Populus tremuloides
Populus tremuloides, the Quaking Aspen or Trembling Aspen, is a deciduous tree native to cooler areas of North America, with the northern limit determined by its intolerance of permafrost. It occurs across Canada in all provinces and territories (with the possible exception of Nunavut). In the United States, it occurs at low elevations as far south as northern Nebraska and central Indiana. Farther west, it grows at high altitudes as far south as Guanajuato, Mexico. In the western United States, this tree rarely survives at elevations lower than 1,500 feet due to the mild winters experienced below that elevation, and is generally found at 5,000-12,000 feet.
The name references the quaking or trembling of the leaves that occurs in even a slight breeze due to the flattened petioles.  Other species of Populus have petioles flattened partially along their length, while the Quaking Aspen's are flattened from side to side along the entire length of the petiole.  This quaking of the leaves produces a soft sound that many consider a hallmark of the Quaking Aspen.
It is a tall tree, usually 20 to 25 meters (66 to 82 feet) at maturity, with a trunk 20-80 cm diameter; records are 36.5 m height and 1.37 m diameter. The leaves on mature trees are nearly round, 4-8 cm diameter with small rounded teeth, and a 3-7 cm long, flattened petiole. Young trees (including root sprouts) have much larger (10-20 cm long), nearly triangular leaves. The flowers are catkins 4-6 cm long, produced in early spring before the leaves; it is dioecious, with male and female catkins on different trees. The fruit is a 10 cm long pendulous string of small (6 mm) capsules, each capsule containing about ten minute seeds embedded in cottony fluff, which aids wind dispersal of the seeds when they are mature in early summer.
Shrublike dwarf clones exist in marginal environments too cold and dry to be hospitable to full-size trees, for example at the species' upper elevation limits in the White Mountains (California).
It propagates itself by both seed and root sprouts, and extensive clonal colonies are common. Each colony is its own clone, and all trees in the clone have identical characteristics and share a root structure. A clone may turn color earlier or later in the fall than its neighbouring aspen clones. Fall colors are usually bright tones of yellow; in some areas, red blushes may be occasionally seen. As all trees in a given clonal colony are considered part of the same organism, one clonal colony, named Pando, is considered the heaviest and oldest living organism at six million kilograms and approximately 80,000 years old.
The leaves of the Quaking Aspen serve as food for caterpillars of various Lepidoptera. See List of Lepidoptera which feed on poplars.

# Dieback
Beginning in 1996, individual North American scientists noticed an increase in dead or dying aspen trees. As this accelerated, in 2004, word spread and a debate over causes began. No insect, disease, or environmental condition is yet specifically identified as a joint cause. Trees adjacent to one another are often stricken or not. In other instances entire groves have died.
Many areas of the Western US have experienced increased diebacks which often attributed to cattle grazing. At high altitudes where grasses can be rare, cattle browse young Aspen sprouts and prevent those young trees from reaching maturity. In these cases, the cattle jeopardize the health of the stand. As a result, some Aspen groves in close proximity to cattle grazing allotments have a very few young trees and are invaded by conifers, which the cattle will not browse. Eventually, the coniferous trees outcompete the Aspen because of a lack of new recruitment, often outcompeting grasses as well.
Because of the vegetative regeneration method of reproduction used by the Aspen, where an entire group of trees are essentially clones, there is a concern that something that hits one will eventually kill all of the trees, presuming they share the same vulnerability. A conference was held in Utah in September 2006 to share notes and consider investigative methodology .