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Warmer temperatures play big role in droughty tree die-offs

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A decade ago, he and Craig Allen, a researcher with the US Geological Survey's field station in the Bandelier National Monument near Los Alamos, N.M., teamed up to explore the effects of a drought that struck northern New Mexico in the early 1950s. Up to that time, it was the most severe drought the area had experienced in 500 years. You can read more about some of the work Dr. Allen does here.

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During that drought, the Ponderosa pine forests died at lower elevations, leaving the field largely to juniper and piñon woodlands. In effect, the boundary between the two moved upslope. The change occurred over five years. And it has persisted -- certainly long enough for the scientists to see its effects when they conducted their study 40 years later. One byproduct of the change: higher rates of soil erosion. The team published its results in the Proceedings of the National Academy of Sciences in 1998.

In the end, the change "was not a surprise," Breshears says. "But it hadn't been well documented."

That study illustrated how rapidly an ecosystem can shift -- for the long haul. Fast-forward to the drought of 2000-2003. This time, bark beetles had entered the picture. And instead of looking at one location, the team looked at the entire Four Corners area, in addition to the site they studied in their earlier work. Now they could document a sudden drought-driven piñon die-off covering some 4,600 square miles.

The kicker: In general the drought of 2000-2003 "was not necessarily drier than the 1950s drought," Breshears says. But it was measurably warmer. Yet it's hard to peg the tree die-off under drought conditions to higher temperatures. Those pesky bark beetles clouded the picture. The team published its results in 2005.

So, says Mr. Adams, it was time to take the notion of temperature as the dominant villain governing tree die-offs during a drought and see if it could be shifted from "it might be" or "it looks like it is" to "yes, it well could have been."

On to the Biosphere 2 lab

So, it was out onto a ranch to dig up 20 reproductively mature piñons of like stature and from similar ground conditions. They went into large nursery pots and thence to the controlled environment of the University of Arizona's Biosphere 2 lab, near Oracle,Ariz. Ten trees were kept at temperatures that tracked those the trees would, on average, experience in the wild. The other 10 were kept in temperatures that were an average of 4.3 degrees Celsius above the first group's. Once the trees adjusted to their new digs, the team stopped watering five trees in each group. No bark beetles, no wildfires, only differences in temperature for the "drought-stressed" trees.

Water-stressed trees in the "global warming" group lasted an average of 18 weeks before dying. Water-stressed trees in the "ambient" environment survived for 25.1 weeks.

Based on a suite of measurements the team took, carbon starvation killed the high-temp piñons. In essence, the tree closed the pores, or stomata, on its needles to conserve moisture it otherwise would have lost through evapotranspiration. The closed stomata prevented the plant from taking up CO2 for photosynthesis. This carbon starvation also undercuts a tree's ability to mount its natural defenses against pests. Ergo, dead trees.

The results appear in today's edition of the Proceedings of the National Academy of Sciences.

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