98 VEGETATION OF A DESERT MOUNTAIN RANGE. 



like stands of evergreen oaks and the latter are treeless (see plate 9b) . 

 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 pachyphloea, 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 

 a 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 

 cHmatic 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, pi. 1. 



