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In Western states, an iconic natural landmark is disappearing. Aspen trees are vanishing in mountains where they usually thrive. In forests from Arizona to Canada, the aspens are not regenerating. Scientists are scrambling to learn why. And towns that depend on the trees to attract tourists are worried.

From Aspen Public Radio, Kirk Siegler reports.

KIRK SIEGLER: The quaking aspen tree, with its signature white bark and deciduous green leaves, is a stark yet beautiful contrast to the West's ubiquitous evergreen. Over half of the region's aspens call Colorado home.

Mr. PHIL BOUDIN(ph) (Forester): So this stand that we're in is just slowly kind of unraveling, you now?

On Colorado's White River National Forest, foresters like Phil Boudin are already noticing the changes.

Mr. BOUDIN: Right here, I mean, the stand we're looking at, the older aspen stand, is doing pretty good right now, compared to the one behind us that we were just looking at that is probably 80, 90 percent dead.

SIEGLER: Hiking up this steep mountain next to the Vale ski resort, Boudin says it's difficult to pinpoint a cause for the deaths, especially since some of these aspens are doing fine.

Mr. BOUDIN: When you're looking at ecosystems, they're so complex in all the different interactions that there's no way that you can easily point to one thing.

SIEGLER: Indeed, scientists are at a loss. Aspens are dying in wet areas, dry areas, at low elevations and high altitudes.

Mr. WAYNE SHEPPERD (Rocky Mountain Research Station): Which is kind of the head-scratcher here, is why is this going on over such an extensive area?

SIEGLER: That's Wayne Shepperd, of the Forest Service's Rocky Mountain Research Station. He says forests are always changing. It's just that usually the changes are so slow, humans don't notice them. The opposite is the case with aspens.

Mr. SHEPPERD: We feel there's probably an underlying reason, but we have not been able to identity that yet.

SIEGLER: Warmer temperatures, a fungus, or even livestock munching on bark and new growth underneath aspens are all on the table.

Dan Binkley(ph), of Colorado State University's Forest Restoration Institute, says drought should be at the top of that list. But he cautions nothing can be ruled out this early on.

Mr. DAN BINKLEY (Forest Restoration Institute, Colorado State University): Whatever this widespread die-off is being caused by, you have to kind of leave room for some surprise or unexpected answers to come along.

SIEGLER: Unlike most trees, aspens aren't supported by one root. Instead, hundreds of trees can be connected to a single root system. That's why when one root system fails, the number of dead aspens can look staggering.

Near the town of aspen, the popular Conundrum Trail winds along the stream flanked by groves of the trees just starting to golden. Thousands of tourists come each year to see these changing colors, tourists like Jack Welch(ph) of Austin, Texas, who sees aspens as a symbol of Colorado.

Mr. JACK WELCH (Tourist): I mean, we're sitting here above Aspen, Colorado amongst new growth aspen not growing back as fast as they should, and it's almost depressing.

SIEGLER: Tourism officials aren't depressed yet, but they are nervous. At this point they aren't sure what the economic effects on the multibillion dollar tourism industry would be if the aspen continue to disappear. They hope scientists like Wayne Shepperd are right.

Mr. SHEPPERD: As a scientist I'm cautiously optimistic that I think this is probably a disturbance to our forested ecosystems that will run its course and probably everything will be fine.

SIEGLER: Still, Shepperd says scientists must try and pin down why the aspens are dying, something they hope to have done by the end of the year. He says they'll then figure out how the die-off might be stopped, or at least slowed down.

For NPR News, I'm Kirk Siegler in Aspen.

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