1936] MAMMALS AND LIFE ZONES OF OREGON 11 



ods after a rain or snow, are more nearly typical desert and more 

 lifeless than any other part of the State. Such level valley bottoms 

 without drainage can produce neither water plants, because they dry 

 up quickly after rainfall, nor dry-land plants, because the occasional 

 sheet of water kills them. Thus devoid of vegetation, they have 

 an animal life limited to occasional antelope, jack rabbits, coyotes, 

 swift foxes, badgers, and lizards that wander over them. 



In the more humid areas under the abundant growth of vegetation, 

 the base soil is generally covered with rich humus of varying depths 

 that is especially attractive to burrowing insects and a great variety 

 of small invertebrates. Consequently this same mold-covered soil is 

 most attractive to numerous species of insectivorous mammals and 

 birds and holds the greater part of the shrew and mole population 

 of the State. 



PLANT COVER 



Types of vegetation or plant associations, as modified by soil mois- 

 ture and other local conditions, have a powerful influence on the 

 distribution of species of animals. Next to altitude or latitude the 

 varying degrees of humidity are most influential in determining the 

 characteristics of these plant associations. In a general way the 

 heaviest timber, as well as dense undergrowth, is found in the areas 

 of heaviest annual rainfall and the lightest vegetation in the areas 

 of lightest rainfall. The annual precipitation ranges from a maxi- 

 mum of about 100 inches in places on the coast and in the mountains 

 to less than 10 inches in the valleys of the southeastern part of the 

 State. The gradual change from dense humid forest and under- 

 growth through open forest, scattered woodlands, sagebrush, and 

 shrubby cover to the low and scattered bunch vegetation of the 

 desert valleys is shown in crossing the State from west to east. Most 

 pronounced is the change following a line drawn from the northwest 

 to the southeast corner. Coastal salt marshes, inland tule swamps, 

 and open grasslands all have a bearing on the distribution of certain 

 species of animal life. 



LIFE ZONES OF OREGON 



Owing to its broad extent, wide range of altitude, and varied 

 physiographic features, Oregon shows great diversity of climate and 

 contains 5 of the 7 primary life zones of the continent. ( See frontis- 

 piece.) Only the hot Lower Austral and Tropical Zones are unrepre- 

 sented. The Upper Sonoran, the arid western subdivision of Upper 

 Austral Zone, characterized by greasewood, saltbush, and rabbitbrush, 

 covers the low warm valleys of the State east of the Cascades, and en- 

 ters the Eogue and Umpqua Valleys west of the mountains. The 

 Transition Zone, characterized by the yellow pine, Oregon maple, and 

 mountain-mahogany, occupies the higher valleys and bench lands 

 east of the mountains and has a more humid subdivision in the val- 

 ley foothills and Coast Kanges west of the Cascades. The Canadian 

 Zone, the zone of spruce, fir, and lodgepole pine, covers the broad 

 high part of the mountains. The Hudsonian Zone, the zone of the 

 whitebarked pine, and dwarfed spruces and hemlocks near timber 

 line, caps or surrounds the highest peaks in a narrow belt, rarely 



