34 VEGETATION OF A DESERT MOUNTAIN RANGE. 
and Abies at 9,000 feet on eastern and western exposures. Above 7,500 
feet Pinus strobiformis ceases to be confined to the proximity of streams, 
and occurs in admixture with Pseudotsuga and Abies, but is not so 
common as they in the heaviest stands of this type of forest. On. the 
north slopes of Mount Lemmon is a small colony of Abies arizonica, 
which is not known from any other locality on the mountain. 
Slopes of due south or southwestern exposure are held by Pinus 
arizonica up to the summit of Mount Lemmon at 9,150 feet, with a 
slight occurrence of Pseudotsuga and Pinus strobiformis above 8,000 
feet. The Pseudotsuga and Abies forest is found in fine development 
at 7,500 feet on steep north exposures, and reaches its maximum devel- 
opment in stature and size of the trees on the north slopes of Mount 
Lemmon at 8,500 to 9,100 feet (see plates 1 and 35). The altitude of 
the Santa Catalina Mountains is nowhere sufficient to admit of the 
occurrence of extended bodies of such forest, nor of their existence on 
southerly slopes. 
In the Fir Forest the last relicts of the Encinal have disappeared: 
Quercus hypoleuca, Quercus reticulata, and Juniperus pachyphlea are 
nowhere to be found in association with Pseudotsuga and Abies, 
although they may grow very near them on opposed slopes. Arbutus 
arizonica, which is more common in the Pine Forest than in the Encinal, 
is likewise absent from the Fir Forest. The deciduous Quercus submollis 
and the widely distributed Populus tremuloides are the commonest 
of the subordinate trees, the latter often becoming dominant over 
areas of an acre or more in extent, where it ultimately gives way to 
conifers. 
The floor of the Fir Forest is much more heavily and continuously 
shaded than that of the densest stands of pine, a circumstance which 
is of great importance in determining the nature of the forest reproduc- 
tion and also in conditioning the character of the shrubby and herba- 
ceous vegetation. The dense shade, the heavy litter, and the high humus 
content of the soil tend to preserve its moisture throughout the arid 
fore-summer (see p. 61), so that the seedling trees and other plants of 
these situations are very far removed from the desiccating influences 
which are operative in the open Pine Forest. The Fir Forest near the 
summits of ridges is somewhat more open than that which is found on 
middle and lower slopes, and this difference is accompanied by a dis- 
similarity in the herbaceous flora of upper and lower slopes. On the 
latter may frequently be found communities of plants which differ little 
in their specific make-up from the communities which occupy flood- 
plains, although they are much less dense. 
The heaviest stands of Abies and Pseudotsuga, like most heavy conif- 
erous forests, are relatively poor in both shrubs and herbaceous plants. 
A few of the shrubs common to the water-courses are to be found also 
in the Fir Forest, such as Jamesia americana, Symphoricarpos oreo- 
