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 canons 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. 



