Change of Aspect with Altitude. 245 



trees named above, deciduous white oak, woods, grasses and 

 sub-alpine herbs. 



In descending the Manning Trail, the first specimens of 

 pinyon {Pinus cemhroides), Garrya wrightii, Yucca sckottii, 

 Nolina eruynpens, the first manzanita chaparral, and 

 other characteristic species denoting the advent of semi-xero- 

 phytic conditions, appear together where the trail turns out of 

 the Arizona pine forest do^^n a south slope, at an altitude of 

 little under 7,500. \\ ith these are associated, also making their 

 first appearance, such other species as Aristida schiedeana, 

 Hedeoma drunwiondii, Oryzopsis fimbriata, Lycurus phleoides, 

 Calliandra reticulata, Parosela albiflora. Here are also the first 

 Chihuahua pine, Arizona white oak, and the first considerable 

 numbers of alligator juniper {Juniperus pachy phlcea) . As we 

 descend, all of these become more or less general, and finally, 

 more or less completely, and many of them in company, seek 

 seclusion on the shadiest slopes, all disappearing before the 

 Lov.er Sonoran is reached, and the first giant cactus rears its 

 head on the sun-scorched opposite side. 



The phenomenon of change of aspect with altitude may 

 also be oV>served in more general features of plant geography. 

 An instance is beautifully shown in Happy Valley. Here sev- 

 eral parallel ridges extending east and west at the same level 

 have only open grass country, dotted here and there with a 

 solitary oak, on the south side, while the oak woods are well 

 developed on the north. The sharp crests of the ridges mark 

 with equal abruptness the change from one to the other type. 

 On the ridge \\ hich marks the northern boundary of the valley 

 also extending east and west and some 2,000 feet higher, the 

 same oak-woods type is now seen on the south side, while im- 

 mediately on the north loom the spires of pine trees. On the 

 opposite side of the Rincons, the scattered outposts of the pine 

 forest, marked by Pinus chihuahuana, appear on the northwest 

 side of the ridges as low as 6,500 feet. On the southerly aspects, 

 however, the first stragglers are detected far up toward Spud 

 Rock, some 1,200 feet higher. 



The desert traveler, in following an arroyo or wash near 

 the transition between the Lower and Upper Sonoran zones, 

 may often find one zone developed on the bluffs on the one hand. 



