|Quaking aspen grove, Lamoille Canyon, Nevada, U.S.|
Populus tremuloides is a deciduous tree native to cooler areas of North America, one of several species referred to by the common name Aspen. It is commonly called quaking aspen, trembling aspen, American aspen, Quakies, mountain or golden aspen, trembling poplar, white poplar, popple, and even more names. The trees have tall trunks, up to 25 meters (82 feet) tall, with smooth pale bark, scarred with black. The glossy green leaves, dull beneath, become golden to yellow, rarely red, in autumn. The species often propagates through its roots to form large groves based on a single rhizome.
Populus tremuloides is the most widely distributed tree in North America, being found from Canada to central Mexico. It is the defining species of the aspen parkland biome in the Prairie Provinces of Canada.
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.
A tall, fast growing tree, usually 20–25 m (66–82 ft) at maturity, with a trunk 20–80 cm (0.66–2.62 ft) in diameter; records are 36.5 m (120 ft) in height and 1.37 m (4.5 ft) 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 (1.6–3.1 in) in diameter with small rounded teeth, and a 3–7 centimeters (1.2–2.8 in) long, flattened petiole. Young trees (including root sprouts) have much larger—10–20 centimeters (3.9–7.9 in) long—nearly triangular leaves.
The flowers are catkins 4–6 centimeters (1.6–2.4 in) 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 (3.9 in) pendulous string of 6-millimeter (0.24 in) 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.
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 (460 m) due to the mild winters experienced below that elevation, and is generally found at 5,000–12,000 feet (1,500–3,700 m). 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.
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 (three million per pound) are only viable a short time as they lack a stored food source or a protective coating.
Beginning in 1996, individual North American scientists noticed an increase in dead or dying aspen trees. As this accelerated in 2004, word spread and a debate over causes began. No insect, disease, or environmental condition is yet specifically identified as a joint cause. Trees adjacent to one another are often stricken or not. In other instances, entire groves have died.
Many areas of the Western US have experienced increased diebacks which are often attributed to ungulate grazing and wildfire suppression. At high altitudes where grasses can be rare, ungulates can browse young aspen sprouts and prevent those young trees from reaching maturity. As a result, some aspen groves close to cattle or other grazing animals, such as deer or elk, have very few young trees and can be invaded by conifers, which are not typically browsed. Another possible deterrent to aspen regeneration is widespread wildfire suppression. Aspens are vigorous resprouters and even though the above-ground portion of the organism may die in a wild-fire, the roots, which are often protected from lethal temperatures during a fire, will sprout new trees soon after a fire. Disturbances such as fires seem to be a necessary ecological event in order for aspens to compete with conifers, which tend to replace aspen over long, disturbance-free intervals. The current dieback in the American West may have roots in the strict fire suppression policy in the United States. On the other hand, the wide-spread decimation of conifer forests by the mountain pine beetle may provide increased opportunities for aspen groves to proliferate under the right conditions.
Because of the vegetative regeneration method of reproduction used by the aspen, where an entire group of trees are essentially clones, there is a concern that something that hits one will eventually kill all of the trees, presuming they share the same vulnerability. A conference was held in Utah in September 2006 to share notes and consider investigative methodology.
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.
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.
- Quaking Aspen by the Bryce Canyon National Park Service
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- US Forest Service Silvics Manual: Populus tremuloides
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- Alberta Forest Genetic Resources Council: Populus tremuloides
- Interactive Distribution Map for Populus tremuloides
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