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Aspen
A Quaking Aspen grove
Scientific classification
Kingdom: Plantae Division: Magnoliophyta Class: Magnoliopsida Order: Malpighiales Family: Salicaceae Genus: Populus Section: Populus
Species
Populus adenopoda Populus alba Populus grandidentata Populus sieboldii Populus tremula Populus tremuloides

Aspens are trees of the willow family and comprise a section of the poplar genus, Populus sect. Populus. There are six species in the section, one of them atypical, and one hybrid:

• Populus tremula: Common, Trembling or Eurasian Aspen (northern Europe & Asia)
• Populus tremuloides: Quaking, Trembling or American Aspen (northern & western North America)
• Populus grandidentata: Bigtooth Aspen (eastern North America, south of P. tremuloides)
• Populus adenopoda: Chinese Aspen (China, south of P. tremula)
• Populus sieboldii: Japanese Aspen (Japan)
• Populus alba: White Poplar (northwest Africa, southern Europe, east to central Asia)
• Populus × canescens: Grey Poplar (hybrid P. alba × P. tremula)

The five typical aspens are all native to cold regions with cool summers, in the far north of the Northern Hemisphere, extending south only at high altitudes in mountains. The White Poplar by contrast is native to much warmer regions, with hot, dry summers. They are all medium-sized deciduous trees reaching 15–25 m tall, exceptionally to 30 m.

Aspens (apart from the aberrant White Poplar) are distinguished by their nearly round leaves on mature trees, 4–12 cm diameter with irregular rounded teeth. They are carried on strongly flattened leaf stems, which enable the leaves to twist and flutter in the slightest of breezes. The juvenile leaves on young seedlings and root sprouts differ markedly from the adult leaves, nearly triangular, showing here the typical leaf shape of most other poplars; they are also often much larger, 10–20 cm long. The five typical aspens are distinguished from each other by leaf size and the size and spacing of the teeth on the adult leaves. White Poplar leaves differ in being deeply five-lobed, covered in thick white down, and having only a slightly flattened leaf stem.

All the aspens (including White Poplar) typically grow in large colonies derived from a single seedling, and spreading by means of root suckers; new stems in the colony may appear at up to 30–40 m from the parent tree. Each tree only lives for 40–150 years above ground, but the root system of the colony is long-lived, in some cases for many thousands of years, sending up new trunks as the older trunks die off above ground. One such colony in Utah, given the nickname of "Pando", is claimed to be 80,000 years old, making it possibly the oldest living colony. Some aspen colonies become very large with time, spreading about a metre per year, eventually covering many hectares. They are able to survive intense forest fires as the roots are below the heat of the fire, with new sprouts growing after the fire is out. However, aspens do not thrive very well in the shade, and it is difficult for aspen seedlings to grow in an already mature aspen stand. Fire indirectly benefits aspen trees, as it allows the saplings to flourish in open sunlight on account of the burned landscape. Lately aspen has increased its popularity in forestry, mostly because of its fast growth rate and ability to regenerate from sprouts, which makes the regeneration of the forest after harvesting much cheaper, as no planting or sowing is required.

Aspens are important food plants for the larvae of Lepidoptera species—see List of Lepidoptera that feed on poplars.

The aspen tree's quivering leaves are, in Christian lore, said to be the result of arrogance at the Crucifixion because the aspen did not tremble like other trees. A German version claims that the aspen was the only tree to refuse to acknowledge the divinity of Jesus. The cross that Christ was crucified on is sometimes said to have been aspen wood. As aspens do not occur in Palestine, this legend is improbable^{[citation needed]}. Another old saying was that aspen leaves are made from female tongues, and their quivering is due to women's inability to stop talking^{[citation needed]}.

Emigrant Basque shepherds in the 19th and 20th century carved texts and figures on aspens of the American Southwest to express their loneliness.

The wood is white, and soft, but fairly strong, and with very low flammability. It has a number of uses, notably for making matches, where its low flammability makes it safer to use (easy to blow out) than most other woods. Shredded aspen wood is also a popular animal bedding, as it lacks the phenols associated with pine and juniper, which are thought to cause respiratory ailments in some animals. Heat treated aspen is a popular material for the interiors of a sauna.

P. tremula (Eurasian Aspen) leaves; adult leaves and shoot left, juvenile leaf right	P. tremuloides (Quaking Aspen) in New Mexico with fall foliage	P. tremuloides (Quaking Aspen) with fall foliage, Jasper National Park, Alberta, Canada	P. tremuloides (Quaking Aspen), La Sal Mountains
P. tremuloides (Quaking Aspen), Little Cottonwood Canyon, Salt Lake County, Utah	P. tremuloides (Quaking Aspen), Klamath Basin National Wildlife Refuges Complex	P. tremuloides (Quaking Aspen), Klamath Basin National Wildlife Refuges Complex	P. grandidentata (Bigtooth Aspen) in Massachusetts

• Aspen Information Resource, U.K.
• Quaking Aspen Forests of the Colorado Plateau