98 VEGETATION OF A DESERT MOUNTAIN RANGE. 
like stands of evergreen oaks and the latter are treeless (see plate 9n). 
Almost equally striking, however, is the contrast between the open 
pine forests on south slopes at 9,000 feet and the heavy, deep-shaded 
stands of fir on north slopes at the same elevation (compare plate 
29a and plate 35). 
Although the influences of slope exposure are operative at all eleva- 
tions they acquire added power with increase of altitude.* In the 
Desert region and the Lower Encinal the uppermost limits of species 
on north slopes and on south slopes are from 600 to 1,000 feet apart 
(Carnegiea, Echinocactus, Quercus emoryi), while in the Forest region 
the upper limits on opposed slopes differ by 1,000 to 2,000 feet (Quercus 
hypoleuca, Juniperus pachyphlea, Arbutus arizonica). Another test 
of the same fact may be had by comparing a north slope at 3,000 feet 
with a south slope at 6,000, and by then carrying the comparison up 
3,000 feet. Between the north slope at 3,000 feet and the south slope 
at 6,000 feet is the strong contrast of Desert and closed Encinal, with 
only a few xerophilous ferns and one small cactus in common. Between 
the north slope at 6,000 and the south slope at 9,000 feet is the very 
close resemblance of two stands of Pine Forest, in one of which are still 
to be seen a few Encinal forms that have disappeared from the other and 
higher one. 
The increased influence of slope exposure at higher elevations is not 
to be attributed to the fact that the species of the Upper Encinal and 
Forest range through greater elevations than do the species of the 
Desert and the Lower Encinal. The number of feet through which 
@ species ranges on south slopes or on north slopes has no necessary 
connection with the difference between its upper limits on north and 
on south slopes. The ability of a large number of plants to range 
through a greater vertical distance in the Upper Encinal and Forest 
than it is possible for the plants of the lower vegetations to do may be 
owing to the ability of the plants of the upper portion of the mountain 
to withstand a greater gamut of conditions than the plants of the basal 
vegetations can. It would, in any event, not be due to the existence 
of more gradual gradients of climatic change at the higher elevations, 
since in every case of the measurement of these gradients they have 
been shown to grow steeper between 6,000 and 9,000 feet than below 
6,000 feet. The increase in the effects of slope exposure with increase 
of altitude can only be ascribed to an increasing differentiation of the 
climatic conditions between north and south slopes at higher elevations. 
An examination of the curves of evaporation and of soil moisture (figs. 
10 and 14) will show that the readings for the highest stations exhibit 
the greatest apartness, at least with respect to the intensities involved. 
* Merriam has illustrated this fact in a diagrammatic profile of San Francisco Peak, but has 
not mentioned it in the text of his paper. See Merriam, C. Hart. Biological Survey of the San 
Francisco Mountain Region, Arizona. U.S. Dept. Agric., North Amer., Fauna No. 3, 1890, pl. 1. 
