By Justin Gillis / Source: nytimes

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Aspen trees, a hallmark of the West, near Brian Head, Utah. Scientists warned of the effects should greenhouse-gas emissions continue apace. Sean Gallup/Getty Images
The beloved aspen forests that shimmer across mountainsides of the American West could be doomed if emissions of greenhouse gases continue at a high level, scientists warned on Monday. That finding adds to a growing body of work suggesting forests worldwide may be imperiled by climate change.

The new paper analyzed the drought and heat that killed millions of aspens in Colorado and nearby states a decade ago. Such conditions could become routine across much of the West by the 2050s unless global emissions are brought under control, the study found.

“I think of aspens as a good canary-in-the-coal-mine tree,” said William R. L. Anderegg, the Princeton University researcher who led the new study, released online Monday by the journal Nature Geoscience. “They’re a wet-loving tree in a dry landscape. They may be showing us how these forests are going to change pretty massively as that landscape gets drier still.”

The study found that large aspen die-offs were a near-certainty only if greenhouse emissions were to continue at the runaway pace that has characterized the last decade. If global emissions are brought under control, the chances will improve that large stands of aspens could be preserved, the paper found.

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Drought and climate stress triggered widespread aspen forest mortality. William Anderegg
In the fall, stands of trembling aspens are among the most breathtaking sights in the West, turning hillsides an iridescent golden hue.

Dr. Anderegg grew up camping and hiking in the aspen forests of southwestern Colorado and was dismayed when the trees started dying a decade ago. He has devoted part of his early scientific career to understanding the dieback — and the implications of it for forests elsewhere.

A central focus of the research has been to get a better handle on exactly how trees die in droughts, crucial for predicting how they will fare as global warming proceeds. Dr. Anderegg’s research on aspens suggests that when the ground gets too dry, air bubbles appear in the tiny tubes that carry water through the tree.

“These air bubbles block the pipes and interrupt water transport, giving the tree a kind of heart attack, basically,” Dr. Anderegg said.

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