Nov. 15, 1914 Revegetation of Range Lands 95 



The forest exhibits three principal rock formations: Basaltic, granitic, 

 and limestone. These give rise to as many different soil types, which in 

 turn very largely determine the character and density of the vegetation. 

 The best and most luxuriant vegetation is found upon the basaltic soils, 

 which cover the greater part of the region, the comparatively recent lava 

 flows from which they originate having buried the original formationsi n 

 some places to a depth of several hundred feet. They are porous and 

 very friable, admit of about average percolation, and retain water well. 

 The granite and limestone soils, on the other hand, are poorly decom- 

 posed and lose moisture rapidly through percolation and evaporation. 

 In consequence the vegetation is usually sparse, only the more drought- 

 resistant plants being able to establish themselves. The limestones, 

 mixed with shales, are the oldest and most restricted of the three forma- 

 tions. The granites, which are of a later period, form the peaks, crests, 

 and soils of the very highest mountains. 



CHARACTER AND DISTRIBUTION OF THE VEGETATION 



Within the great altitudinal range between the lower valleys and 

 higher mountains of the region wide differences naturally exist in the 

 physical conditions which govern plant growth, and therefore in the 

 character and composition of the growth itself. On the other hand, 

 physical and climatic conditions, and consequently the vegetation, are 

 strikingly similar within certain altitudinal limits, making it possible to 

 divide the region into four climatic zones. Following Merriam's classi- 

 fication' these are: 



Transition zone (yellow-pine association) 3,000 to 4,500 feet. 



Canadian zone (lodgepole-pine association) 4.500 to 6,800 feet. 



Hudsonian zone (whitebark-pine association) 6,500 to 8,500 feet. 



Arctic-Alpine zone (Alpine-meadow association) above 8,000 feet. 



The altitudinal limits of these zones are not absolutely marked, since 

 altitude does not wholly determine the character and composition of the 

 vegetation. Hence, the limits given above should be considered only 

 approximate for a given locality in this latitude. 



The Transition Zone. — The Transition zone contains a number of 

 coniferous tree species, the most characteristic being western yellow pine 

 {Pinus ponderosa). Toward the upper limits of the zone yellow pine 

 g[ives way to Douglas fir (Pseudoisuga taxijolia) and lowland fir {Abies 

 grandis). As a rtile, the timber is open, with considerable undergrowth 

 of average palatability and nutritiousness, if grazed relatively early 

 (PI. XII). 



Among the most characteristic and abundant herbaceous species is 

 pine-grass {Calamagrostis rtihescens). Other species which furnish a large 



* Merriam, C. Hart. Life zones and crop zones of the United States. U. S. Dept. Agr., Div. BioL 

 Survey, Bui. lo, 79 p., 1 map. 1898. 



