Change of Aspect with Altitude. 243 



to emerge upon east and west sides, and associates with aspen, 

 Nuttall willow, and Pteridium aquilimim pubescens. Its com- 

 panion, the aspen {Populus tremuloides) , though descending to 

 about 8,000 feet with the white and Douglas firs in places simu- 

 lating most closely its northern habitat, appears for the first time 

 on the side of the afternoon sun on the southwestern flank of 

 Fly Peak at an elevation of 9,200 feet. No rocky talus, often 

 giving rise to colonies of mesophytic species out of place, is 

 present, and the slope is compapratively smooth and open. Here 

 is also met Holodiscus australis, of similar, though less pronounced 

 ha! itat affinities. Though best developed about shady boreal 

 nooks and northerly slopes at this altitude, Salix nutiallii is 

 also encountered here, while on the north side of IMonument Peak 

 it descends to 8,400, and in certain gulches on the north side of 

 the Rincon Mountains to 8,400 feet. At the latter altitudes it 

 is limited to the aspect named. 



In the last named mountains the reversal of aspect between 

 altitudinal limits was found clearly repeated. During an ascent 

 of Rincon Peak, the blue oak {Quercus ohlongijolia) was first met 

 at 4,000 feet, on northerly slopes of the spur flanking Rincon Creek 

 on the south. It continued with slight interruption in its char- 

 acteristic open stand on top of the spur, confined itself at length 

 to the sunnier aspects, and disappeared at last on the most 

 open west and southwest aspects. In ascending the Manning 

 Trail to the northern Rincons, it is first seen hugging the north- 

 west slopes of the gulches, marking here the first outposts of 

 the Upper Sonoran zone at approximately 4,500 feet. About 

 300 feet higher it becomes general in its orchard-like disposition, 

 and already at 5,600 to 5,700 feet the last blue crowns of this 

 pretty oak become confined to the last open, grassy areas on 

 slopes receiving the most sunlight. At approximately 4,500 feet 

 Quercus emoryi made its first appearance in rocky gulches and 

 north slopes in the southern P-incons, but in the chaparral belt 

 bet.'. een 6,500 and 7,000 feet its final occurrence was on steep 

 southv. est and west slopes only. Ascending the Manning Trail, 

 which lies open to desert winds on the southwest mountain side, 

 it first appears a little higher on the northwest side of the trail- 

 bearing ridge, and its last representatives are seen at 7,000 feet, 

 on a striclty southern slope. On the southern Rincons, Quercus 



