ASPEN IN THE CENTRAL ROCKY MOUNTAIN REGION 39 
pends on the higher stumpage rate obtained. Since the mine-prop 
market is limited and conifer props are preferred, it is not often 
possible to sell aspen to the mines. 
A conifer log scaling 100 board feet and worth $0.20 at $2 stump- 
age contains about 17 cubic feet and is worth $0.0118 per cubic foot. 
An aspen prop 16 feet long is worth $0.08 at 0.5-cent per linear foot 
stumpage, and contains 4.3 cubic feet and is worth $0.0186 per cubic 
foot. The aspen is not only worth almost double the conifer, but in 
utilization for props very much less of the tree is wasted. 
In general, the income from aspen is lower than from conifers on 
the same site. Aspen is a wood distinctly valuable for only a few 
purposes, and the entrance into this region of industries using aspen 
is problematical. Conifers, on the other hand, always have a broad 
general usefulness, and a permanent demand independent of special 
markets. There can be no point, therefore, in trying to maintain an 
aspen management type. 
TRANSFORMATION OF ASPEN TYPE 
The natural transformation of the aspen type over into a conif- 
erous type is a very slow process. The rate of invasion of the 
conifers can very clearly be noted in places where single old trees 
stand in areas of pure aspen. In protected flats the seed spreads 
fairly uniformly from the parent trees to a distance of about 150 
feet in all directions. On exposed ridges it will spread as far as 400 
feet to the leeward and very slightly in other directions. In a 
45-year-old aspen stand, where this rate of seeding in was carefully 
studied, the conifers were still only about 8 to 12 inches tall. It 
will not be less than 80 to 100 years before these trees begin to seed 
in turn to the leeward. The average progress of the conifer 
stand is therefore somewhat about 3 feet per year; but this is occa- 
sionally accelerated by seed that has by some chance been carried 
much farther than usual, and is forming a new point from which 
seedlings spread in turn. On the assumption that conifers found 
in the aspen zone will bear seed at 80 years, most areas ought to be 
well seeded in with reproduction in three tree generations or about 
250 years in the Douglas fir-white fir zone, and 150 years in the 
Engelmann spruce-alpine fir zone. Certain areas in the lower zones 
may require more than 250 years to be transformed on account of 
their great extent and purity. On aspen areas at high elevations 
(site quality 5), the replacement by a conifer stand will practically 
never take place, conifers scattered among the aspen being the most 
that can be expected. 
Transformation can be hastened artificially but at a rather high 
cost. Artificial regeneration has a certain advantage in that the 
more valuable species, Engelmann spruce and Douglas fir, can be 
given the advantage in each zone, while in natural invasions alpine 
fir and white fir are much more numerous. The cost of planting 
amounts to about $25 per acre, which if it is to be recovered in one 
rotation amounts to an annual charge of 12 cents on a 200-year 
rotation, no interest charges included. It is evident that a decided 
advantage of conifers over aspen must be demonstrated before any 
general plan for planting up aspen areas to conifers is embarked 
upon, and there must be a reasonable assurance that the advantage 
