26 • BULLETIN 680, U. S. DEPARTMENT OF AGRICULTURE. 



fir is associated more often with lodgepole pine and Englemann 

 spruce, while western yellow pine grows with it at the lower eleva- 

 tions and alpine fir at its upper limits of range. The southern 

 Rockies and adjacent dry interior mountains, into which Douglas 

 fir extends, are characterized by climatic conditions differing greatly 

 from those of the middle and northern parts of this chain of moun- 

 tains, so that here Douglas fir is variously associated with limber 

 pine, Mexican white pine, bristle-cone pine, Engelmann spruce, 

 white fir, cork fir, Gambel oak, and aspen, 



Douglas fir is moderately tolerant of shade in its earlier growth, 

 becoming less so with age. Tolerance, however, varies with the 

 locality and region, being greatest under the conditions produc- 

 tive of the best growth. The tree endures more shade than western 

 yellow pine, sugar pine, western white pine, and lodgepole pine, 

 but less than western hemlock, western red cedar, white and alpine 

 firs, incense cedar, and Engelmann, blue, and Sitka spruces. Partial 

 shade and shelter are very necessary in the early life of Douglas 

 fir wherever the conditions for growth are least favorable. At the 

 North, where moisture is abundant, it thrives in the open from 

 early youth, while in drier interior regions it prefers the shade of 

 weeds, brush, etc. Dense pure or mixed stands in the Northwest 

 have clean trunks for about two-thirds of the length, while trees of 

 open forests and those in the Rocky Mountains are clean-trunked 

 for only about one-third their length, or, in scattered stands, carry 

 branches almost to the ground. The trunks clean themselves of 

 side branches slowly even in dense stands, which indicates con- 

 siderable tolerance of side shade; on the other hand, young trees in 

 dense stands grow rapidly in height, showing their need of over- 

 head light. 



Douglas fir is generally a prolific seeder, producing seed nearly 

 every year, with specially good local seed years at intervals of three 

 or four years. The seed is matured at about the same time through- 

 out the tree's range. Throughout the range of Douglas fir its seed 

 formation is nearly as abundant and its power of reproduction 

 nearly as great as in the case of western yellow pine. At best the 

 seed has a moderately high rate of germination and persistent vital- 

 ity. Not infrequently, however, the seed has a rather low per- 

 centage of germination, which may be due in some measure to the 

 fact that a large number of seeds are destroyed by insects. Many 

 are also eaten by birds and rodents. Germination and early de- 

 velopment of seedlings are best on warm, moist, pure mineral soU, 

 or in one mixed with humus. Reproduction is rare on thick duff 

 or vegetable matter, but abundant in humid regions after this layer 

 has been burned off, or thorougliiy broken up by logging operations. 

 Unburned, logged areas in this moist region are more commonly 



