BULLETIN 1291, U. S. DEPARTMENT OF AGRICULTURE 
FORM 
Aspen is a full-boled tree, having excellent form on the best sites 
but deteriorating into a crooked shrub on poor sites. The form of 
the tree is not constant, but changes with height although not with 
diameter, becoming fuller boled as height increases, as indicated by 
the absolute breast-height form quotient (diameter inside bark at a 
point halfway between breast height and the top of the tree divided 
by diameter, breast height, inside bark). This is expressed as fol- 
lows: At a height of 40 feet the form quotient is 0.64; at 50 feet, 
0.64; at 60 feet, 0.65; at 70 feet, 0.67; at 80 feet, 0.73. 
Mature aspen 12 to 14 inches in diameter at breast height and 50 
feet high has a fuller bole than any of the conifers associated with it 
except white fir, as indicated by its form quotient of 0.64 for 50-foot 
aspen. The quotient for Englemann spruce, alpine fir, and Douglas 
fir is 0.62; that for white fir is 0.65. In amount of clear-length as- 
pen is extremely variable, although as a rule in fully stocked stands 
the crown is limited to a mere wisp of branches in the top, with dry 
branches extending down perhaps 10 feet. The twigs die early and 
soon break off in good dense stands ; in poor sites they are more per- 
sistent, grow larger, and occupy more of the bole of the tree, giving 
it a dark, dirty, unkempt appearance, in striking contrast to the 
smooth trunks of thrifty trees. Typical conditions of this sort on 
sites 1, 3, and 5 are shown in Plates III and IV. 4 
Pronounced stump swelling is shown in aspen at an early age, 
amounting to as much as an inch at the base of trees of 8-iner/diam- 
eter breast high. By the time the breast-high diameter is 16 inches 
the swell at stump height is nearly 2 inches. It begins to affect 
diameter-breast-high measurement directly when the latter reaches 
10 inches, at 16 inches contributing as much as half an inch to this 
diameter. 
The relation of diameter to total height varies with sites— the 
poorer the site the shorter the trees of a given diameter (Table 2). 
Table 2 — 
-Average height of aspen c 
f different dia 
meters 
, on sites 1 to // 
Diameter breast 
Total height 
Diameter breast 
height 
Total height 
height 
Site 1 
Site 2 
Site 3 
Site 4 
Sitel 
Site 2 Site 3 Site 4 
Feet 
40 
46 
51 
57 
62 
66 
70 
73 
75 
76 
Feet 
35 
41 
46 
51 
56 
59 
63 
66 
68 
70 
Feet 
32 
36 
41 
45 
46 
51 
54 
57 
58 
GO 
Feet 
30 
34 
37 
39 
41 
43 
45 
40 
48 
Fed 
78 
79 
80 
81 
81 
82 
82 
83 
83 
Feet 
71 
72 
73 
74 
74 
74 
75 
75 
75 
Feet 
61 
62 
62 
Fed 
6 inches 
16 inches... 
17 inches 
18 inches 
9 inches 
19 inches... 
20 inches . 
21 inches 
12 inches.. 
22 inches 
13 inches 
DARK 
The bark is one of the striking characteristics of the aspen tree, 
it being smooth, thin, and white or yellowish, except on larger trees, 
where it becomes dark and furrowed toward the base. This change 
is brought about sometimes on relatively small trees by sheep nib- 
* Sec Table G, i>. 13, for criterion of sites. 
