Populus tremuloides

Appearance

A tall, fast growing tree, usually 20–25 m at maturity, with a trunk 20–80 cm in diameter; records are 36.5 m in height and 1.37 m in diameter.

The bark is relatively smooth, colored greenish-white to gray, and is marked by thick black horizontal scars and prominent black knots. Parallel vertical scars are tell-tale signs of elk, which strip off aspen bark with their front teeth.

The leaves on mature trees are nearly round, 4–8 centimeters in diameter with small rounded teeth, and a 3–7 centimeters long, flattened petiole. Young trees have much larger—10–20 centimeters long—nearly triangular leaves.

The flowers are catkins 4–6 centimeters long, produced in early spring before the leaves; it is dioecious, with male and female catkins on different trees. The fruit is a 10-centimeter-long pendulous string of 6-millimeter capsules, each capsule containing about ten minute seeds embedded in cottony fluff, which aids wind dispersal of the seeds when they are mature in early summer.

The quaking aspen is the State Tree of Utah.

Naming

The quaking or trembling of the leaves that is referred to in the common names is due to the flexible flattened petioles. The specific epithet, ''tremuloides'', means similar to ''Populus tremula'', the European aspen.
Some species of ''Populus'' have petioles flattened partially along their length, while the aspens and some other poplars have them flattened from side to side along the entire length of the petiole.

Distribution

The northern limit is determined by its intolerance of permafrost. It occurs across Canada in all provinces and territories, with the possible exception of Nunavut. In the United States, it can be found as far north as the southern slopes of the Brooks Range in Alaska, and it occurs at low elevations as far south as northern Nebraska and central Indiana. In the western United States, this tree rarely survives at elevations lower than 1,500 feet due to the mild winters experienced below that elevation, and is generally found at 5,000–12,000 feet . It grows at high altitudes as far south as Guanajuato, Mexico.

Shrub-like dwarf clones exist in marginal environments too cold and dry to be hospitable to full-size trees, for example at the species' upper elevation limits in the White Mountains.

Habitat

It propagates itself primarily through root sprouts, and extensive clonal colonies are common. Each colony is its own clone, and all trees in the clone have identical characteristics and share a single root structure. A clone may turn color earlier or later in the fall than its neighbouring aspen clones. Fall colors are usually bright tones of yellow; in some areas, red blushes may be occasionally seen. As all trees in a given clonal colony are considered part of the same organism, one clonal colony, named Pando, is considered the heaviest and oldest living organism at six million kilograms and approximately 80,000 years old. Aspens do produce seeds, but seldom grow from them. Pollination is inhibited by the fact that aspens are either male or female, and large stands are usually all clones of the same sex. Even if pollinated, the small seeds are only viable a short time as they lack a stored food source or a protective coating.

Uses

Aspen bark contains a substance that was extracted by indigenous North Americans and European settlers of the western U.S. as a quinine substitute.

Like other poplars, aspens make poor fuel wood, as they dry slowly, rot quickly, and do not give off much heat. Yet they are still widely used in campgrounds because they are cheap and plentiful and not widely used in building lumber. Pioneers in the North American west used them to create log cabins and dugouts, though they were not the preferred species.

The leaves of the Quaking Aspen and other species in the genus ''Populus'' serve as food for caterpillars of various moths and butterflies. See List of Lepidoptera that feed on poplars.

In Canada, it is used mainly for pulp products such as books, newsprint, and fine printing paper. Aspen is especially good for panel products such as oriented strand board and waferboard. Its lumber is light in weight and is used for furniture, boxes and crates, core stock in plywood, and wall panels.

Between logging for fuel, building, and pulp, and clearing for agriculture, the area of aspens declined dramatically in the nineteenth and twentieth centuries.