Change of Aspect with Altitude. 241 



the other species noted, it descends farthest in the bottom of the 

 V-shaped draws and gulches, and ascending, gradually climbs 

 the siopes on either hand. Going higher, it presently replaces 

 the oak brush on east and west slopes. It is the most abundant 

 forest tree on many slopes of lesser gradient thence upw ard, is 

 largely replaced by oak chaparral on the austral side of Horse 

 Ridge, reaches quite to the summit of the high crest westward, 

 and almost to the top of Mt. Wrightson. While at these eleva- 

 tions hardly producing a continuous forest, it is quite general 

 on all sides of the higher mountains, occupies many slopes al- 

 most exclusively with Oiiercus reticulata or Muhlenbergia gracilis, 

 while in moister, colder places Pimis strobijormis s imetimes 

 successfully disputes its ground. If the mountains were high 

 enough, there is little question that the same concentration on 

 southerly aspects that obtains among the oaks, vv ould ultimately 

 appear in the pines as well. In fact, at the same elevation in the 

 Chiricahua Mountains the Engelmann spruce and Dcuglas fir 

 replace it entirely on a number of areas on the north side of the 

 several main summits. 



The \^ idely prevalent, low and thorny evergreen underbrush 

 known as Ceanothus jendleri was first noted in small numbers, ^■et 

 as a distinct constituent element, on a west slope well covered 

 with thrifty oak brush, the deciduous Schmaltzia, and other 

 Transition species, at an altitude not exceeding 6,000 feet, 

 indicating a habitat comparatively mesophytic for this aspect 

 and level. In the still more mesophytic forest ensuing it v. as 

 absent, but immediately upon getting over the crest of Morse 

 Ridge, 2,000 feet above, a cover of shrubs of this species, pene- 

 trable only with difficulty, took complete possession of the inter- 

 spaces in the evergreen oak chaparral very similar to that t)elow. 

 On the north side opposite, the smooth, grassy floor of the 

 Douglas fir forest, entirely devoid of the chaparral species, bore 

 as conspicuous plants only lupine and a slender, deciduous 

 wafer ash (Ptelea). 



Another example is the madrone (Arbutus arizonica), 

 found on the most favorable slopes of Stone Cabin Can^•on at 

 the unusually low altitude of 5,500 feet, in the middle of the L'p- 

 per Sonoran. It is here a large shrub, or a small tree, but a 

 large tree occurred at the lowest limit in a thicket in the bottom 



