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 
