• 14 



species with which it is associated. These regions are the north 

 coast region, the Sierra region, the northern, the central, and the 

 southern Rocky Mountain regions. (Fig. 3.) The first two embrace 

 the range of the coast fomi of the tree and the last two that of the 

 mountain form. The northern Eooky Mountain region may be 

 called a transition district in which the two forms yery probably 

 intermingle. 



Though well defined at the geographical extremes of the Douglas 

 fir range, the regions merge into one another in other parts. Each 

 region may be further diyided into local and altitudinal forest types, 

 but each has characteristics sufficiently pronounced to distinguish it 

 as a whole from the others. Climate is the factor fundamentally 

 responsible for these yariations in forest composition. 



XORTH COAST REGIOX. 



The north coast region extends from the crest of the Cascade 

 Mountains, in southern British Columbia. AYashino-ton. and Oreo:on, 

 to the sea. and southward to moist localities in the California coast 

 ranges. In the exceedingly humid climate and moist soils of this 

 region all trees reach large size, and Douglas fir attains its best 

 deyeloj^ment in size and abundance. About half of the heayy forests 

 in this region are Douglas fir, which often 'forms extensiye and prac- 

 tically pure stands of great density. Its chief associates are western 

 and black hemlocks. Sitka sprtice. giant arboryita?, western white 

 pine, and lowland, amabilis. and noble firs. Of these, western hem- 

 lock deseryes the most attention because of its abundance and ag- 

 gressiyeness in reproducing imder Douglas fir and with its heays^ 

 foliage excluding the latter from the forest. On the coast of south- 

 ern Oregon and northern California the great redwood ty2:)e of forest 

 is predominant. 



Modified by the preyailing warm sea winds and protected by the 

 Rocky mountains and the Cascades from northeastern cold wayes, the 

 climate of the north coast region is generally mild and ttniform, with 

 frequent fogs and gradual and moderate changes in temperature. 

 The summers are cool and the winters mild, with an interyal of six 

 or seyen months between killing frosts. In the northern part of this 

 region rainfall and eleyation are indirect rather than direct agents in 

 limiting the distribution of Douglas fir. The more exacting species 

 with which it associates are able to exclude it from the more moist 

 situations to relatiyely dry places, where it can compete successfully 

 with them. This does not mean that Douglas fir is unable to thriye 

 in moist localities pro^'ided the drainage is good, for its best deyeloj)- 

 ment is made where rainfall is heayiest. Xear sea leyel west of the 

 Olympics, and from 1.500 to 2,500 feet aboye the sea on the western 



[Cir. 150] 



