12 



tains its lower limit is considerably above the sea. In parts of the 

 Klamath and Trinity National Forests it grows as low as 900 feet 

 above sea level, but is apt to be scrubby and scattered up to 2,000 feet, 

 while none of its scant representation in the Santa Lucia Mountains 

 is found below 2,500 feet. On the western slope of the Sierra Nevada 

 Mountains it is sometimes found in river valleys as ,low as 800 or 900 

 feet above sea level, but is usually scattered and stunted below 2,000 

 feet, and reaches its best development between altitudes of 3,000 and 

 6,000 feet, while in Mariposa County, near the southern limit of its 

 range, it grows, more or less stunted, up to 7,500 feet. 



The mountain form at its northern limit, near Tacla Lake, in Brit- 

 ish Columbia, grows at an elevation little over 2,000 feet, which is 

 about the common level of the country. In Idaho, Montana, and 

 northern Wyoming it is found at altitudes of between 2,400 and 9,000 

 feet ; in southern Wyoming, northern Colorado, Utah, and northern 

 Nevada, between 6,000 and 9,000 feet ; and in southern Colorado and 

 Utah, Arizona, New Mexico, and western Texas, between 7,000 and 

 11,000 feet above the sea. At the extremes of altitude, especially in 

 the south, the growth consists chiefly of straggling individuals, and 

 at the highest elevations the trees become very much gnarled and 

 stunted. Good development is usually confined to favorable locali- 

 ties Yv'ithin a narroAv range of 1,000 or 1,500 feet about midwa}^ be- 

 tween the extremes. 



RELATION OF EXPOSURE TO DISTRIBUTION. 



Douglas fir is less exacting in regard to exposure than Engelmann 

 spruce, alpine fir, and other species with which it associates, but 

 nevertheless, in common with all tree growth, it reaches better devel- 

 opment, in the arid portions of its range, on northern exposures than 

 on southern, especially at low altitudes, since the more shaded north- 

 ern slopes offer more moisture. This is particularly true of the 

 Rocky Mountain form in its extension through the mountains of the 

 Southwestern States and northern Mexico. In the north and at high 

 altitudes throughout its range, how^ever, where the question of mois- 

 ture is not so pressing, and where trees may even be forced to the 

 southern exposures by frosts and cold winds, Douglas fir is found on 

 the warmer southern slopes. In such situations the growing season 

 may be considerably longer on the southern sides of mountains than 

 on the northern. 



The coast form grows, as a rule, at lower elevations on the western 

 sides of mountain ranges than on their eastern slopes, owing to the 

 influence of moisture-laden winds from the sea. Especially in the 

 southern portions of its range, Douglas fir of both forms grows best 

 and reaches its lowest altitudinal limits in sheltered situations, such 

 as protected canyons, slopes, and benches. 



[Cir. 150] 



