CORRELATION OF VEGETATION AND CLIMATE. 97 
THE ROLE OF TOPOGRAPHIC FEATURES IN DETERMINING DEPARTURES 
FROM THE NORMAL ALTITUDINAL GRADIENT OF VEGETATION. 
The normal or ideal gradient of vegetation is disturbed by three 
sets of topographic influences: (a) that of slope exposure, (b) that of 
the surface flow or underflow of streams and arroyos and the high 
soil moisture of flood-plains, and (c) that of location with respect to 
ridges, slopes, or valley bottoms, which may be designated briefly as 
the influence of topographic relief. These three sets of topographic 
features do not bring into operation any factors, nor any intensities 
of the common factors, which are not involved in the normal vertical 
gradients of physical factors, although in some cases they bring about 
new combinations of factors not exactly duplicated at other elevations 
under the conditions of the hypothetical normal gradient. In the 
description of the vegetation there have been frequent allusions to 
these three sets of departures from the ideal gradient of vegetation. 
Instrumentation has also been described which throws light upon the 
operation of slope exposure and of topographic relief. The influence 
of streams has been very obvious in its nature and has not been investi- 
gated instrumentally. 
THE ROLE OF SLOPE EXPOSURE. 
The importance of slope exposure in determining the vertical limits 
of species, and in thereby determining the vertical range of types of 
vegetation, has been a matter of observation and comment among 
almost all writers on the vegetation of the western United States. 
Although the phenomenon is of universal occurrence throughout the 
extra-tropical portions of the globe it is rendered particularly striking 
in regions where there are transitions from desert or grassland into 
forested country. In any region like the Santa Catalina Mountains, 
with their steep climatic gradient and varied topography, the opera- 
tion of the factors involved in slope exposure is such as to present an 
alternation of vegetistic regions, causing constant departures of the 
vegetation from the theoretical norm to the norm of higher or lower 
portions of the mountain. 
Slope exposure is a ‘‘factor” in differentiating the vegetation of 
opposed slopes at all elevations. Even at altitudes between 2,000 and 
3,000 feet among the volcanic hills of the Tucson region, there are 
conspicuous differences between the south slopes, with their heavy 
stands of Carnegiea, Encelia farinosa, and Opuntia bigelot, and the 
north slopes with their abundant individuals of Parkinsonia microphylla 
and Lippia wrightit and their heavier growth of perennial grasses.* 
The difference between northern and southern exposures is most con- 
spicuous between 4,000 and 5,000 feet, where the former have orchard- 
* See Spalding, V. M. Distribution and Movements of Desert Plants. Carnegie Inst., Wash., 
Pub, 113, 1909. 
