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Where have all the aspen gone?

October 2, 2006 By David Brill This article courtesy of Nature News.

David Brill talks to forestry expert Wayne Shepperd, at the Rocky Mountain Research Station at Fort Collins, Colorado, about the mysterious death of these trees in the western United States.

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There have long been stories about declining aspen numbers, but you've now realised things have been much worse over the past few years. What is happening?

Aspen here in the western United States regenerate by sprouting from the root system of existing trees, rather than growing from seeds. Normally if a parent tree dies you would expect to see thousands of sprouts coming back up immediately from the roots. But what we're seeing now is that the roots appear to be dead and we're not getting any new sprouts.

This hasn't been reported before in the literature. It's a very quick thing I've never seen anything quite like it.

How bad is the problem?

Here in Colorado, which probably has around 2 million acres [810,000 hectares] of aspen, we do annual aerial detection flights. Last year in the San Juan forests, there was upwards of 30,000 acres affected out of about 300,000 [12,000 hectares out of 120,000 hectares]. So that's around 10% of their aspen. We're just completing the flights for this year so we don't know how many additional acres will be affected.

What's causing the die-off?

The general indication is that trees are stressed, and then factors such as diseases and insects that attack stressed trees are killing them off. But we're not sure what that the cause of that underlying stress might be. It's so widespread that we can't point to one factor.

My feeling is that it's a natural phenomenon, rather than an invasive disease that is new to these ecosystems. It could be climatic much of the western US has been in fairly severe drought for the past few years but we can't point the finger strictly at this because some of the trees that are dying are in very wet areas.

Where is this happening?

I've been travelling around this summer and almost everywhere I go I see signs of it. It's running from Canada nearly down to Mexico.

What is being done about it?

Prescribed burning is being used to restore aspen in some parts. Parent trees produce auxin, a growth hormone that inhibits the roots from sprouting. If we burn down the mature trees the buds on the roots can sprout and start growing. In many cases just protecting the trees with a fence to keep grazing animals out will result in regeneration.

What will you do next?

We're still very much in the early stages of trying to investigate this die-back. A lot of people who are doing research on aspen are very concerned about it. There was an aspen workshop symposium at Utah State University earlier this month, and now there's a meeting here at the Rocky Mountain Station [on 28 September] to begin to plot a strategy.

Aspen is an iconic species that everybody wants to protect and see flourish, so there's a momentum building now. I would expect quite a few more researchers to start looking at this in the next year.

What does the future hold for the aspen?

In the worst-case scenario we're probably going to see aspen disappearing from some landscapes that it previously occupied for thousands of years, and there isn't anything we can do about it. Personally I don't think all the aspen in western North America are going to disappear, but I think we will see changes in distribution. You have to accept that Mother Nature is bigger than we are, and these changes inevitably happen.

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