Change of Aspect with Altitude. 239 



sometimes dense, again very open, all the way to the high 

 summits on all southeast, south, and east aspects, reaching the 

 ultimate summit of Mt. Wrightson within a few yards of its 

 height of 9,432 feet above sea. On the northwest side of what 

 once may have been a crater's rim it is absent, and on certain 

 parts of this elevated and extremely sharp crest extending a 

 little east of north, it is cleanly replaced by a deciduous oak 

 {Quercus submollis) as soon as the edge is crossed. Here we see Q. 

 reticulata always with the small, revolute, oblong leaf and small 

 stature, dwindling to less than two feet on the summits. 



This change of aspect is equally well exemplified in the oc- 

 currence of other oaks, and to a lesser extent also in their change 

 of form. Arizona White Oak {0. arizonica) appears first in the 

 Upper Soncran zone, on the north side of a ridge lying at an 

 altitude of about 4,500 feet near the McBeth ranch, but is en- 

 tirely absent from the grassy south side. Thence upward, as 

 usual, it is almost everywhere present, linking the Upper Sonoran 

 and Transition zones, and at about 8,200 feet its uppermost out- 

 posts finally disappear from a southeast slope. It undergoes a 

 change of form with ascent, from the size of a tree with large, 

 obovate, often toothed leaves, frequently found in the lower 

 gulches, to small but always erect brush form, growing five or 

 six feet high in mixture with other shrubs, bearing a smaller, 

 thinner, more oblong, less coriaceous leaf. Many slight varia- 

 tions occur in this oak, here as elsewhere, but in this region the 

 transition from one form to another is usually so gradual as to 

 escape notice in the field, making it an unquestionably homo- 

 geneous species. So that any ill-founded segregates that might 

 be made would not disturb the unity of type for purposes of the 

 present comparison of its different aspect habitats. The same 

 may be said of the other, less variable, and very distincet s: ecies 

 of oak. 



The whiteleaf oak, in the Stone Cabin Canyon, likewise 

 makes its first appearance on north slopes, with the advent of 

 the Transition zone, it is heavily represented thence upward, 

 especially in the cooler, better protected gulches, where it pro- 

 duces tall, vigorous shrubs with stems of post size, and often fine, 

 tall trees. It becomes general on various aspects, but is almost 

 displaced by the Douglas fir on the north side of Horse Ridge. 



