10 
BULLETIN 1291, IT. S. DEPARTMENT OF AGRICULTURE 
tion that varies widely with slope and exposure; also in the lodge- 
pole pine region aspen commonly extends through the tension zone 
at the lower limit of the lodgepole type, thence in sheltered locali- 
ties and along streams out into the sagebrush type for considerable 
distances. In Arizona. Utah, and Colorado, the lower limit is 
about 7,000 feet ; in Idaho and Wyoming, 6,000 feet. 
The upper limit has a mean annual temperature around 35° F., 
about equivalent to southern Manitoba, and the lower limit 45° F., 
comparable with southern Xew England. However, the tempera- 
ture extremes are very much less in the Eocky Mountains than in 
the mid-continent. Winter minima are not frequently much below 
— 10° F. for more than a few days at a time at the upper limits of 
aspen, while summer maxima rarely exceed 85° F. The upper limit 
seems to be marked by insufficient summer growing temperatures. 
Rainfall in the aspen type shows considerable variation. In the 
central Rocky Mountain region of extensive aspen stands, this cli- 
matic factor is of first importance in limiting the lower extension 
of the type. In other regions where aspen occurs more sporadically, 
it is so frequently found on sites which have exceptional soil mois- 
ture that rainfall is a secondary consideration in determining the 
extent of the species. On the other hand, aspen will grow on 
average sites in Utah where there are no water supplies except from 
rainfall and where the annual precipitation is as low as IT inches, 
of which only a little more than 5 inches falls in the growing sea- 
son. In the spruce-fir type there may be as much as 30 inches per 
year. Where precipitation is low the stands reach poor develop- 
ment and become decadent at an early age, although even here they 
are decidedly superior to the thicket type of aspen, found at high 
elevations, which is little more than brush. Stands intermingling 
with sagebrush at low elevations are of site quality 3 and 4. 5 
Table 3. — Mean temperatures in the aspen zone, in degrees Fahrenheit 
Station 
Jan. 
Feb. 
Mar. 
Apr. 
May 
June 
July 
Aug. 
Sept. 
Oct. 
Nov. 
Dec. 
An- 
nual 
Colorado: 
Cheeseman 
Estes Park 
30.7 
22.7 
25.7 
16.6 
16.3 
20.8 
15.7 
20.2 
29.3 
25.9 
23.7 
19.2 
17.7 
22.9 
17.8 
21.7 
36.8 
31.9 
29.5 
28.9 
23.0 
27.8 
25.0 
31.3 
43.7 
36.6 
33.7 
38.4 
31.6 
36.5 
33.6 
41.0 
50.9 
44.4 
42.9 
45.6 
42.8 
44.0 
42.4 
49.5 
60.9 
55.7 
52.9 
55.1 
47.8 
53.1 
51.8 
56.8 
65.5 
62.1 
57.6 
60.1 
54.6 
57.8 
55.9 
64.3 
68.6 
60.6 
56.0 
59.0 
52.8 
55.9 
54.2 
62.3 
58.2 
52.4 
49.9 
52.2 
45.7 
50.6 
47.5 
53.3 
47.8 
40.8 
39.9 
41.6 
36.8 
40.2 
32.4 
43.0 
40.1 
33.2 
33.6 
30.8 
27.1 
31.0 
25.9 
32.1 
29.7 
30.2 
24.9 
15.3 
15.9 
21.2 
15.3 
19.9 
46.8 
41.4 
39.2 
Lake Moraine _.- 
Silverton 
Telluride 
Wagon Wheel... 
Idaho: Irwin.. - 
38.6 
34.3 
38.5 
34 8 
41.3 
Utah: 
East Portal, 
Strawberry 
Tunnel 
Soldier Summit.. 
Winter Quarters- 
Wyoming: 
Afton 
Alta - 
11.4 
17.7 
16.5 
14.6 
21.8 
18.2 
21.9 
28.4 
25.3 
31.5 
39.5 
35.7 
41.9 
46.6 
43.4 
50.7 
53.5 
51.3 
57.1 
60.5 
58.2 
54.6 
60.8 
56.7 
47.4 
51.1 
47.9 
39.4 
41.2 
36.5 
27.2 
29.0 
26.8 
15.1 
17.5 
15.5 
344 
39.0 
36.0 
17.1 
17.2 
18.9 
18.0 
27.5 
24.7 
38.2 
37.0 
46.9 
45.3 
55.7 
53.4 
61.1 
59.1 
59.9 
57.6 
52.7 
50.1 
42.0 
41.1 
29.7 
28.9 
17.4 
17.2 
38.9 
37.5 
c See Table G, p. 13, for criterion of sites. 
