TOPOGRAPHY AND LIFE ZONES OF OREGON [31] 



associations are strikingly obvious, while others are so intricate and 

 obscured by other phenomena that recognition is difficult and in some 

 instances almost impossible. Differences of opinion and interpretation 

 confuse the beginning student. It is not the intention here to go into 

 detail concerning these biotic communities, but rather to paint a simple 

 word picture that can be understood by those who are just beginning 

 their study of Oregon's fascinating out-of-doors. 



Broadly speaking, North America is divided into bands of character- 

 istic associations of plants and animals that extend in a generally east- 

 and-west direction across the continent. If the continent were perfectly 

 flat, these bands would tend to lie in arcs with the ends on the oceans 

 and the arc sprung northward toward the center of the land mass. A 

 glance at the general life-zone map will show that this idealistic line is 

 warped out of place by altitude, presence or absence of considerable bodies 

 of water, mountain ranges, rainfall, temperature, and many other factors, 

 with the result that, particularly in a mountainous State such as Oregon, 

 these zones are broken up into irregular blocks, tongues, and islands that 

 lie scattered among the mountains and along the rivers in apparent con- 

 fusion. This lack of uniformity, when once understood, is not confusion, 

 however, but rather conformity with the series of laws that are the basis 

 of the science of ecology. 



Plants are most frequently used as life-zone indicators, as they are avail- 

 able the year around and are not quite so flexible in their response to 

 environmental change as are birds and mammals. Every species of plant 

 and animal has a definite range within which one may confidently expect 

 to find it. Some of these species have a range that is almost world-wide. 

 A good example is the Short-eared Owl, which is found not only over 

 much of North America but also over the northern part of the Old World. 

 Such forms, which apparently are little affected by environmental differ- 

 ences, are useless for determining life zones. Others are so definitely 

 limited by environment that their presence in any numbers is at once 

 indicative of certain zones. Yellow pine, which characterizes the Transi- 

 tion Zone of eastern Oregon, is an excellent example. 



The zone pattern of vegetation types is controlled by many factors, 

 prominent among which are soil depth, fertility, acidity, alkalinity, 

 water supply, temperature both summer and winter slope, exposure, 

 altitude, and latitude. If there could be found an isolated mountain 

 peak, such as Mount Hood, formed as a perfect cone, with slopes at equal 

 angles in all directions, with no valleys or ridges, and with exactly similar 

 soil, rainfall, and other conditions from sea level to the peak, the life 

 zones would be arranged in the form of circular belts around the moun- 

 tain. These belts would be tilted downward on the colder northern slope 

 and upward on the warmer southern slope, due to the difference in ex- 

 posure. This ideal arrangement is never found, however, for zones are 



