108 VEGETATION OF A DESERT MOUNTAIN RANGE. 



species of the Desert (for elevations see page 37). On a high ridge 

 tributary to Bear Canon have been found the highest individuals of 

 Opuntia sp., the highest species of that genus on the mountain, and 

 Mamillaria grahami, the highest-ranging plant of the Desert. The 

 individuals which most nearly approach these highest stations for 

 Opuntia and Mamillaria have been found on south exposures about 

 600 feet lower, in the Bear Canon drainage. 



On an exposed ridge, with a considerable inclination to the south, 

 at 7,800 feet are found the highest individuals of Pinus cembroides, 

 Juniperus pachyphloea (with one known exception). Yucca schottii, 

 Echinocereus poly acanthus, and Arctostaphylos pungens. In this station 

 the influences of slope exposure and of topographic relief are combined, 

 thereby bringing about the pronounced conditions that are expressed 

 in the highest occurrence of 5 species of the Upper Encinal. On the 

 ridges above Marshall Gulch are found the highest occurrences of 

 Quercus hypoleuca and Quercus reticulata, both of which forms extend 

 further down the south faces of these east-and-west ridges than they 

 do down the north faces. 



When Desert plants are found on the ridges of the Encinal region 

 they fail to appear on the south-facing slopes just below these ridges. 

 When the plants of the Encinal are found at their highest locations 

 on ridges of the Forest Region they are also absent on the south-facing 

 slopes just below the ridges. This does not appear to be the case with 

 respect to the highest occurrences of plants which are believed to have 

 their true climatic limit just below the sunamit of Mount Lemmon, 

 such as Quercus hypoleuca and Quercus reticulata. 



The extent by which the highest individuals on ridges exceed the 

 highest individuals on south slopes is never more than 500 to 600 feet, 

 except in the case of Pinu^ cembroides, in which it is about 700 feet. 

 Opuntia sp. and Mamillaria grahami, which have their upper limit in 

 the vicinity of 7,000 feet, agree in this respect with Opuntia versicolor, 

 Echinocactus wislizeni, and Fouquieria splendens, which have their 

 limit in the vicinity of 5,500 feet. 



Perhaps the most common explanation of the highest occurrence of 

 species on ridges is that the soil is driest in such situations and therefore 

 offers to plants from lower elevations a habitat more like that in which 

 they are abundant. The principal objection to such an explanation 

 is the unquestionable fact that a somewhat more moist soil is not 

 inimical to the plants of the Desert nor to the plants of the Encinal. 

 Neither is there a sufficient difference between the soil moisture at 

 the bottom of a slope and on the ridge at the top of the slope, in the 

 arid seasons, to cause a differentiation of the vegetation. 



The explanation of the phenomenon may be sought partly in the 

 existence of cold-air drainage, which is at least responsible for the 

 absence of Desert and Encinal plants from the bottoms of canons at 



