Known for their brilliant gold leaves that tremble even in light breezes, quaking aspens are the most widely distributed tree species in North America. While scattered throughout all the western states, most of the aspen forest in the U.S. is found in Colorado and Utah.
But researchers predict that these distinctive trees will decline in distribution in the Colorado Rocky Mountains due to climate change over the next century.
Researchers have noted a phenomenon called sudden aspen decline (SAD) across the western U.S., where some quaking aspens (Populus tremuloides) are experiencing a pattern of widespread mortality across their range. It’s thought to be related to disease and insects and worsened by climate change and human land use.
“With their shallow root systems, aspen are particularly intolerant to drought, and may become more vulnerable to SAD when it's warmer and drier,” the study’s senior author Jelena Vukomanovic, assistant professor in the North Carolina State University Department of Parks, Recreation and Tourism Management, tells Treehugger.
“Because they're particularly suited to mountain climates with cold winters, their suitable habitat is also shifting upward in elevation and northward in latitude. Some researchers think this will lead to aspen disappearing from the most Southern and the driest parts of their range.”
For the study, researchers used computer modeling to simulate how the distribution of the trees would decline under three possible scenarios: if temperatures don’t change; under a 4-degree temperature increase with 15% less precipitation; and with a 4-degree decline and 15% more precipitation
In each of the three scenarios, they modeled whether the trees were visible from nearly 33,000 vantage points along three scenic byways in Colorado: Cache la Poudre, Trail Ridge Road, and Peak-to-Peak Highway. They also included how insects, wildfire, and wind would impact tree growth and distribution. Quaking aspens are not tolerant of drought and shade, the researchers point out, but they are often quick to colonize an area after it is burned.
The findings showed that aspens would decline in all three climate scenarios.
“Our models predict the total area of aspen will decrease over the next 100 years,” Vukomanovic says. “Climate warming causes double the decrease in aspen compared to if the climate stays the same. We found the aspen decline seen from scenic drives will be greater than declines overall (approximately 22% fewer visible aspen in 100 years compared to today), and that with a warming climate, aspens will move up to higher, cooler elevations.”
They found that changes depended on elevation with greater increases at lower elevations under all three scenarios where trees are colonizing recently burned areas.
The findings were published in the journal Ecoystem Services.
The Importance of Aspens
Aspens are important because they offer habitat for various wildlife, including hare, moose, black bear, elk, deer, ruffed grouse, migratory birds, and a variety of smaller animals, according to the U.S. Forest Service. These animals eat the bark, leaves, buds, and twigs of the trees, reports the National Wildlife Federation.
And the trees are iconic in the area for their beauty and their unusual trembling leaves. They turn large areas of the hillsides bright gold and are the main feature of the Rocky Mountains’ striking fall foliage.
“Aspen are revered in mountain landscapes for their scenic beauty and the immersive sensory experiences they provide, such as the way their two-toned leaves flutter in the wind, the distinct whispering sound the leaves make, the visual complexity their white trunks provide, and the feeling of walking among a large clonal stand,” Vukomanovic says.
“They have symbolic and historic value to indigenous people in the region, and are a fundamental aspect of the landscape's character and identity. While past research has modeled aspen change across the continent, these future forecasts of aspen are not often considered from the human visibility perspective. Combining viewscape modeling with aspen forecasting gives us a detailed personal perspective of when and where climate change will affect the cultural and tourism-related value of aspen.”
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