Global Warming: Yet Another Threat to Southwest's Iconic Pinyon Pine?

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.

"The way you would compensate for that, if it's possible, is to have additional regeneration. It's like a bank account – trees in and trees out. Now what we're seeing is, wow, both of those are going in the wrong direction," says Dr. Breshears, who did not take part in the study.

Buried in the results are hints that the situation could be a bit less dire than the overall numbers suggest. The two Oklahoma plots showed no significant change between the two periods the data cover. And where the decrease in seed-cone production was significant, the pinyons that were most vulnerable were those at higher elevations, where warming is most pronounced.

That suggests that the trees at lower elevations may be better adapted to warming so far, Redmond says.

Still, "the results were not what I was expecting at all," Redmond says. The magnitude of warming's apparent effect overall, and the trends showing the highest rates of decline among the stands experiencing the largest increases in temperatures came as a surprise, she says.

Concern over the future of the Southwest's forests extends beyond the obvious – lumber, changes in the behavior of the region's fragile watersheds, and the region's overall biodiversity. A large-scale change in plant types also could reinforce the warming the region is projected to experience with climate change.

"The climate modelers will tell us that one of the first variables they have in terms of the land surface is how much tree cover there is," Breshears says. "With some of these die-off events, we're seeing a huge change in tree cover."

Experiments have shown on relatively tiny plots that when trees are removed, temperatures increase, he says.

Much remains to be done to see whether the large die-offs the region has experienced also reinforce warming, he continues.

"But based on other things we know, it's certainly reasonable to expect that here could be feedbacks to the climate system," he says.

"We're losing a lot of woodlands quickly, and we have a lot of questions about what will come back and how fast it will come back" to areas hit by extensive wildfires or die-backs, he says. "To what extent within the course of our lifetime might we be losing some of these forests and woodlands?"

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