ii ii i ii mi 111 iiiii 111 1 ■ nun it ii iiiii iiiii mil mi mi 
ASPEN IN THE CENTRAL KOCKY MOUNTAIN EEGION 
13 
nection with site quality. Therefore a classification of sites is 
necessary before a presentation of data regarding the tree is under- 
taken. 
Height growth has been adopted as the criterion of site quality 
in this bulletin. Results shown under growth, yield, etc., are 
identical, however, whether height or annual volume increment is 
chosen as the criterion of site, as there is extremely close correla- 
tion between the two. 
Five site qualities have been recognized, based upon the height 
of the dominant trees in the stand and the age of the stand. The 
dominant rather than the average height was chosen as the basis be- 
cause otherwise a determination both of the age of the stand and of 
the average diameter would be required. Table 6 gives the age- 
height relations on each site. 
Table 6. — Criterion of site quality — Aspen 
Age 
Height of dominant trees 
Age 
Height of dominant trees 
Site 1 
Site 2 
Site 3 
Site 4 
Sitel 
Site 2 
Site 3 
Site 4 
10 years .... .. 
Feet 
13 
25 
37 
47 
57 
66 
73 
77 
Feet 
11 
19 
29 
38 
48 
55 
62 
67 
Feet 
8 
15 
23 
30 
37 
44 
50 
55 
Feet 
6 
13 
19 
24 
30 
35 
40 
44 
90 years .- 
Feet 
80 
81 
82 
83 
83 
83 
83 
Feet 
71 
74 
75 
76 
Feet 
60 
63 
66 
Feet 
47 
20 years - 
100 years 
49 
40 years .- 
120 years 
50 years 
130 years 
150 years 
80 years 
Trees less than 40 feet tall at maturity belong to the unmerchant- 
able-thicket type of aspen of site quality 5. 
In general, site quality 1 is found on moist flats having deep, 
perhaps rather heavy, soil. The trees reach good size, and are clean- 
boled and straight. (PL III.) 
Site quality 2 includes the great bulk of the stands of good quality 
in the central Utah region. In general, these sites are relatively 
not so well watered and are more likely to be on slopes, as com- 
pared with site quality 1. The trees reach saw-log size before de- 
cay renders them valueless. 
Site quality 3 includes situations similar to quality 2 as to topo- 
graphy, but usually at elevations below the best altitudinal zone for 
asjDen. It also occupies steep slopes within the optimum zone. 
(PI. IV, fig. 1.) 
Site quality 4 is found nearly everywhere at the lower limits of 
aspen, and also on south slopes at higher elevations. It usually 
bears a dense pole stand which rarely develops trees of saw-log 
size. 
Site quality 5 includes great areas of aspen on south and west 
slopes, on poor soils, and particularly at elevations above the zone 
of best development of the species. (PI. IV, fig. 2.) 
These site classifications can not be directly translated into terms 
of site classifications for coniferous trees, as there is a great dif- 
ference in the habit of growth of conifers and aspen, and they do 
not respond to changes in their environment in the same ways. A 
