ABQJOURNAL NORTH: Tree Deaths Tied to Warming

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Tuesday, April 14, 2009

Tree Deaths Tied to Warming

By John Fleck
Copyright © 2009 Albuquerque Journal
      Circumstantial evidence has long pointed to rising temperatures as a tree killer in New Mexico and the rest of the West. Now, University of Arizona scientists say they have proof.
       And that means waves of tree deaths like northern New Mexico saw in 2002-2003 are likely to become more common in the 21st century as temperatures increase, according to new research being published this week.
       If the research holds up, it suggests the possibility that places like northern New Mexico could have little or no piñon-juniper woodland later this century, said Henry Adams, the new study's lead author.
       In a warming world, modest droughts could be enough to kill trees, when it used to take the kind of extreme drought that only happens once or twice a century, said David Breshears, one of Adams' colleagues.
       That could make it hard for forests to get re-established, the scientists said, leaving a landscape of grasslands.
       The new research, involving controlled laboratory tests of the effect of heat on trees, adds to a growing body of evidence that increasing temperatures are already killing forests in the western United States, said University of Arizona ecologist David Breshears. The tree death is likely to get worse because of global warming, said Breshears, one of the leaders of the research team whose work is being published this week in the Proceedings of the National Academy of Sciences.
       The new research is “a pretty unequivocal demonstration” of the role rising temperature has in killing trees, said Craig Allen, a U.S. Geological Survey biologist who was not involved in the new research.
       With forecasts predicting higher temperatures in the 21st century, the new research shows that northern New Mexico's forests are at increasing risk of the sort of widespread die-off seen in 2002 and 2003, said Allen, who is based at Bandelier National Monument in northern New Mexico.
       In the wild, it is often the bark beetle that delivers the final blow after some combination of high temperatures and lack of precipitation leaves trees near death. But out in the woods, there is no way for scientists to conduct a controlled experiment to see what role each factor plays.
       To overcome that problem, the scientists, led by Adams, a graduate student, transplanted piñon trees from northern New Mexico to Biosphere 2, a giant environmentally controlled greenhouse in the foothills of the Santa Catalina Mountains outside Tucson.
       Originally built in the late 1980s for an experiment in which a group of human subjects were to live in a sealed environment, Biosphere 2 is now used by University of Arizona scientists for ecological research.
       Adams and his colleagues used it to simulate global warming, keeping some of the trees at temperatures that matched their natural environment and heating part of Biosphere 2 to simulate temperatures forecast for later this century.
       They then put some of the trees under drought stress, cutting off their water supply, to see if those in higher temperatures died sooner. They did. Meanwhile, trees given normal water rations survived, suggesting to the scientists that transplantation was not to blame.
       Drought plays a key role, the scientists found, but increased temperature means a much more modest drought is enough to kill the trees.
       In the past, droughts severe enough to kill off piñon-juniper woodlands came a couple of times a century. In New Mexico, the droughts of the 1950s and the early 2000s were sufficient to cause widespread tree mortality. In the years between, the woodlands then have a chance to slowly re-establish themselves, giving the trees time to mature to reproductive age. “It takes decades to get tree cover back up and going,” Breshears said.

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