VEGETATION OF THE SANTA CATALINA MOUNTAINS. 33 



The pure or nearly pure stands of Pinus arizonica which occur be- 

 tween 8,000 and 9,000 feet are increasingly poor in the evergreen oak 

 shrubs, which have disappeared at the latter altitude. The clumps of 

 young Quercus submollis give the forest its only deciduous element at 

 this altitude, and the low patches of Ceanothus, so common at 8,000 

 feet, give way at 9,000 feet to Symphoricarpos oreophilus and to the 

 much less frequent Holodiscus dumosus. Very many of the commonest 

 herbaceous perennials of the Pine Forests which lie between 7,000 and 

 8,000 feet do not reach 9,000 feet, or are replaced in the physiognomy 

 of the forest by closely related species. On the summit and southern 

 slopes of Mount Lemmon the commonest herbaceous plants are: 

 Kceleria cristata, Dugaldia hoopesii, Erigeron neomexicanum, Pteris 

 aquilina var. pubescens, Gnaphalium decurrens, Hieracium lemmoni, 

 Senecio sp., Antennaria marginata, Silene greggii, and Helianthella 

 arizonica. Throughout the higher Pine Forest Arceuihobium divari- 

 catum and Arceuihobium robustum are common on the trunks and limbs 

 of Pinus arizonica. 



At about 6,800 feet Alnus acuminata, Acer interior, and Quercus 

 submollis become frequent along streams (see plates 30 and 31). The 

 first two are confined to this habitat throughout their vertical range, 

 while the oak, which is the only deciduous member of the genus in 

 these mountains, is found even in some of the driest situations above 

 7,600 feet. Quercus submollis occurs characteristically either as single 

 trees of considerable size, up to 40 feet in height and 4 feet in girth, 

 or else as crowded circumscribed groups of young trees, which doubt- 

 less owe their juxtaposition to the accidents of seed dispersal. Salix 

 taxifolia is also a common streamside shrub above 6,800 feet, and 

 in certain portions of the mountain Rosa fendleri is abundant in the 

 proximity of streams. 



Herbaceous plants are to be found in increasing numbers at or near 

 the banks of streams between 6,000 and 7,400 feet. Prominent among 

 them are: Juncus arizonicus, Aquilegia chrysantha, Thalictrum fendleri 

 var. wrightii, Scrophularia sp., Trifolium pinetorum, Fragaria ovalis, 

 Potentilla thurberi, Hypericum formosum, Lobelia gruina, Agrimonia 

 brittoniana var. occidentalis, Gaura suffulta, and Tagetes lemmoni. 



THE FIR FOREST. 



Between 7,000 and 7,400 feet is a rapid change in the character of 

 the forest stands on northerly slopes, due to the increasing occurrence 

 of Pseudotsuga mucronata and Pinus strobiformis, the lower limits of 

 which have already been mentioned, and to the appearance of Abies 

 concolor. These three species occur in mixed stands together with 

 Pinus arizonica on northerly slopes up to about 7,500 feet, above which 

 elevation the latter becomes a very infrequent tree on slopes facing 

 directly north, although it still occurs in admixture with Pseudotsuga 



