ASPEN" Itf THE CENTRAL E0CK1Y MOUNTAIN REGION 7 
bling the lower bark. The thickness varies regularly with diameter, 
except near the base of the tree, where the bark is always relatively 
thicker than it is higher up, and increases in thickness more rapidly 
especially when the trees are 12 inches in diameter or over and fur- 
rowing and development of a dark, ridged bark begins to take place. 
Bark, usually whitest at low elevations, frequently appears yellow 
or brownish in exposed sites at high altitudes. The color differences 
are not entirely matters of site and exposure, however, as in some 
places distinctly white-barked and yellow-barked races of aspen are 
found adjoining one another with a clear line of demarcation be- 
tween them. This line can frequently be traced for long distances, 
particularly in the spring, since the yellow-barked race puts out 
leaves earlier than does the white. Leaves on yellow-barked trees 
may be about the size and shape of spade pipes on playing cards 
when the buds on the white-barked trees first show green. The 
white-barked trees usually appear straighter and taller than the yel- 
low ones on the same site (3). 
ROOT SYSTEM 
The root system of aspen is of particular interest, on account of 
the dependence of the species upon root suckers for reproduction. 
The typical root system of aspen consists of numerous shallow roots 
radiating from the base of the stump, one or two of which, particu- 
larly in small trees, are distinctly larger than the rest and are the 
original roots upon which the tree formed as a sucker. The root- 
spread as a rule is downward and outward to a depth not greater 
than 2 feet, and from this lowest point the roots pursue an undulat- 
ing course, rising to the surface of the ground and sinking again by 
turns. The ultimate feeding roots are found in all levels down to 2 
or 3 feet, although most frequently from 6 inches to 2 feet- 
Sprouts appear where the roots rise close to the surface, especially 
where two very shallow roots cross and the upper is brought very 
near the top of the soil. Certain small roots appear to be devoted 
primarly to reproduction, since they run for long distances in the 
shallower soil layers without much change in diameter or furcation, 
and with practically no feeding rootlets. \Vhen a sprout grows on 
such a root, a rapid thickening takes place near the point of at- 
tachment of the sucker and many feeding rootlets develop, particu- 
larly on that part of the root away from the parent tree. These 
usually remain functional for a long distance from the living tree, 
but rot off when the parent tree dies. Sprouts frequently are found 
near aspen stumps, but generally these come from roots of distant 
trees brought near the surface in crossing the large stump roots. 
These reproductive roots have been found also in old decayed stumps 
of conifers which they have penetrated, working up close to the top 
surface and turning sharply back down through the stump into the 
soil again. At the top point where the root makes a loop, a sprout 
or group is sent out through the top of the stump. (Figs. 2 and 3.) 
CLIMATIC REQUIREMENTS 
Aspen is a tree capable of growing in a wide variety of sites in a 
cool climate, particularly one characterized by long winters and short 
growing seasons. It is a trans-continental species through Canada, 
