2 BULLETIN 1291, U. S. DEPARTMENT OF AGRICULTURE 
larky of Colorado, especially the " west slope," indicates that the 
conditions there, as far as aspen is concerned, are virtually identical 
with those of the central Utah region. 
DISTRIBUTION 
Aspen (Populus tremuloides Michx.) 1 is the most widely distrib- 
uted broadleaf tree of North America. Its range extends from 
southern Labrador to Hudson Bay (southern shores) and north- 
westward to the Mackenzie River, near its mouth, and Alaska 
(Yukon River); southward to Pennsylvania (mountains), north- 
eastern Missouri, southern Nebraska, and throughout the western 
mountains to northern New Mexico and Arizona and central Cali- 
fornia, Lower California (San Pedro Martir Mountains), and 
Mexico (mountains of Chihuahua) (14). 2 
In the mountains of the western United States aspen is found 
widely distributed at middle elevations. It is of slight importance 
throughout the greater part of this area, however, becoming a promi- 
nent component of forests 3 only in Utah, Nevada, Colorado, and 
parts of northern New Mexico, and Arizona (fig. 1). 
In either region it is impossible in forest management to consider 
aspen independently of the associated conifers. In the East this 
competition makes aspen an especially ephemeral type, occupying, 
as a rule, recent burns. In the West aspen appears to be perma- 
nent over large areas owing to the lesser aggressiveness of the coni- 
fers; but even here there is always a successional tendency working 
in aspen stands, tending more or less successfully to transform them 
into conifer stands. 
Silvicultural practices can aid or hinder this natural process, 
and in every aspen forest under management the silvicultural treat- 
ment must be considered first of all from the standpoint of its effect 
upon the related conifers as well as upon the aspen. Accordingly, 
this bulletin, while dealing with aspen primarily, includes much 
parallel work on conifers associated with aspen, in order to be of 
value in the solution of this most fundamental problem of the man- 
agement of aspen areas. 
The essential uniformity of aspen throughout its wide range is 
emphasized by comparing the results of these studies with earlier 
work on aspen in New England {15). It is found that while growth 
is more rapid in New England, deterioration and decay take place 
1 Other names in common local use are quaking asp (sometimes written quakeenasp), 
asp, quakers, and mountain asp. The scientific name P. tremuloides aiirea (Daniels) 
has been proposed to apply to the western aspen, the chief distinguishing character 
being the striking golden color of the leaves in the fall. Tidestrom recognizes this 
variety as a distinct species, P. aurea (Tidestrom). P. cereAdiphyUa (Brit.) is a tree 
similar to common aspen, except as to leaf characters. It has been found only in 
Upper Hoback Basin, Wyo. 
3 The italic numbers in parenthesis, occurring throughout the bulletin, refer to litera- 
ture cited in the bibliography on p. 46. 
3 The aspen type has been defined by the Forest Service (1013) as: "A stand con- 
taining 60 per cent or more of aspen, often nearly pure, but also with various conifers 
in mixture. Areas with a temporary type of aspen .... should be classified .... 
according to that stage of type development which will be used in management." Prin- 
cipal associated trees include western yellow pine [Piruus ponderosa), Douglas fir 
{Paeudot8uga tuzifolia), lodgepole pine (Pinus contorto), and Engelmann spruce {Picea 
enyelmcvnni) . 
