20 



its form and rcite of growth a decided preference for deep rather 

 than shallow soils, and avoids dry, light sand, as well as heavy clays. 

 It grows best on fresh sandy loam or loamy sand, and reaches its 

 largest size on deep, porous,^ loamy soil, well watered and at the same 

 time well drained. 



On dry, sandy soils the characteristic associate of Douglas fir 

 almost throughout its range is yellow pine. In southern Oregon and 

 in California, Port Orford cedar on the coast and incense cedar in 

 the mountains also share sandy soils with it. ^Miere loamy ingre- 

 dients prevail, the coast form of the tree mixes with Sitka spruce on 

 fresh soils, while on dry soils in the Rocky Mountains and California 

 white fir mixes with Douglas fir. In the northern Eocky Mountains 

 Douglas fir gives way to lodgepole pine on coarse, fresh, gravelly 

 soils, and to Engelmann spruce and alpine fir in poorly drained 

 situations and clayey soils. It thrives in northern Montana and 

 Idaho on the deep, fresh, porous loams on which western larch makes 

 its best growth. 



Douglas fir also adapts itself readily to varying conditions of 

 moisture in soil and atmosphere. The coast form reaches its largest 

 size in the heavily watered north coast region, and is scarcely smaller 

 where it grows in the heavy fogs of the California Sierra Xevada. 

 Here it makes its best growth in humid situations, associated with 

 such moisture-loving species as western hemlock and giant arborvita^. 

 In southern Oregon it has been found growing at the edges of ocean 

 inlets, where the least rise submerges its roots in brackish water. In 

 dr}^ situations its growth is slower and its size smaller than on fresh 

 but well-drained soils. Atmospheric humidity is essential to its best 

 height development, and in dry air with fluctuating moisture content 

 it does not equal, even on the best soils, the height it attains on the 

 poorest soils in humid regions. 



The Rocky Mountain form is much less susceptible to injury from 

 drought than the coast form. It grows best, however, on cool north- 

 erh^ slojDes and in protected canyons at fairly high altitudes. Such 

 situations afford the greatest degree of atmospheric humidity and the 

 most soil moisture. On slopes fully exposed to the sun, both soil 

 and air moisture are scant, and Douglas fir is proportionately stunted 

 and knotty. In the lodgepole pine type of forest, it gives place to 

 the pine in moist situations, but is able to succeed the pine on dry soil. 



Its adaptability to soil and moisture conditions often saves Douglas 

 fir from exclusion by other aggressive species, more tolerant of shade, 

 but also more exacting in regard to soil and moisture. In western 

 Washington and Oregon, Sitka spruce, western hemlock, giant arbor- 

 vitse, and lowland and amabilis firs, which can all endure more shade, 

 are able to monopolize the soils which best suit them, and tend to 



[Cir, 150] 



