Understanding Tree Death

Dec 30 2012, 02:16 IST
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SummaryScientists examine the role of hunger and thirst to predict when and how massive die-offs will happen

SANDRA BLAKESLEE

Everywhere, trees are dying. The boreal forests of Canada and Russia are being devoured by beetles. Drought-tolerant pines are disappearing in Greece. In North Africa, Atlas cedars are shrivelling. Wet and dry tropical forests in Asia are collapsing. The Amazon basin has just been hit by two severe droughts. And it’s predicted that trees in the American Southwest may be gone by the end of this century.

But as this astonishing transformation of landscapes continues, scientists have a confession to make: They do not fully understand how trees die. Certainly warmer temperatures, lack of water and insects play a role. But in each region hit by heat, drought or bugs, some trees remain standing.

Why do some trees die while others survive? What happens deep inside a tree under stress? How slowly or quickly do different species die?

Nate McDowell, a staff scientist at the Los Alamos National Laboratory in New Mexico, aims to find answers. Like a doctor trying to learn why his patient is sinking into a coma, McDowell, a plant physiologist, has set up a kind of intensive care unit for trees to find out precisely how they die, although unlike his physician counterparts, McDowell is nudging his patients toward an early death.

By speeding up aspects of climate change—more heat, less water—he hopes to document every spike in their coffin. And then do an autopsy.

There are two competing theories explaining tree death, McDowell said. They die of thirst. Or they starve to death. But exactly how these processes occur, and how they relate, remains to be shown with scientific rigour. In McDowell’s outdoor experiment, the biggest of its kind in the world, 63 pinyon and juniper trees are being monitored intensely for how they breathe, make food, take up or release water, fight off insects and cope with air that is warmer than usual. Of those, 32 are enveloped in Plexiglas and steel chambers, tops open to the sky and hooked up to a tangle of devices that measure every aspect of their metabolism.

Like a hospital ICU, the experimental site in Los Alamos is noisy. Machines click and roar as they pump warm air into 12 of the chambers, replicating the seven-degree rise in temperature predicted to occur in coming years. Seven of these chambers are also water-deprived. Five chambers serve as controls, with no added stressors. The remaining trees are being monitored outside the chambers, with and

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