ASPEN IN THE CENTRAL ROCKY MOUNTAIN REGION 
31 
Table 18 indicates that on good sites (1 and 2) the question of 
disease as affecting rotation can be dismissed, unless aspen saw tim- 
ber is to be raised, in which case the rotation will have to be deter- 
mined by the length of time the timber will remain reasonably sound 
rather than by the culmination of increment — that is, gross incre- 
ment — without any cull factor to allow for decay as is shown in this 
table. As the sites grow poorer the culmination of increment ad- 
vances to higher age classes and the pathological rotation drops, so 
that on sites 3 and 4 the soundness of the timber will be the prime 
consideration in determining the rotation. 
The yield of conifers associated with aspen can not be closely 
gauged for reasons already mentioned. Several stands of aspen 
and conifers growing under the same conditions side by side have 
been studied, with the results shown in Table 19. 
Table 19.— 
Comparison of yields of 
conifers and q 
spen 
Site quality 
Age of stand 
Present volume 
Mean annual 
increment 
Volume, average 
tree 
Aspen 
Conifer 
Aspen 
Conifer 
Aspen 
Conifer 
Aspen 
Conifer 
1... 
68 years . _ . 
40 years . . . 
60 years _ . 
105 years.. 
Uneven... 
....do 
Cubic feet 
4,000 
1,020 
1,100 
660 
270 
Cubic feet 
3,757 
1,719 
1,028 
1,642 
1,482 
Cubic feet 
58.8 
25.5 
18.3 
15.1 
6.3 
Cubic feet 
35.8 
21.3 
15.1 
13.9 
9.03 
Cubic feet 
12.3 
1.8 
1.42 
.59 
.45 
Cubic feet 
15.2 
2 
13.7 
3 
8.5 
4 
44 and 88 
years. 
....do 
____do 
8.7 
5 
9.4 
Table 19 indicates that the mean annual increment of aspen and 
conifers is approximately the same when middle-aged aspen stands 
and all-aged conifers stands are compared, that is, the actual amount 
of wood produced is about the same for both species, although 
conifers have a small advantage on poor sites. The two respond in 
a very different way, however, as the growth conditions become 
poorer. Aspen trees become much reduced in individual develop- 
ment as the number of trees per acre increases, approaching as a 
limit the familiar aspen thicket type. (PI. VIII.) The individual 
development of conifers is reduced much more slowly, but decrease 
in the number per acre leads to open park-like stands at the limits 
of the coniferous type. As far as the production of merchantable 
saw timber is concerned, the conifers naturally have an immense 
advantage. 
THINNING 
The extreme density of many young aspen stands suggests at 
once the possibility of getting a much better production of larger- 
sized wood by judicious thinnings. As a matter of fact, the pos- 
sibility of getting stimulated growth by thinning does not seem to 
be very encouraging, although no stands exist which have been 
properly thinned from their youth to maturity. The chief practical 
difficulty lies in the small size of the material which must be re- 
moved, as it is very apparent that thinnings must be started early in 
the life of an aspen stand. A series of plots in aspen averaging 
from 3 inches to 7 inches in diameter (40 to 70-year-old stands) 
has been thinned, and the results five years later have been observed. 
