for National Geographic News
Soil-scorching droughts are nothing new to the U.S. Southwest. But the one that hit the region in 1999and still persistshas been different from past droughts: It has been hotter.
It has also caused what is arguably the most extensive die-off of trees ever documented by modern science.
Upward of 45 million piñon pine trees have died in New Mexico in the last three years, according to the U.S. Forest Service. New Mexico, which claims the short, nut-bearing piñon as its state tree, has been hardest hit by the drought.
New research suggests that it was higher-than-normal temperatures and not just the lack of water that produced the large-scale die-off.
David Breshears, a natural resources professor at the University of Arizona, said the piñon mortality in the Southwest should be a warning to the rest of the world: What is happening here can happen anywhere global warming is making itself felt.
"It's the type of thing we can expect more of with global warming," Breshears said. "There is reason to believe other systems could get whacked the way the Southwest did."
The Heat Factor
Breshears said temperatures during the current drought have been on average 3ºF (1.7ºC) higher than they were during in the 1950s, the most recent dry spell in the Southwest for which there is temperature data.
The heat weakened the trees, making them susceptible to bark beetle invasions, he explained.
When they are healthy, piñon can fend off beetle attacks by producing sap. But the heat limited the trees' ability to defend themselves.
The beetles dealt the trees a fatal blow by boring into them, reproducing and feeding off parts of the piñon.
Researchers point out that some piñon died in the drought of the 1950s but not in numbers similar to the current die-off.
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