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Drought Causing Record Forest Destruction in U.S. Southwest

Hope Hamashige
for National Geographic News
December 5, 2005
 
Soil-scorching droughts are nothing new to the U.S. Southwest. But the
one that hit the region in 1999—and still persists—has 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.

"We know that older trees died during the 1950s, but most of the middle-aged trees survived," said Craig Allen, an ecologist with the U.S. Geological Survey (USGS) in New Mexico. "This drought took out almost everything."

The link to climate change, Breshears said, should give pause, because the devastation to the Southwest is not only far-reaching but it swept the system in a short time.

"What we have here is a clearly documented case about how big and how fast die-offs can be in response to climate change," Breshears said.

The early signs of climate-induced die-offs may already be happening, said Neil Cobb, an ecologist at Northern Arizona University in Flagstaff. He notes that bark beetles have invaded the damp environs of Alaska and British Columbia and are killing trees there.

"It is becoming a real concern in British Columbia, because the lodgepole pine are starting to die," he said.

"In the Kenai Peninsula [in Alaska], the trees are starting to turn red."

Long-Term Effects Unknown

Most experts agree that the ecosystem of the Southwest will be forever changed by this historic die-off. But it may be years before anyone understands exactly how those changes will play out.

"It is extremely hard to predict what is going to happen to this ecosystem," Allen, the USGS ecologist, said.

Piñon is slow-growing, he noted, and so while some smaller trees have survived, it will be decades before the piñon forest rebounds.

Allen said it is still too soon to tell how the massive die-off will affect soil erosion and the risk of wildfire.

"It could mean there will be an increase in surface fires, which are easier to control than the crown fires," Allen said. "There is the potential for increased soil erosion in some places, but we can't say yet if that is happening."

Likewise the long-term effect on wildlife that depend on the piñon forest, such as the piñon jay, remains to be seen.

Not Out of the Woods

Winter is a critical time in the Southwest. As the first chills set in there is speculation about whether temperatures will drop enough and snowfall will be significant enough to lessen the effects of the drought.

The signs are not good.

"It has been predicted that the drought will extend to at least 2010 and possibly 2030," said Terry Rogers, an entomologist with the U.S. Forest Service in Albuquerque.

"A lot of what happens will depend on the snow this winter. But the fall was warm, and there has been very little snow so far."

The piñon deaths are subsiding, Rogers noted, but only because so much of the forest is already dead.

But that doesn't necessarily mean the changes are over. As the drought persists and temperatures remain high, the fear is that new tree species will weaken and succumb to the beetles.

Already the beetles are showing up in Arizona's ponderosa pine forests, in Utah's spruces, and in Colorado's Douglas fir.

"We have already seen some juniper mortality, and there could be more," said Breshears of the University of Arizona. "If it gets severe enough to kill wet, high juniper, that is really scary, because the juniper is a tough plant."

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