Aspen is a common name for certain tree species; some, but not all, are classified by botanists in the section Populus, of the Populus genus. Species These species are called aspens: Populus adenopoda – Chinese aspen (China, south of P. tremula) Populus davidiana – Korean aspen (Eastern Asia) Populus grandidentata – Bigtooth aspen (eastern North America, south of P. tremuloides) Populus sieboldii – Japanese aspen (Japan) Populus tremula – Eurasian aspen (northern Europe and Asia) Populus tremuloides – Quaking aspen or trembling aspen (northern and western North America) Habitat and longevity Aspen trees are all native to cold regions with cool summers, in the north of the northern hemisphere, extending south at high-altitude areas such as mountains or high plains. They are all medium-sized deciduous trees reaching tall. In North America, the aspen is referred to as quaking aspen or trembling aspen because the leaves "quake" or tremble in the wind. This is due to their flattened petioles which reduce aerodynamic drag on the trunk and branches. Aspens typically grow in environments that are otherwise dominated by coniferous tree species, and which are often lacking other large deciduous tree species. Aspens have evolved several adaptations that aid their survival in such environments. One is the flattened leaf petiole, which reduces aerodynamic drag during high winds and decreases the likelihood of trunk or branch damage. Dropping leaves in the winter (like most but not all other deciduous plants) also helps to prevent damage from heavy winter snow. Additionally, the bark is photosynthetic, meaning that growth is still possible after the leaves have been dropped. The bark also contains lenticels that serve as pores for gas exchange (similar to the stomata on leaves). Aspens are also aided by the rhizomatic nature of their root systems. Most aspens grow in large clonal colonies, derived from a single seedling, and spread by means of root suckers; new stems in the colony may appear at up to 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", has been estimated to be as old as 80,000 years, if