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). 



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. 

 Bidens sp. 

 Cerastium sericeum. 

 Dalea polygonoides. 



Drymaria sperguloides. 

 Drymeria tenella. 

 Muhlenbergia sp. 



