Drought, higher temperatures, hungry elk threaten aspen trees near Flagstaff

FLAGSTAFF — Beneath the scenic yellow and red leaves of soaring aspen trees in the Kachina Wilderness, forest ecologist Mike Stoddard is looking down. His concern isn't the brilliant fall foliage but what's growing beneath the forest floor. Seeking the markers for trees plotted two decades earlier, Stoddard is hoping data from the past will inform the future of these beloved groves.

Comparing recent findings with data collected over 20 years ago, Stoddard and other researchers at Northern Arizona University's Ecological Restoration Institute published a new study in October that revealed a troubling trend in the recovery of aspen trees on the southern exposures of the San Francisco Peaks.

The study found that environmental factors related to climate change, such as drought and extreme heat, along with animal grazing and wildfire management, have contributed to the decline of aspen trees in the area. This decline could have larger ecological implications on the overall health of the forest, Stoddard warns, especially because some of the trees are not growing back.

Known as a disturbance-loving species, aspens are stimulated by large surface level disturbances, like wildfires. Despite the trees' resilience and speedy recovery, researchers found a decline in young aspens reaching maturity due to prolonged drought and unmanaged animals grazing.

Without active management, researchers warn that these compounding factors could lead to a decline in the aspen population on the San Francisco Peaks, a popular spot for locals and tourists, and the loss of a diverse forest ecosystem. Once the living aspen trees die, there may not be any new growth to take their place.

A 'canary species' for the forest Aspens are the most widely distributed trees in North America, and their most southern range occurs in pockets on the Sky Islands of the American Southwest and Mexico . The trees diversify forest species, serve as habitat for bird nesting and provide stability for the ecosystem by holding in moisture.

Although aspens can produce seeds, the most successful method of reproduction occurs when new sprouts, known as "suckers," grow out of the root system of an existing tree. The new sprout is a clone of the original tree, and many cloned aspen trees can grow from the same root system.

Aspen clones are some of the largest organisms on the planet. A single aspen clone, known as the "Pando" clone, is located in Fishlake National Forest in southern Utah and measures over 100 acres. This clone network is recognized as the "Most Massive Plant" by Guinness World Records.

This type of regeneration is also why aspen trees thrive after wildfires. Aspen roots can lie dormant under the soil as a fire clears the area, and new suckers will rise to the surface to quickly reforest the affected area faster than other types of trees.

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In the San Francisco Peaks, this successful regeneration can be seen in the area affected by the Pipeline Fire of 2022. In the two years that have followed the fire, the aspen stands have regenerated new sprouts that have grown without competition from other tree species and are now too tall to be grazed by elk.

But the aspen trees on lower elevations of the San Francisco Peaks are experiencing a bottleneck effect in their growth, according to the NAU researchers. The suckers are growing back after disturbances, but cannot reach maturity due to elk grazing and lack of resources from prolonged drought and rising temperatures.

Described as a "canary species" by Stoddard, these groves are providing clues to the deteriorating health of the forest on the peaks and all aspens in the species' southern range.

"Aspen communities on southern exposures at these lower elevations are dwindling," said Stoddard. "It's important that we monitor these ecosystems at the edges. They're going to provide information on how things are changing."

Aspens contribute to Flagstaff's fall colors Every year, tourists flock to Flagstaff in September and October to enjoy the outdoors and take in the vibrant fall colors of the aspen trees. Discover Flagstaff, the city's tourism bureau, displays a "Leaf-ometer" on its website when the leaves are brightest. The website received nearly 80,000 views in October and was featured on Phoenix-based television stations.

"It was the most popular page on our site during the month," said Ryan Randazzo, media relations manager at Discover Flagstaff in an email to The Republic, "Overall traffic to our site was up 17% for October this year ... and the Leaf-ometer was the primary driver."

Tourism to Flagstaff provides $565 million of economic impact to the city, and employs approximately 8,000 people.

"While we cannot say that fall colors are the only reason people visit Flagstaff in September and October — we have many things to see and do here year-round — we have seen hotel/motel occupancy in those months increase during the last 20 years," Randazzo said.

Fewer colors: Fall leaves look a little muted? Warmer, drier conditions are changing autumn in Arizona

The future of the aspens at risk Beneath the towering white trunks of the aspens, Stoddard looks to the thin plants protruding from the ground for clues to the fate of the peaks' forest.

"This is the future," said Stoddard, grasping an inconspicuous growth reaching just above his height. Once the aspen clones grow past a certain height threshold, the likelihood they will reach maturity greatly increases.

Nearby, swaths of new aspen growth in the study area have been abruptly stunted by grazing elk.

The Rocky Mountain elk that populate the San Francisco Peaks are not native to Arizona. In 1912, 83 elk were transplanted into Arizona from Yellowstone National Park, and the statewide population has grown to nearly 35,000 animals, according to the Arizona Game and Fish Department.

Without the presence of natural predators, the elk population near Flagstaff is controlled through the issuance of hunting permits. State and federal agencies are limited in their ability to fence off aspen stands in the wilderness area, so forest managers have turned to these hunting permits as a tool to help the young aspen in reaching maturity.

In the aftermath of the 2010 Shultz Fire, which burned through over 15,000 acres in northwest Flagstaff, the U.S. Forest Service and Arizona Game and Fish entered into a cooperative agreement that increased the availability of elk hunting permits to aid in the regeneration of aspen trees.

"Land managers, wildlife managers and foresters saw the need to protect new aspen shoots," Stoddard said, "so they came together to create an initiative to increase hunting pressure on elk so they could stimulate some of this regeneration and help them mature into tree size."

While it's too early to determine whether the measure was effective, Stoddard is optimistic after seeing the aspen trees rebound in the burn scar of the 2022 Pipeline Fire, an area not included in the study.

Although aspen trees can thrive on disturbances like wildfires, the compounding pressures from prolonged drought and uncontrolled elk grazing have put other aspen stands on the peaks at risk of disappearing in the next century.

"If (the aspens) are not putting their resources into the future, it's just a matter of time," Stoddard said. "They need disturbance, but not all disturbances are equal to regeneration."

John Leos covers environmental issues for The Arizona Republic and azcentral. Send tips or questions to .

Environmental coverage on azcentral.com and in The Arizona Republic is supported by a grant from the Nina Mason Pulliam Charitable Trust.

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