CONIFERS OF THE ROCKY MOUNTAIN REGION. 25 



OCCURRENCE AND HABITS. 



In the Kocky Mountain region Douglas fir occurs generally at 

 elevations between about 4,000 and 11,000 feet, the lower elevations 

 being at the North, while in the central and southern Rockies the 

 altitudes attained are from about 6,000 to 11,000 feet (Map No. 6). 

 In the northern Pacific coast country it occurs at sea level. It 

 grows best and in greatest abundance where precipitation and 

 relative humidity of the air are greatest. Except at high elevations 

 and at the northern limit of range, Douglas fir prefers northern to 

 southern exposures, and sheltered slopes, canyons, benches, etc., 

 to exposed situations. At the North, and at high elevations, warmer 

 southern exposures are preferred, for heat, not moisture, then 

 becomes the controlling factor in its growth. The lower limit of 

 vertical range in drier regions is determined by lack of moisture, 

 and the upper limit of distribution is marked chiefly by lack of 

 heat. Douglas fir occurs at higher levels on the east sides of moun- 

 tains than on the west sides, higher elevations also being reached 

 on southern slopes than on northern exposures. But it is more 

 abundant on western slopes than on eastern slopes, and likewise 

 more frequent on northern than on southern exposures — except 

 at the far North, where heat is the essential factor. 



Douglas fir is adapted to a great variety of soils, including nearly 

 all with sufficient moisture, from the border of brackish seacoast 

 water to soils in which, among other trees, only the drought-en- 

 during western yellow pine grows. It prefers fresh, well-drained, 

 porous, deep, loamy soils, avoiding saturated, poorly drained, 

 heavy soils. Good soil and abundant atmospheric and soil moisture 

 arc necessary for the best growth, but with abundant moisture, 

 quantity and quality of soil are less important, and vice versa. 

 Douglas fir grows faster and larger on poor gravels and sand in the 

 humid Puget Sound country than on the best soils of the Rocky 

 Mountains, where it is subject to dry air and deficient precipitation; 

 likewise, it is not so large on the best soils of the drier California 

 mountains, even though the climate is mild and heat and sunshine 

 are sufficiently abundant for other tree growth. 



Douglas fir forms large pure forests and often nearly pure stands, 

 but chiefly it is associated with a number of other species. In the 

 northern Rocky Mountain region it grows with western white pine, 

 western yellow pine, lodgepole pine, western red cedar, lowland 

 white fir, western hemlock, Engelmann spruce, alpine fir, and 

 western larch. Here the climatic and soil conditions are similar 

 in great measure to those of the northern Pacific slope country, 

 and account for the presence of so many typically west-slope trees. 

 In the middle Rockies, where the climate and soils are dry, Douglas 



