Identifying the Aspen Tree

The aspen tree (Populus tremuloides) is the most widely distributed tree species in North America, ranging from Alaska to Newfoundland and down the Rocky Mountains to Mexico. Utah and Colorado, in particular, are home to the largest portion of the natural acreage of aspen in the world.

Aspen trees are described as an important, community-dependent keystone species within their natural range. They are the most visible of western North American hardwoods, providing understory biodiversity, wildlife habitat, livestock forage, specialty forest products, and highly desirable scenery. Here, we discuss how to identify aspen trees in the forest, as well as growing conditions and common pests.

Description and Identification

Aspen trees go by several different names: trembling aspen, golden aspen, quiver-leaf aspen, small-toothed aspen, Canadian aspen, quakie, and popple. Aspen is the only transcontinental broadleaf tree growing from Newfoundland to California and Mexico.

Aspen trees tend to have rough bark that ranges in color: greenish or yellowish, grey, white, or a mix. These trees are medium-sized, growing between 20 and 80 feet tall. The leaves are circular or triangular, each has a long, flattened stem. The thin, damage-prone bark is light green and smooth with bands of warty bumps. Sometimes aspens are mistaken for birch because of the light color of their bark. Curiously, this bark is responsible for photosynthesis, a task usually performed by a tree's leaves. According to the National Wildlife Federation, "In winter, when other deciduous trees are mostly dormant, quaking aspens are able to keep producing sugar for energy."

This species is often associated with Douglas fir timber, as it is a pioneer tree for fires and logging, taking root more quickly than other species once an area has been cleared. Aspens can even persist through multiple wildfires in the Central Rocky Mountains, dominating a space until less fire-resistant conifers have a chance to get established.

The tree has the most wind-sensitive leaves of any broadleaf species, and this is reflected in its common names—the leaves "tremble" and "quake" during moderate winds. The wood has commercial value for furniture parts, particle board, furniture, matches, boxes, paper pulp.​​

Natural Range

Aspen trees grow singly and in multi-stemmed clones over the widest distribution of any native tree species in North America. They occur in pure stands on sandy, gravelly slopes.

Their ability to clone is particularly fascinating. Aspens are able to regenerate by sending shoots and suckers up from their lateral roots, resulting in many genetically identical trees. This aggregate is known as a clone. As the U.S. Forest Service explains, "The members of a clone can be distinguished from those of a neighboring clone often by a variety of traits such as leaf shape and size, bark character, branching habit, resistance to disease and air pollution, sex, time of flushing, and autumn leaf color. A clone may turn color earlier or later in the fall or exhibit a different fall color variation than its neighboring aspen clones, thus providing a means to tell them apart."

A clone can grow up to 100 acres in size and can live for much longer than the lifespan of individual trees within it. When a tree that's part of a clone dies, it's eventually replaced with new growth and the clone lives on—for up to tens of thousands of years. There is one famous clone in Utah, known as Pando, that's recognized as the largest living entity on Earth. It has nearly 50,000 stems growing out of a single root system, covers 100 acres, and weighs an estimated 13 million pounds. Pando is thought to have come from a single seed that sprouted at the end of the last Ice Age, around 2.6 million years ago.

The aspen tree is found in Canada, northwestern Alaska, and southeast through Yukon and British Columbia. Throughout the western United States, you are most likely to come across an aspen tree in the mountainous regions of Washington and California, southern Arizona, the Trans-Pecos region of Texas, and northern Nebraska. From Iowa and eastern Missouri, it ranges east to West Virginia, western Virginia, Pennsylvania, and New Jersey. There are also aspen trees in the mountains of Mexico, as far south as Guanajuato.

They tend to grow in lower altitudes in the north and higher altitudes in the south. "Quaking aspens are conspicuously absent from the Southeast because there are no high-elevation mountains where it can live."

Silviculture and Management

Care must be taken when transplanting aspen trees, according to the book "Native Trees for North American Landscapes," written by Guy Sternberg and James Wilson. Any problems during the transplant may cause pests or disease, such as "cankers, insect attack, bark blemishes, and premature death." The authors recommend using root cuttings when establishing the tree in its location.

These trees grow quickly if planted in the right place. Buy trees from a nursery, as these may have fewer issues with disease than if you take a wild cutting. The tree needs moist, well-drained, and slightly acidic soil. Under favorable conditions, an aspen can add as much as five feet in height annually. Plant on northern or eastern slopes; aspens do not like drought or excessive heat. Fertilize once per year and add mulch to protect roots and keep them cool.

Common Pests and Diseases

Aspen trees are vulnerable to infection from a number of sources. Common problems include fungal diseases like Cytospora, leaf spots, and other cankers that can cause trunk rot. Insects that plague aspen trees include aphids, sawflies, poplar twiggall flies, and more.

While beautiful trees, aspens are very sensitive to many environmental problems. They host more than five hundred species of parasites, herbivores, diseases, and other harmful agents. As a result, these trees have been a disappointment to many when planted in the landscape.