The Aspen (Populus tremula) is a species of poplar native to cool temperate regions of Europe and Asia, from the British Isles east to Kamchatka, north to inside the Arctic Circle in Scandinavia and northern Russia, and south to central Spain, Turkey, the Tian Shan, North Korea, and northern Japan.
It is a medium-sized deciduous tree growing to 10–25 m tall, with a trunk up to 1 m diameter. The bark is pale greenish-grey and smooth on young trees with dark grey diamond-shaped lenticels, becoming dark grey and fissured on older trees. The adult leaves, produced on branches of mature trees, are nearly round, slightly wider than long, 2–8 cm diameter, with a coarsely toothed margin and a laterally flattened petiole 4–8 cm long. The flat petiole allows them to tremble in even slight breezes, and is the source of its scientific name. The leaves on seedlings and fast-growing stems of root sprouts are very different, heart-shaped to nearly triangular, and often much larger, up to 20 cm long; their petiole is also less flattened. The flowers are wind-pollinated catkins produced in early spring before the new leaves appear; they are dioecious, with male and female catkins on different trees. The male catkins are patterned green and brown, 5–10 cm long when shedding pollen; the female catkins are green, 2–4 cm long at pollination, maturing in early summer to bear 10–20 capsules each containing numerous tiny seeds embedded in downy fluff. The fluff assists wind dispersal of the seeds when the capsules split open at maturity.
It is a very hardy species and tolerates long, cold winters and short summers.
Like other aspens, it spreads extensively by root sprouts, which may be produced up to 40 m from the parent tree, forming extensive clonal colonies.