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 rehef. 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, 

 wdth 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 bigelovii, and the 

 north slopes with their abundant individuals of Parkinsonia microphylla 

 and Lippia wrightii 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. 



