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Research Predicts Vulnerable Forests

Warming temperatures make forests like this one, destroyed by last year’s Las Conchas Fire, far more vulnerable. (Journal file)

FOR THE RECORD: This story  incorrectly reported the length of the current drought, as measured by a team of Los Alamos researchers. According to the scientists, the drought of 2000-12, in terms of its combination of tree-stressing dryness and heat, is the fifth worst in the last thousand years. The story described it as being the drought of 2008-2012.

Park Williams remembers the forest when he visited Mesa Verde more than two decades ago as an inquisitive third-grader on a family vacation.

“Back then, it was wet and green,” the 31-year-old scientist recalled.

Leaning over a data-strewn laptop, Williams pointed to the 1980s on a graph of Southwestern climate. “Since then, we’ve found it was probably the wettest decade in a thousand years,” he said.

It was a very different story when a grown-up Williams, with a newly minted Ph.D. and a curiosity about how forests respond to a changing climate, arrived at Los Alamos National Laboratory in April 2011. It was the heart of the hot drought that fueled the devastating Las Conchas forest fire. After the wet spell of the ’80s, the forests around his new home were suffering a remarkable flip in the region’s climate, drought conditions rarely seen in the last thousand years.

New research from Los Alamos National Laboratory scientist Park Williams suggests that the worst forest-killing droughts of the past in the Southwest will become the norm of the future as the climate changes.

Using tree ring records, Williams and his colleagues have concluded that the drought of 2000-12 2008-12, in terms of its combination of tree-stressing dryness and heat, was the fifth worst in the last thousand years.

More striking than the current drought, though, is the forecast embedded in Williams’ graphs. If climate trends follow even the most conservative projections from scientists who study the effects of rising greenhouse gases, the work by Williams and his colleagues suggests a warming climate will push the Southwest’s forests by the middle of the 21st century into a regime in which the worst tree-killing drought conditions of the last thousand years become the norm.

Writing last month in the journal Nature Climate Change, Williams and a team of colleagues offered a sobering projection: Our changing climate is pushing the forests of the Southwestern United States toward conditions “unfamiliar to modern civilization.”

“I think it’s an incredibly important paper,” said University of Arizona researcher Dave Breshears. “We’re trying as a scientific community not to be alarmist, but we’re seeing results that are consistently alarming.”

We should not expect an inexorable slide into megadrought, said US Geological Survey ecologist Craig Allen, a co-author on the paper. In short-run periods of years or even a decade or more, natural variability will likely lead to a rebound in forest conditions, but in the long run the trend is clear, Allen said.

Williams, whose academic expertise is in the use of tree rings to understand climate, had been studying Western forests before he came to Los Alamos. Lanky, with a scraggly beard and a Patagonia backpack, he looks like the field scientist he is, collecting tree ring samples from remote forests.

But for much of the drought analysis, Williams put down his tree coring equipment, looking at the forests instead through a computer in a hunt for patterns in large data sets of tree ring measurements, satellite-collected plots of changing vegetation, fire records and climate simulations.

He found that even small temperature increases, like those we have seen in recent decades, push ponderosa, piñon and other arid climate trees to the brink, leaving them vulnerable to bark beetles, fire and drought-starved death, Williams and his colleagues wrote. And New Mexico and its neighbors are ground zero for the problem.

“In the Southwest, there’s just this bullseye,” Williams said.

University of Arizona climate researcher Jonathan Overpeck said you can already see the results.

“Clearly warming is already making the impact of drought on Southwest vegetation worse,” Overpeck said in an email. The results mean global warming “will hit vegetation harder than we thought,” he said.

Williams’ is only the most recent research to come to this conclusion. Overpeck and colleagues Jeremy Weiss and Julio Betancourt also published related research last month that attacks the problem from a different direction, but comes to the same conclusion – even small increases in temperature can turn ordinary droughts into forest killers. And as long ago as 2005, Breshears published a study that fingered warming temperatures in the great northern New Mexico piñon die-off during the drought of 2002-03.

Scientists have long known that trees, especially in arid climates, grow less in years with less rain and snow. Williams and his colleagues used measurements of the annual rings put on by growing trees to show that warmer temperatures also play a role, especially during the spring and summer months. Years with little precipitation and warm temperatures saw the most bark beetles, tree mortality and fire, Williams and his colleagues found.

Weiss, Betancourt and Overpeck found the same thing when they compared tree mortality in the Southwest during the drought of the 1950s to the warmer drought of the 2000s.

The problem, Weiss explained in a University of Arizona writeup of the research, is the way warmer air acts like an “atmospheric sponge.” The warmer the air, the more moisture the atmospheric sponge soaks up as it evaporates more of the rain and snow before the trees have a chance to drink it up with their roots. Plants also lose more water from their leaves, or are forced to shut down to conserve water, according to Williams.

“From a plant’s standpoint, it’s kind of a double whammy,” ecologist Allen said.

For Williams, forest death is not some abstract concept in the future. Working now at Los Alamos National Laboratory, he sees it every day in the beleagured piñon-juniper woods peppered with already dead ponderosa pines he passes every day on his drive from his Santa Fe home to work.

“These are things that we’re seeing already,” he said.