Aspen Trees

The brilliant foliage of the quaking aspen (Populus tremuloides) provides some of the most iconic and striking scenery in Colorado’s Rocky Mountains. These altitude-loving deciduous trees grow up to fifty feet tall, and their leaves turn a vibrant gold, red, and orange during the fall. Each year, fall aspen colors draw thousands of tourists called “leaf peepers” to the mountains, making the aspen a lucrative part of Colorado’s economy.

Beyond their status as one of Colorado’s most recognizable and sought-after trees, aspens play an important role in maintaining the state’s biodiversity, as they support a range of wildlife that conifers and other trees do not. Important as they are, aspen trees are susceptible to a range of insect infestations and the effects of a warming climate, which have contributed to a phenomenon known as “sudden aspen decline” among stands in various parts of the United States. Still, aspens are likely to remain numerous throughout the state for some time, owing to their ability to quickly colonize areas recently burned by wildfire.

Appearance and Life Cycle

Aspens are ubiquitous across Colorado’s mountains, growing anywhere between 5,000 and 11,500 feet in elevation. They have straight, smooth trunks with grayish-white bark and distinctive ellipsoidal leaves that flutter in even the slightest breeze, which is why they are called “quaking” aspen (the root of its scientific name, tremuloides, means “tremble”). They can grow up to fifty feet tall, but most are much shorter.

Like their riparian cousins the cottonwoods, aspen trees are members of the poplar family. Trees in this family grow from a single root structure called a rhizome. These lateral-growing roots send up sucker shoots that become trees, giving the appearance of multiple trees in a stand. Such aspen stands are among the largest single organisms in the world.

A single rhizome can produce dozens of trees, all of which will share the same genetic markings—they will turn the same color in the fall, at the same time, for example. In order to achieve the genetic diversity that creates their array of fall colors across the Rockies, aspens also reproduce sexually. Aspens are dioecious, meaning that individual trees are either male or female. In the spring, before their leaves grow, female aspen trees produce small, white flowers that release cottonlike seeds, while male trees release pollen that rides the breeze and pollinates the females’ seeds.

Aspens grow quickly and can survive in a range of soil types and environs. However, they prefer rocky, well-drained soils in open landscapes. This is why they are particularly prominent in areas burned by wildfire. Aspen trees not only have high moisture content and can easily survive wildfires, but they and wildflowers are usually among the first to move into a burned area, beating out slower-growing and more shade-tolerant species. It takes between twenty and thirty years for an aspen stand to regenerate after a fire, a far faster rate than for conifers.

Once a new aspen tree takes root, it will develop its rhizome and start sending out sucker shoots, copying itself over and over until it becomes a stand. Different stands can be distinguished by their leaf shape and color, bark color and texture, or when their leaves start to change.

Ecology

In some places, aspens represent deciduous islands in a sea of evergreens; they colonize areas where conifers have burned, fallen, or been otherwise removed. But if there are enough of them, aspens  can also form ecosystems of their own, becoming a “keystone” species whose existence and survival dictates the survival of other species. Aspen trees produce ten times the amount of forage that conifers do, which means they attract far more animal life. Deer, elk, cattle, and sheep are all known to browse aspens, and large populations of elk in particular have posed threats to aspen stands in places like Rocky Mountain National Park. Beavers cut down aspens for their dams, stimulating sucker production. Aspens also support and are vulnerable to a range of insects, including aphids, poplar borers, aspen leaf miners, and leafhoppers.

Aspens in Indigenous Culture

Aspen trees have great historical and cultural significance in North America. For example, the Nuche (Ute) people used the bark of aspen trees to carve glyphs detailing important cultural stories. One Nuche legend seeks to explain why the aspen quakes. In the story, the aspen failed to pay homage to a great spiritual entity that was to visit Earth, dooming it to quake for all time.

Aspen bark contains salicylates, compounds that provide medical functions similar to aspirin. The Nuche and other Indigenous people used medicinal preparations from aspen trees to treat stomach pain, colds, fevers, heart problems, and venereal disease. Indigenous people also consumed the tree’s inner bark as a treat in cakes, as syrup, or in its raw form. The trees themselves were often used in construction of lodges and tipis.

Current Uses

Today the wood from aspen trees is used in many products, from toothpicks to furniture and paper. Its soft yet relatively strong wood is also used to make matches, saunas, children’s toys, and even chopsticks. While other, stronger woods are better choices for furniture and housing construction, the aspen’s wide geographic range throughout North America makes it a readily available source for those smaller wood products.

Sudden Aspen Decline

In the early 2000s, scientists began noticing that aspen stands across the American West and Canada were not regenerating as they should be after disturbances such as fires or storms. As of 2007, some 13 percent of Colorado’s aspen trees were reported to be experiencing “sudden aspen decline” (SAD), which refers to “rapid dieback and mortality” among stands. The most prominent cause is stress from prolonged drought, which weakens the trees’ natural defense against predators and affects their resilience after fires and other disturbances. In some places, overpopulation of ungulates such as elk also contributes to SAD, as overbrowsing reduces sucker growth and prevents stand regeneration.

Future

Despite the ongoing occurrence of SAD and threats from elk and insect pests, aspens are likely to thrive in Colorado for some time. This success is directly correlated to another effect of climate change in Colorado—the state’s changing experience with wildfires, which are becoming larger and more intense as the twenty-first century goes on. Wildfires clear out conifers, creating new habitat for aspen and other shorter, quick-growing species. So, while Coloradans should not take aspens for granted, they are likely to remain an important part of Colorado’s ecology and economy into the future, even as warming temperatures are likely to change some aspects of their proliferation and life cycle.