30 VEGETATION OF A DESERT MOUNTAIN RANGE. 
steepness of the slope and its soil characteristics, particularly with 
respect to the soil moisture supply (see plate 21). af 
Much more gradual and inconspicuous is the transition from the 
Pine Forest to that in which Abies concolor (white fir) is the dominant 
tree. This type of Forest occupies the northern slopes of the highest 
summits and ridges of the range from 7,500 feet upward, but there are 
no elevations in the Santa Catalinas sufficiently great to bring the Fir 
Forest onto the south slopes. 
Throughout the Pine Forest there are trees, shrubs, and herbaceous 
plants which may be found in the Encinal, at least in its upper portion, 
but only in the lowest edge of the Pine Forest may plants be found 
which suggest the genera or vegetation types characteristic of the desert. 
A single cactus (Echinocereus polyacanthos), a Yucca, and an Agave are 
the sole representatives of the succulent and semi-succulent forms of 
the lower elevations, and they are rare above 7,000 feet and absent above 
7,800 feet. 
The Pine Forest is not, however, without vegetational features which 
suggest the effects of a climate not far removed in character from that 
of the desert. The openness of the lowest stands of Pinus arizonica, 
the high mortality among the seedlings of the pine, the character of 
the foliage of the shrubs and herbaceous perennials, and the deep-seated 
root systems of the latter plants, all point to the existence of a pre- 
carious soil-moisture supply and to atmospheric conditions conducive 
to active transpiration. In the Fir Forest none of these features is 
observable, and the vegetation as a whole presents a much more 
mesophilous aspect. 
In the Forest region the winter is a season of almost absolute rest, 
save for the photosynthetic activity which is doubtless carried on by 
the conifers, and possibly by the evergreen oaks and shrubs. The 
deciduous trees and shrubs are leafless from early or mid October until 
April or May, and only a few herbaceous perennials are active during 
this period, such as the evergreen species of Pyrola and the early vernal 
plants, such as Frasera. The amount of activity on the part of the 
perennial herbaceous plants during the arid fore-summer is largely 
dependent on the amount of winter precipitation and the date of its 
termination. In the lower portion of the Pine Forest it often happens 
that almost all activity is in abeyance until the first rains of the humid 
mid-summer, while in the upper Pine Forest and in the Fir Forest it 
is always possible to find a majority of the common herbaceous plants 
in activity in May and June. There is a notable scarcity of annual 
plants above 6,000 feet, and the only ones that have been detected in 
the Forest region are: 
Androsace arizonica. Drymaria sperguloides. 
Bidens sp. Drymeria tenella. 
Cerastium sericeum. Muhlenbergia sp. 
Dalea polygonoides. 
