Ranging as it does from Alaska to Mexico and from the Pacific 

 coast to western Texas, Colorado, Nebraska, and Manitoba, western 

 aspen is perhaps the best-known tree of the western mountains, where 

 it is a characteristic feature of the landscape. It was in 1911 that the 

 American aspen, previously known solely as Populus tremuloides. was 

 definitely separated by Tidestrom 2 into an eastern and a western 

 species. The eastern form retains the name Populus tremuloides and 

 the western form is now known as P. tremuloides aurea. 3 The species 

 and its varieties are most widely recognized as aspen but less com- 

 monly are designated as American aspen, poplar, popple, quaking 

 aspen, quaking asp, trembling aspen, and trembling poplar. The 

 term "aspen", wherever it occurs in this description, refers to the 

 species tremuloides as a whole, including its western varieties. 



Populus tremuloides. as previously known, ranges from Labrador 

 and Hudson Bay to Alaska, southward through Canada and over 

 most of the United States, except the South Atlantic and Gulf 

 States, and into northern Mexico. As now understood, the Ameri- 

 can aspen is botanically separated into five closely related forms: 

 (1) American aspen (P. tremuloides} in its type form, and (2) 

 kidney leaf aspen (P. tremul-oi' des renifor'mis), both mainly eastern 

 forms; (3) western aspen (P. tremuloi'des au'rea), the common and 

 widely distributed western form of the species; (4) redbud aspen 

 (P. tremuloi'des cercidi'phylla), a locally restricted form of western 

 Wyoming; and (5) Vancouver aspen (P. tremuloi'des vancouv- 

 eria'na], a form confined to the locality of Vancouver Island. 2 

 From a practical standpoint there is little need for separating the 

 forms, as the ordinary observer will not appreciate the characteristics 

 noted in distinguishing these varieties, because they consist chiefly 

 of relatively trivial differences in flowers, size and shape of leaves, 

 and in autumnal coloration. 



Western aspen is naturally a forest-forming variety, which often 

 covers large areas with dense, practically pure stands. It is com- 

 monly considered a temporary type, but when it becomes established 

 on fire-denuded areas, where all coniferous seed trees have been 

 killed over extensive areas, thick stands may dominate for many 

 years. In the northern part of its range, western aspen occurs from 

 sea level up to an altitude of from 2,500 or 3,500 feet, and farther 

 south it ranges mostly between elevations of 6,000 and 11,000 feet., 

 It attains its best development on rich, moist, loam soils, but also 

 occurs in wet soils and on dry, gravelly hillsides. On the better sites, 

 it develops into a large tree occasionally 90 feet high and 30 inches 

 or more in diameter. 



Western aspen grows typically in open stands and frequently 

 under such favorable soil conditions as to be associated with a varied 

 and luxuriant undergrowth, which is highly palatable to livestock. 

 Furthermore, it is sufficiently palatable to sheep and cattle, which 

 browse extensively on the leaves and twigs, especially during the 

 fall, as to be a material factor in the grazing capacity of many 



2 Tidestrom, I. NOTES ON POPULUS, PLINIUS. Amer. Midland Nat. 2: [29J-35, illus. 

 1911. 



3 Sudworth, G. B. POPLARS, PRINCIPAL TREE WILLOWS, AND WALNUTS OF THE ROCKY 

 MOUNTAIN REGION. U. S. Dept. Agr. Tech. Bull. 420, 112 pp., illus. 1934. 



