8 
BULLETIN 1291, IT. S. DEPARTMENT OF AGRICULTURE 
extending almost to the Arctic Ocean. In the Rocky Mountain 
region it is found in the altitudinal zones characterized by Douglas 
fir and Engelmann spruce. At its upper limit it usually fails to 
reach such extreme altitudes close to the climatic timber line as are 
occupied by limber pine (Pinus flexilis), bristle-cone pine, (P 
Le<|er7d 
*£r : Dors at end of roots indicats disappearance inTo ground* 
• Sprout 
Fig. 2. — Aspen root systems excavated to the d^jrtJi of 1 foot. — Reference is made to dif- 
ferent areas on this figure by means of Nos. 1 to 10 along the left edge and the letters 
A to J along the top. The area referred to may be located by running horizontally 
from the number given and perpendicularly from the letter and noting where they 
cross. For example, the large stump near the bottom of tbe figure lies in F-10 and 
G— 10. Roots from which trees in the old cut-over stand originated as sprouts are 
shown in A-9 and from 7-C to 9-D. Tbese roots are much larger than any others on 
these trees. Rootlets apparently devoted primarily to reproduction, being free from 
feeding rootlets, run from D-10 to E-7 and from F-7 to H-9. When sprouts occur 
on such roots rapid thickening takes place and many feeding rootlets are sent out as 
in C-6, 1-10, and B, C-4, 5. On larger roots this does not happen, as in F-7 and 
1-9. Root collar suckers are not frequent, but appear at A-9 and D-9 
aristata), white bark pine (P, albicaulis), or even alpine fir (Abies 
lasiocarpa). It is, however, found in favorable sites only a few 
hundred feet lower, and may reach very good development in the 
higher parts of the spruce-fir type. The absolute altitude readied 
naturally varies with latitude. In the San Francisco Mountains of 
Arizona aspen reaches an altitude of 9,500 feet, in Colorado as high 
