Stands of the white-barked trees - famous fixtures in Western mountain landscapes - have been declining in Utah and other regional states for decades. But in recent years, most noticeably this summer in Colorado, there have been mysterious mass die-offs. In some places, stands are dying even as close neighbors continue to thrive.

More than 100 researchers gathered at Utah State University on Tuesday to discuss ways to determine the root causes of the growing problem. The conference, ''Restoring the West: Aspen Restoration,'' runs through today.

Aspens reproduce using a wide-reaching root system that sends out tree clones that grow nearby. Even if the parent tree dies, the root system can survive and create new clones.

In some cases in Colorado, entire root networks have perished, said Wayne Shepperd, a researcher with the U.S. Forest Service's Rocky Mountain Research Service in Colorado.

"If we're losing roots," he said, "that's going to change the amount of aspens on the landscape."

Since 2003, Utah has seen die-offs across thousands of acres, said John Guyon, a U.S. Forest Service researcher based in Ogden.

This summer, Colorado has seen similar die-offs, Shepperd said.

Aspens, native to the higher elevations of the West, face several challenges, but it is unclear which problem or set of problems might be responsible for the die-offs. Cytospora cankers, a plant disease; insects, such as the large aspen tortrix; and climate change are all possible culprits.

"It's really scary how many cytospora cankers we're finding out there," said Guyon. Normally, these cankers are not considered to be a deadly problem for aspens.

Aside from saving the aspens to preserve ecological diversity in the West, the die-offs could harm water resources. In some areas, conifers are encroaching on aspen territory. There may be differences in how watersheds containing different trees process water.

"There may be significant loss of water resources that could be coming out of these watersheds," said Ron Ryel, a USU researcher on wildland resources.

Researchers have found differences between the sizes of snowpacks near aspens and conifers, which can affect how much water is available for use in the summer.

The aspen decline is an issue from Arizona to Alberta, Canada. Utah has lost as estimated 50 percent of its aspen trees since European colonization of America, Ryel said.

Despite the gloomy outlook for aspens, the conference contained a few bright spots for restoration of the trees.

Southern Utah's Fishlake National Forest has had some success prompting aspens to grow new clones. Fires and commercial harvesting were among a few activities that triggered some aspen root systems to sprout new clones, some of which have grown into trees.

glavine@sltrib.com