FLOWERING PLANTS AND FERNS OF ARIZONA 19 



of its dominant plants, link it closely with the same type of vegetation 

 in California, although there are very few species common to both 

 regions. 



A number of shrubs, semishrubs, and other plants are infrequent in 

 the chaparral or only locally abundant, including, in order of abun- 

 dance: Rhus ovata, Cowania stansburiana, Quercus palmeri, Arcto- 

 staphylos pringlei, Garrya flavescens, Nolina microcarpa, Berberis 

 haematocarpa, Agave parryi, Mimosa biuncifera, Eriodictyon angusti- 

 -folium, Aplopappus spp., Eriogonum wrightii, Quercus gambelii, Brick- 

 ellia californica. 



Western Xeric Evergreen Forest. — This type of vegetation is 

 widely distributed in central and northern Arizona, forming a zone 

 which surrounds the mesic evergreen forest or occurs in large and 

 small isolated stands. Its limits are seldom sharply defined, for it 

 stretches down in attenuated form into the grassland, and its charac- 

 teristic trees extend upward to altitudes at which they mingle with 

 the larger trees of the mesic forest. So far as its dominant plants are 

 concerned it has the simplest composition of any of the vegetations 

 of the Southwest. There is a wide floristic difference between the 

 xeric forest of southern Arizona, often composed solely of evergreen 

 oaks, and that of central and northern Arizona, composed almost 

 wholly of juniper and pinyon. Also, there are greater variations in 

 density and stature in the xeric forest than in any of the other types 

 of vegetation, the low and open stands being at lower altitudes, and 

 the dense tall ones at higher elevations. 



The type of xeric evergreen forest in which the oaks are dominant 

 is nearly confined to the eastern part of the State south of the Gila 

 River, where rainfall is somewhat greater in summer than in winter. 

 It occurs chiefly on hills and mountain slopes between altitudes of 

 4,000 and 6,500 feet, and in many localities extends from the moun- 

 tain base out onto the upper bajada but does so only above 5,000 feet . 

 The most extensive oak forests are found in the foothills of the larger 

 mountains of the southeastern counties. The commonest tree is 

 Quercus emoryi (bellota), which appears to be the most drought 

 resistant of the evergreen oak trees. Quercus arizonica (Arizona oak) 

 and Q. oblongifolia (blue oak) are less abundant than the bellota at 

 lower elevations, but are equally common above 5,500 feet. Juniperus 

 monosperma (oneseed juniper) is of sporadic occurrence and is 

 seldom an important associate of the oaks, but J. pachyphloea 

 (alligator juniper) is abundant, usually mingled with the oaks, 

 sometimes occurring in limited nearly pure stands, or again growing 

 in company with Pinus cembroides (nut pine). 



The oak type of xeric forest is rarely closed, being commonly an 

 open or a very open community, with many associated shrubs, 

 succulents, and semisucculents. Among the commonest of these 

 associates in Cochise, Graham, Pima, and Santa Cruz Counties are: 

 Yucca elata, Y. schottii, Nolina microcarpa, Dasylirion wheeleri, Agave 

 palmeri, Cercocarpus breviflorus, Mimosa biuncifera, Dalea wislizeni, 

 Rhus trilobata, Echinocactus wislizeni, Opuntia spinosior, Garrya 

 wrightii, Arctostaphylos pungens, Aplopappus spp. 



Juniper and pinyon mingle extensively with the evergreen oaks in 

 the xeric forest of southern Arizona, but on passing northward the oaks 

 become less frequent. Throughout the northern half of the State 

 juniper is more abundant than pinyon below 6,500 feet. Above that 



