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, in which their respiratory function resembles that of 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 as being 80,000 years of age; if validated, this would likely make it the
oldest living colony of aspens. Some aspen colonies become very large with time, spreading about per year, eventually covering many hectares. They are able to survive
forest fires, because the roots are below the heat of the fire, and new sprouts appear after the fire burns out. The high stem turnover rate combined with the clonal growth leads to proliferation in aspen colonies. The high stem turnover regime supports a diverse herbaceous understory. Aspen seedlings do not thrive in the shade, and it is difficult for seedlings to establish 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, devoid of other competing tree species. Aspens have increased in popularity as a forestry cultivation species, mostly because of their fast growth rate and ability to regenerate from sprouts. This lowers the cost of
reforestation after harvesting since no planting or
sowing is required. Aspen populations have been declining in some areas; this "sudden aspen death" has been attributed to factors as diverse as
climate change exacerbating drought and modifying precipitation patterns; recruitment failure from herbivory or grazing preventing new trees from coming up after old trees die; and to successional replacement by conifers due to fire suppression altering forest diversity and creating conditions where aspen may be at less of an advantage. In contrast with many trees, aspen
bark is
base-rich, meaning aspens are important hosts for
bryophytes and act as food plants for the
larvae of butterfly (
Lepidoptera) species. Young aspen bark is an important seasonal forage for the
European hare and other animals in early spring. Aspen is also a preferred food of the
European beaver.
Elk,
deer, and
moose not only eat the leaves but also strip the bark with their front teeth. ==Image gallery==