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Global warming: Yet another threat to Southwest's iconic pinyon pine?

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For instance, a new study of a recent widespread die-off of aspens in Colorado identified a three-year drought and high summer temperatures as the trigger for the die-off, which affected some 17 percent of the state's aspen forests. The work, published online Jan. 25 by the journal Global Change Biology, was led by researchers at the Carnegie Institution for Science's Department of Global Ecology. The results suggest an increasing vulnerability of aspens to future large-scale die-offs if climate projections pan out for the area.

In addition, researchers are concerned about the future of the Southwest's Ponderosa forests, especially after record-breaking fires in Arizona and New Mexico in 2011 torched hundreds of thousands of acres of the trees.

Ponderosa also experience masting events, in which seed-cone production in a single masting year can rival the total of all the seed cones produced between masting years.

Even without climate change, some of the burn scars are so extensive that some researchers say some burned-out areas may come back as oak and brush, rather than forests. Anywhere from three to seven years can pass between masting years. This can give more aggressive plant species time to recolonize large burned areas and out-compete any Ponderosa seedlings that trickle in from unburned spots.

So far, the only study looking at the potential impact of temperatures on Ponderosa seed-cone production hasn't found any, Ms. Redmond says.

But that study looked at only two stands, while the study she and her colleagues – Frank Forcella with the US Department of Agriculture and University of Colorado ecologist Nichole Barger – examined covered nine stands over a wide geographic range.

Their work "is a really important study," says Dave Breshears, an arid-lands ecologist at the University of Arizona in Tucson who has documented pinyon die-offs in the Southwest.

"We're getting increasingly confident that these types of woodlands are very sensitive to a warmer, drier climate," he says.

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