88 VEGETATION OF A DESERT MOUNTAIN RANGE. 



CORRELATION OF VEGETATION AND CLIMATE IN THE 

 SANTA CATALINA MOUNTANS. 



The earlier chapters of the present paper have described the salient 

 features of the vertical distribution of vegetation in the Santa Catalinas, 

 and also some of the principal gradients of climatic change. Both the 

 vegetation and the climate have been shown to exhibit progressive 

 changes with increase of altitude, and these changes have been found 

 to undergo hastening or retardation under the influence of topographic 

 irregularities. It will be the object of the following pages to correlate, 

 in so far as possible, the altitudinal changes of vegetation and climate, 

 in an effort to determine roughly some of the physical factors which 

 are of critical importance in limiting the vertical ranges of the types 

 of vegetation and of their characteristic species. 



THE NORMAL ALTITUDINAL GRADIENT OF VEGETATION. 



Any attempt to ascribe vertical limits to the Desert, Encinal, and 

 Forest, or to state the vertical limits of individual species, is met at 

 once by the omnipresent importance of slope exposure in determining 

 these limits. The altitudinal range of vegetations and species may be 

 determined by examining only slopes of south exposure, or only those 

 of north exposure, and the two examinations would agree closely as 

 respects the vertical ranges, but would disagree by approximately 

 1,000 feet with respect to the upper and lower limits of the vegetations 

 or species. It is impossible to determine the normal character of vege- 

 tation at a given altitude by seeking level ground, for it will be found 

 only in the flood-plains, subject to the influence of a high soil moisture, 

 or on a ridge, subject to equally special conditions. It is also impos- 

 sible to visit adjacent valleys or plateaus lying at the same elevation 

 and to find on them vegetation which is subject to the same climatic 

 and soil conditions. For some purposes it is desirable to consider the 

 vertical stages of vegetation under ideal conditions, as affected by 

 altitude without the complications due to topographic features. It 

 is then possible to hypothecate a norm of vertical stages of vegetation 

 by averaging the altitude of any given limit as separately determined on 

 north and south slopes, or it is possible to take into consideration only 

 the altitudinal changes of south slopes or of north slopes, taken alone. 



It has been shown that the influence of topography on the vegetation 

 is chiefly (sometimes solely) to carry the common types of vegetation 

 above or below the elevations at which they are universal. The 

 influence of topography on the gradients of climate is of the same char- 

 acter; the topographic relief causes no wholly new factors to come into 

 play, but serves merely to carry the physical conditions of the Desert 

 into the Encinal, for example, or to bring the conditions of the Forest 



