Aspens, apart from the aberrant White Poplar (Populus alba), are distinguished by their nearly round leaf on mature trees, 4-12 cm in diameter with irregular rounded teeth. They are carried on strongly flattened leaf stems, that cause the leaves to twist and flutter in slight breezes. The juvenile leaves on young seedlings and root sprouts differ markedly from the adult leaves, being 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.
The unusual ability of the leaves of Populus to twist and bend due to the flattened petioles may not be fully understood. It is thought to help protect the trees from severe winds, perhaps by helping dissipate energy more uniformly throughout the canopy. It is also thought to improve the rate of photosynthesis throughout the tree by reducing the exposure of the outer leaves to extreme sunlight (thus reducing photoinhibition) by presenting the leaves at an oblique angle to the sun throughout the day, while at the same time allowing more light through to the lower leaves which are generally over-shaded. This would enable leaves throughout the tree to photosynthesize more efficiently. Another theory that has been advanced is that the constant motion aids the tree's growth: the constant movement is thought to increase the intake of air by the leaves, and hence the rate of carbon fixation from the air's carbon dioxide.
The Quaking Aspen of North America is known for its leaves turning spectacular tints of red and yellow in the autumn, popular among tourists.
The five typical aspens are all native to cold regions with cool summers, in the north of the Northern Hemisphere, extending south at high altitudes in the mountains. The White Poplar, by contrast, is native to warmer regions, with hot, dry summers. These trees are all medium-sized deciduous trees ranging 15–30 metres tall.
All of the aspens (including the White Poplar) typically grow in large clonal 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 metres from the parent tree. Each individual tree can live for 40–150 years above ground, but the root system of the colony is long-lived. In some cases, this is for thousands of years, sending up new trunks as the older trunks die off above ground. For this reason it is considered to be an indicator of ancient woodlands. 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 of aspens. Some aspen colonies become very large with time, spreading about a metre per year, eventually covering many hectares. They are able to survive forest fires, since the roots are below the heat of the fire, with new sprouts growing after the fire burns 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, since it allows the saplings to flourish in open sunlight in the burned landscape. Lately, aspens have an increased popularity in forestry, mostly because of their fast growth rate and ability to regenerate from sprouts, making the reforestation after harvesting much cheaper, since no planting or sowing is required.
In contrast with many trees, aspen bark is base-rich, meaning that aspens are important hosts for bryophytes and act as food plants for the larvae of butterfly (Lepidoptera) species—see List of Lepidoptera that feed on poplars.
Aspen wood is white and soft, but fairly strong, and has low flammability. It has a number of uses, notably for making matches, where its low flammability makes it safer to use than most other woods. Shredded aspen wood is used for packing and stuffing, sometimes called excelsior (wood wool). It is also a popular animal bedding, since it lacks the phenols associated with pine and juniper, which are thought to cause respiratory system ailments in some animals. Heat-treated aspen is a popular material for the interiors of a sauna.
Aspens and other members of the Populus genus contain salicylates, compounds related to aspirin. Leaves and leaf buds of aspens have been used to treat burns, irritations, aches, and swollen joints. Bitter herbal tea from bark and leaves has been used to treat mild urinary tract inflammations. The Ojibwe used the inner bark of the trunk as a poultice, and the Cree ate the inner bark in the spring as a mild purgative.