1936] MAMMALS AND LIFE ZONES OF OREGON 19 



These critical frost conditions apply not only to eastern Oregon 

 but to a more extensive area of the Great Basin, involving parts 

 of eastern California, Nevada, Utah, and southern Idaho. They must 

 be understood if the area is to be developed to its best possibilities. 



Detailed information on climate and crop conditions, on recom- 

 mended farm practices, and on suitable crops for the various sections 

 of Oregon can be obtained in publications of the United States De- 

 partment of Agriculture and of the Oregon Agricultural College. 



TRANSITION ZONE 



The Transition Zone, as the name implies, lies between the Austral, 

 or Sonoran Zone to the south or at lower altitudes, and the Canadian 

 Zone to the north or at higher elevations. It derives a part of its 

 fauna and flora by the overlapping of species of the lower and higher 

 zones. In Oregon and the Pacific Northwest generally, it is unusu- 

 ally broad and well defined with many restricted species of its own. 



In Oregon the zone may be divided into four well-marked subdivi- 

 sions based on different degrees of humidity, varying from an annual 

 precipitation of approximately 100 inches down to 10 or 15 inches. 

 The humid division lying west of the Cascades is characterized gen- 

 erally by heavy forests of pine, hemlock, Douglas fir, western hem- 

 lock, and many Abroad-leaved trees (pi. 9) . Along its western edge lies 

 the narrow coast strip, rarely more than a few miles in width, extend- 

 ing from southwestern British Columbia to northwestern California 

 and well marked by the Sitka spruce and accompanying species 

 of plants and animals. Along the eastern side of the Cascades and 

 over the Blue Mountain plateau with a much reduced rainfall, the 

 semiarid or semihumid division of the zone is characterized by open 

 forests of yellow pine (pi. 10, A). On the high plains and plateaus 

 of the southeastern part of the State the more arid division of the 

 zone is devoid of real timber and characterized by mountain-mahog- 

 any (pi. 10, #), sagebrush, and the broad-leaved balsamroot. Al- 

 though blending into each other, these subdivisions of the zone must 

 be treated separately to be understood and recognized. 



COAST STRIP OF TRANSITION ZONE 



The coastal strip, or fog belt, as it is sometimes called, gets the 

 first sweep of the damp, cool, but never very cold ocean winds. It 

 has a remarkably even climate throughout the year, with an annual 

 rainfall of 80 to 100 inches. While almost free from frost in winter, 

 it is cold and damp in summer, thus allowing an unusually even 

 temperature with an almost complete overlapping of Transition and 

 Canadian Zone species. It has been mapped first as one zone and 

 then the other, but the best authorities acknowledge it to be a mixture 

 or overlapping of the two. 



To the southward the rainfall decreases slightly, and south of the 

 mouth of the Coquille River the shores are steeper, more abrupt, and 

 consequently drier and slightly warmer. Hence a considerable num- 

 ber of California plants extend up the coast as far as the Coquille 

 and not beyond. On plants alone Peck (1925a, p. 35) divides the 

 strip at this point but the subdivision is not strongly marked in other 

 forms of life. 





