Global Warming Kills Trees

Before I slipped away for vacation last week, I was able to write this (sub. req.):

New Mexico’s massive pi?on die-off of 2002 and 2003 might be a harbinger of life here in a warming world, new research suggests.

High elevation forests that had survived previous droughts saw as much as 90 percent pi?on mortality, a team of researchers led by University of Arizona ecologist Dave Breshears reported Monday in the Proceedings of the National Academy of Sciences.

“Across a whole landscape, this system got whacked,” Breshears said in a telephone interview.

Drought weakened the trees enough that bark beetles could kill them, but warmer temperatures appear to have played a key role, the scientists found.

The temperature difference? 1 to 2 degrees Fahrenheit above the long-term average? might not sound like much. But the scientists say it made the difference, leading to tree death in areas relatively unaffected by the drier drought of the 1950s.

“This is a different kind of response than we saw following the 1950s drought,” said Breshears, who has been studying pi?on woodlands since the 1980s. “This drought was hotter.”

Breshears, in an interview, was careful not to blame the 2002 die-off on human-caused global warming, saying no one event can be unequivocally linked to the planet’s long-term rising temperature trend.

But he said the dramatic drought-induced changes in the Southwest’s landscape since the turn of the 21st century are consistent with global climate change projections.

“We’re more likely to get more frequent, more intense droughts,” Breshears said.

For those of you not in the U.S. southwest, the pi?on/juniper woodland is a dominant middle-elevation forest type that was decimated a few years back – whole hillsides turned from their usual green to a frightening greyish brown. Like dwindling snowpacks, it’s a warming effect that you can see on the ground, now, but until Breshears et al. did this work, no one had established the link. It’s still a controversial link, as the comments from Julio Betancourt in my story suggest. (Betancourt, a terrific scientist who I respect on these issues and who probably knows the drought of the 1950s better than anyone, believes we don’t enough about 50’s dieoff to be sure the current dieoff is worse.) But caveats notwithstanding, it’s nevertheless important work of the sort that matters – climate science helping us understand impacts on the ground.