300 



FORREST SHREVE 



On the northeast face of the mountain the pines are first 

 encountered at about 6800 feet, while on the southwest face there 

 are very few groves below 7600 feet. This difference is partly 

 due to slope exposure and partly to the well-known influence 

 which is exerted on the vertical distribution of mountain vege- 

 tation by the elevation of the basal plain or valley. Extensive 

 areas between 7000 and 8500 feet are occupied by oak scrub, 

 with groves of pines only in the most favorable situations. 



The predominant pine is Pinus ponder osa. The closely allied 

 form known as Pinus arizonica has not been detected in the 



Fig. 5. Forest on a shoulder of the north slopes of Mt. Graham at 10,300 feet. 

 The trees are Picea engelmannii, Pinus strobiformis and Abies arizonica. 



Pinaleno Mountains, although in the Santa Catalina range it 

 forms the great bulk of the yellow pine forest, Pinus ponderosa 

 being local and infrequent. Above 8600 feet the pines are con- 

 fined to southern exposures and at about 9500 feet they reach 

 their upper limit. The northern slopes between 8000 and 9500 

 feet are chiefly occupied by Pseudotsuga taxifolia, Abies co?icolor 

 and Pinus strobiformis, all trees of general occurrence throughout 

 the high mountains of southern Arizona. On going above 9600 

 feet these trees become less frequent. The highest elevation is 

 reached by Pinus strobiformis, which occurs on dry ridges at 

 10,300 feet, where its resemblance to P. flexilis is very strong. 



