Decline in aspens pushes Arizona forest officials to look for answers

FLAGSTAFF, Ariz. — By some measures, aspens are the largest organisms in the world, as entire groves spanning thousands of trees can be interconnected through elaborate root systems.

The trees, which can live up to 100 years, draw people from across the Southwest to Coconino National Forest each autumn with their yellow and reddish leaves.

But in recent decades, the number of aspens in Arizona’s higher elevations has declined sharply. A 2001 study by the U.S. Forest Service said that Arizona has seen a 96 percent decline in aspen acreage since 1900, the largest by far among states with significant aspen populations.

Mary Lou Fairweather, a Flagstaff-based forest pathologist for the Forest Service, said the decline has intensified in recent years.

“The last couple decades there was more of a concern because we had this huge mortality event that was totally tied to the drought in 2002-2003,” she said.

Fairweather said the drought and the high temperatures that accompanied it killed more than 90 percent of aspens in some parts of the state from 2002 through 2007.

She also pointed to a shortage of fires, both wildfires and controlled burns, as another factor impeding regrowth. While it might seem counterintuitive, Fairweather said that aspens can survive most low-temperature fires, but the absence of fires has led to an influx of ponderosa pines and other conifers.

The pines compete with aspens for water, sunlight and space, and they typically win, said Shawn Martin, silviculturist for the Flagstaff ranger district of the Forest Service.

“I like to think of it like you have a small turkey and you have to feed 20 people: Someone’s going hungry,” Martin said. “And the aspens are typically the losers in that battle.”

Aspens are also a favorite food for elk, which were reintroduced to the area in the first half of the 20th century. Though elk graze primarily on grass, aspens become a delicacy for hooved animals when no healthy grass is available, according to Steve Clark, executive director of the Arizona Elk Society.

“To elk, aspen is candy,” Clark said. “And it’s a very nutritious candy as well.”

Fairweather said that fully grown aspens can regrow after being eaten but that immature aspens make for easy targets for browsing herbivores.

Much of the discussion surrounding how to protect aspens centers around keeping elk away from the trees. Martin said the Forest Service, in conjunction with the Arizona Game and Fish Department and private groups, has been working on a variety of methods to curtail that browsing.

The most straightforward strategy is putting fences around groves to keep elk away. Fairweather said that aspen fences became popular in Arizona in the 1980s, after the Forest Service found that cutting down mature aspens in order to make way for new growth didn’t work. By and large, the fencing has been successful at keeping the elk away, she said.

But fences have their problems as well. Martin said that they are easily damaged, both by falling trees and by hunters who are unwilling to go around them. Though the Forest Service works with Friends of Northern Arizona Forests, a volunteer group, to keep track of damaged fences, Martin said the costs can pile up.

“For a long time, the Forest Service just didn’t have the resources or the personnel to do that,” he said.

Other strategies include using individually fenced cones to grow “browse-resistant” aspen saplings that elk aren’t interested in eating. Forestry officials have also discussed tearing up the ground around aspen groves to simulate a natural disturbance and encourage growth.

However, perhaps the method that forestry groups are most excited about is “jackstrawing,” cutting down pine trees and using them as a natural fence around aspens.

“We’re using natural materials that are on-site,” said Steve Rosenstock, the habitat program manager for Arizona Game and Fish. “And over time, by the time the trees have grown up sufficiently large to be protected, the barrier will start to break down so we don’t have a permanent structure to stand on the land.”

Mary Lou Fairweather said that while aspens aren’t in any real danger of going extinct in northern Arizona they continue to face threats ranging from elk to climate change.