulents range through more 
than 1,000 to 1,500 feet in habitats of the same topographic character. 
At higher elevations a number of common plants extend more than 
1,500 feet in situations of the same character, as for example Pinus 
arizonica, which ranges through nearly twice that altitude on dry 
southern slopes. 
A vertical range of 4,700 feet is exhibited by Vitis arizonica, which 
occurs in several arroyos and cafions at 3,000 feet and is found in the 
same habitat throughout the Desert and Encinal regions of the moun- 
tain, reaching its highest observed station at 7,700 feet in a steep dry 
arroyo in the Pine Forest. Although the habitat of Vitis is superficially 
identical throughout its range, it is found at 3,000 to 5,000 feet only 
in the largest arroyos, in which it is able to draw upon much greater 
and more constant supplies of soil moisture than are available in the 
small arroyos to which it is confined at the upper edge of its range. 
