The soft sound of quaking aspen leaves trembling on the slightest breeze is the sound of summer in the mountains of the West. But that sound has become softer, and researchers believe the color of fall may be fading, too. This because of an extraordinarily rapid dieback of the aspen, a phenomenon biologists are calling SAD, or Sudden Aspen Decline.

In northern Arizona, Forest Service officials are reporting some 60 percent to 95 percent mortality in the low-elevation aspen groves, around 7,000 feet, of the Kaibab and Coconino national forests. Among the researchers diagnosing the sick trees is Northern Arizona University forestry graduate student Tom Zegler.

"We are concerned because it's a tree that brings people into the woods. Its aesthetic values are high; it is one of the only trees in the West that turns colors in the fall. Aspen also have an extremely high ecological value. Per acre they provide for a greater diversity of wildlife than the sea of ponderosa pine trees around them. And because aspen allow more diffused light to reach the forest floor than other trees in northern Arizona, a greater diversity of understory plants can grow beneath them."

Recent drought years and warmer temperatures may be playing a role in SAD, making the trees more vulnerable to a suite of pests and pathogens. The year 2000 was the worst drought year on record in Arizona until that record was broken in 2002. And the drought years aren't over. Most climate models predict a warmer and much drier future climate in the Southwest and extreme events, such as droughts, are expected to increase in frequency and magnitude.

"Aspen in Arizona have been hit by a multitude of stressing agents over the last decade," said Forest Service Southwest Region Plant Pathologist Mary Lou Fairweather. "In addition to drought, there was a late-season frost event in 1999 and several years of defoliation by the western tent caterpillar. Aspen became so stressed that a canker causing fungus and a wood borer, normally considered opportunistic organisms that only infest weakened trees, were associated with mortality of entire stands of trees."

Aspen's role in the ecosystem is to grow quickly after a large land disturbance, like a wildfire, that opens an area to sunlight. The sun-loving aspen trees shoot up and provide shade for other plants to grow. In 80 years or so, mixed conifers eventually grow taller than the aspen and begin to take over the area. Aspen only grow to be about 150 years old, but researchers say what's happening now is too fast for natural plant succession.

Ironically, on a place called Aspen Hill south of Williams, researchers have been unable to find any aspen except in a small grove that has been fenced off from animals, because the deer and elk aren't helping the aspen any either. "We found more than 98 percent of aspen trees surveyed are browsed by ungulates," said forester Mike French of the Kaibab National Forest.

Researchers say herbivores are attracted to aspen trees the same way kids are attracted to ice cream. "Compared to conifers, aspen are tasty and more nutritious. The leaves are easier to digest and the chlorophyll-containing bark is itself a food source," said Zegler.

Graduate student Katie Ireland from the Ecological Restoration Institute at NAU is exploring the long-term dynamics of aspen stands. She is particularly interested in the influence of fire, which has been suppressed in the American West since settlement in the late 1880s.

"Fire exclusion is thought to be one factor in longer term decline of aspen throughout the western U.S. Mature aspen stems produce a hormone, auxin, which prevents the root system from producing new suckers. Fire acts to promote aspen regeneration by removing the mature aspen stems, thus removing the hormonal influence and promoting the growth of new aspen suckers."

Through tree rings, Ireland is aging the living aspen trees, determining when the last fires occurred and the impact fire had on tree or sucker growth, as aspen rarely grow from seeds. Most often they clone themselves from a sucker system beneath the ground. In the fall it's easy to see the clones because they change the same color at the same time.

Researchers don't know if SAD has damaged the root systems — whether through climate change, insects, diseases, browsing animals or lack of frequent fire — to the point that aspen groves won't sprout new suckers. If a root system dies, they fear that particular DNA could be lost forever.

"Since we don't know why SAD is occurring, and what will bring back the aspen, land managers don't want to run fire through or cut down a stand without knowing if the groves will regenerate," said Zegler. "If they burn the whole site or cut them all down, they may be able to shut off that hormonal response to the root suckering system and stimulate new growth. But what if nothing grows back? I'm hoping our data will help them."

The Kaibab National Forest engaged the School of Forestry and the ERI in a joint research effort to find out what's happening to the aspen and whether this popular deciduous tree is fading permanently from the landscape of its lower elevation range.

Bonnie Stevens is the program director for public education and information at the Ecological Restoration Institute at Northern Arizona University.