8 BULLETIN" 14 9 9, U. S. DEPARTMENT OF AGRICULTURE 
weaker, the latter persisting, nevertheless, for long periods, even 
though they may make an extremely slow growth. 
ENGELMANN SPRUCE TYPE 
The Engelmann spruce type, as considered here, includes those 
stands which contain 50 per cent or more of Engelmann spruce with 
varying mixtures of alpine fir, white fir (Abies con-color), lodgepole, 
limber, and bristlecone pines, whitebark pine (Pinus aibicaulis), 
Douglas fir, and blue spruce. The occurrence of bristlecone pine in 
mixture with Engelmann spruce is limited to Colorado and Utah; 
whitebark pine is found in association with Engelmann spruce only 
in Montana, Idaho, and Nevada. Alpine fir is almost invariably 
present, but under natural conditions seldom predominates. Spruce 
is only occasionally found in pure stands. 
Engelmann spruce occurs typically in uneven-aged stands. It 
withstands shade well, and reproduction is nearly always present. 
Seedlings persist even in dense shade; and though they may make 
very slow growth, they respond readily when released. 
Toward the lower limits of tree growth the type follows stream 
courses; at intermediate elevations it spreads to other cool, moist 
situations; from here on up to timber line (10,000 feet or higher) 
it occupies any site. For the protection of watersheds it is by far 
the most important type in the region. 
Maximum stands are found at elevations varying from 9,000 to 
10,000 feet. While yields of 60,000 feet an acre have been reported, 
the average for the commercially timbered areas would run about 
15,000 board feet to the acre. 
DOUGLAS FIR TYPE 
The Douglas fir type includes stands containing 50 per cent or 
more of this species, and also the small and scattered areas of the 
western yellow pine type within the region. Lodgepole and limber 
pines, western yellow pine (Pinus ponder osa) , Engelmann spruce, and 
alpine fir are the species generally found in varying proportions in 
the Douglas fir type. Other species, such as white fir, bristlecone 
pine, and blue spruce, are associates in the southern portion of the 
region. 
The type ranges from the lower limits of tree growth up to ap- 
proximately 9,500 feet. The best stands are found at altitudes vary- 
ing from 7,500 to 8,500 feet. In contrast to the lodgepole pine type, 
Douglas fir usually occurs in stands of comparatively small area on 
cool, north slopes at lower elevations and on warmer sites at the 
higher altitudes. In the northern part of the region, particularly 
in Idaho, the type extends over larger continuous areas, and there 
attains its best development for the region. Stands in Idaho run 
as high as 35,000 board feet to the acre, but the average volume on 
the better-timbered areas is about 10.000 board feet. The volume 
on the commercially timbered lands in the southern portion of the 
region averages only 5,000 to 7,000 board feet an acre. 
The stands, which are usually uneven-aged, ranging from seedlings 
to large overmature trees, contain large quantities of alpine fir or 
white fir, inferior species that in the seedling stage are better able to 
