1936] MAMMALS AND LIFE ZONES OF OREGON 23 



4,000 feet; on the Snake River Valley side on northeast slopes from 

 about 2,800 to 4,800 feet ; and on southwest slopes from about 3,500 to 

 5,500 feet. On the south side of the mountains, with a higher base 

 level, the zone extends on southwest slopes near Burns from about 

 4,500 feet up to 5,500 feet in the mountain north of there, and on 

 especially steep, dry slopes up to 6,000 feet. 



Despite the varying levels of this division of the zone, the climatic 

 conditions are fairly uniform with an average annual rainfall of 

 approximately 20 inches, and it is sufficiently cool to allow generally 

 a fair depth of snow on the ground for several of the winter months. 

 Apparently the only climatic distinction from the treeless arid sub- 

 division of the zone is the slightly greater humidity and the resultant 

 difference in plant and animal life. 



MAMMALS 



Of mammals some of the most characteristic species of the semi- 

 humid Transition Zone are the Rocky Mountain mule deer, Rocky 

 Mountain elk, Klamath chipmunk, yellow-bellied chipmunk, Oregon 

 and golden-mantled ground squirrels, Gambel's white-footed mouse, 

 brown pocket gopher, and others less restricted to the division. 



BIRDS 



Of breeding birds the semihumid division of Transition Zone is 

 characterized in part by Richardson's grouse, pygmy owl, MacFar- 

 lane's screech owl, Rocky Mountain hairy woodpecker, white-headed 

 woodpecker, western wood pewee, Oregon towhee, mountain tanager, 

 Audubon's warbler, and pygmy nuthatch. 



PLANTS 



The semihumid division of Transition Zone is best characterized by 

 the ponderosa pine, generally growing in clean open forests of great 

 beauty and value. These forests reach their greatest perfection in 

 the Upper Deschutes and Klamath country, but are also well devel- 

 oped over much of the Blue Mountain Plateau. Other characteristic 

 trees of the division are the western tamarack (Larix occidentalis) , 

 western birch (Betula fontinalis), and many willows along the 

 streams. The shrubby vegetation is represented by the bitterbush 

 (Purshia tridentata), squawcarpet (CeanotJius prostratus) , buck- 

 brush (Ceanothus velutinm) , snowberry (Symphoricarpos racemo- 

 sus), bearberry (Arctostaphylos uva^ursi), and barberry (Berberis 

 repens) . 



ARID TRANSITION ZONE 



The arid subdivision of Transition Zone in eastern Oregon covers 

 the high valleys, plateau tops, and lower mountain slopes of approxi- 

 mately the southeastern quarter of the State. It includes the moun- 

 tains and plateaus east of Goose and Summer Lakes and south of the 

 Blue and Maury Mountains, among which the Steens and Warner 

 Mountains are the highest. In flora and fauna it is essentially a part 

 of the Great Basin division of the zone, too arid for timber growth, 

 with an average annual rainfall below 15 inches. 



