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The short, bushy pinyon pine thrives in the arid climate of the U.S. Southwest, where there may be little or no rain for months or even years. Yet, a drought that began in 2000 killed some 10 percent of the pines in the Four Corners region where Arizona, Colorado, New Mexico and Utah meet—even in moister, high-elevation areas—and scientists wondered if the warmer temperatures resulting from climate change might be the cause.
So ecologist Henry Adams, a doctoral candidate at the University of Arizona (U.A.) in Tucson, and his colleagues decided to test the effect of higher average temperatures on the pinyon, Pinus edulis. The researchers transplanted 20 trees from New Mexico to Biosphere 2—a 7.2-million-cubic-foot (203,900-cubic-meter) dome in the desert that can recreate any climate—and subjected 10 of them to ambient desert temperatures and the other half to temperatures 7.7 degrees Fahrenheit (4.3 degrees Celsius) warmer, which is near the middle of the range of average temperature increase forecast by the U.N. Intergovernmental Panel on Climate Change in 2007. They then cut off the water to five trees in each temperature group.
Adams and his colleagues report in this week's Proceedings of the National Academy of Sciences USA that the drought-stricken trees in the hotter temperatures died in just 18 weeks, surviving seven weeks less than those subjected to dry conditions in today's thermal range.
In response to drought, the pinyons close the tiny openings on their needles known as stomata to conserve water but, by doing so, also block their supply of carbon dioxide, thus shutting down photosynthesis, Adams says. The pines in a warmer world then ran through their stored supplies of water and energy faster than those in today's climate.
"After three weeks with no photosynthesis happening, eventually the trees run out of stored energy and die," Adams says. "They are closing down their stomata to try to wait out a drought."
That is bad news for the pinyon, one of the most prevalent forms of vegetation in the Southwest; droughts lasting four months, like the one in 2000, occur five times as frequently, even without taking into account any future drying effect of climate change.
"We assumed the upcoming climate will be like the past 100 years—actually, probably, there will be more drought," says ecologist David Breshears of U.A., who participated in the study. "If you go up to five tree mortality events a century for trees that take decades to grow and establish, it's tough for these trees."
It's also possible that the pinyon's sensitivity to temperature is shared by other trees and plants. A study published in Science earlier this year by ecologist Phillip van Mantgem of the U.S. Geological Survey found that all tree species in the western U.S. were dying more frequently—tree mortality has doubled in the past 30 years.
"These results are surprising in how very sensitive the mortality [rate] is to temperature," Breshears notes of the piñon. "If this is a common phenomena then we could be in for a lot more change than has even been projected so far."
Given that trees play a key role in keeping carbon dioxide levels in check, widespread die-offs could exacerbate the climate change that caused them—and render the Southwest's terrain even more desolate.
"These [die-offs] are events that transform the landscape," Breshears says, noting that pinyon pines also face other threats such as bark beetles, which could exacerbate the problem. "It's possible the pinyon is a canary in a coal mine, but the question is a lot broader than just the piñon pine."