As forests disappear, examining how trees die

Sandra Blakeslee / New York Times News Service /

LOS ALAMOS, N.M. — 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 shriveling. Wet and dry tropical forests in Asia are collapsing. Australian eucalyptus forests are burning. 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.

The experiment is badly needed, said Craig Allen, a leading expert on forest ecology for the U.S. Geological Survey who is not involved in the research.

“Without better understanding the mechanisms of tree death, it is not possible to reliably predict when and where the next massive die-offs will occur on this planet,” he said.

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 rigor.

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. There is even an instrument that can peer inside trees, revealing fluxes of water and nutrients flowing inside them.

Like a hospital ICU, the experimental site is noisy. Machines click and roar as they pump warm air into 12 of the chambers, replicating the 7-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 without ambient water.

Each tree is hooked up to a variety of sensors, probes and lines that monitor vital signs. Some measurements are taken every 10 minutes, others once a month.

To monitor how trees might succumb to thirst, researchers are measuring water flow inside each trunk. Normally ropes of water molecules are pulled up from the soil and roots by the atmosphere, moving through very small channels called xylem. When the air is warm, it exerts a greater pull on the water, increasing tension. If the tension gets high, the rope breaks and air is introduced. Like an embolism that can kill a person, air bubbles can block the flow of water. A tree can dry out and die.

A recent study found that 70 percent of 226 forest species from 81 sites worldwide now live on the edge of this so-called hydraulic failure.

To observe how trees might starve to death, researchers are measuring how much carbohydrate, or food, each tree makes from the carbon dioxide taken up by its needles during photosynthesis and then how much the tree consumes to grow or maintain its tissues. There should be at least an even balance.

The experiment began in June. So far the bulk of evidence is consistent with the idea that the trees are drying out or starving, McDowell said.

There has been “a big reduction in the conductance of the xylem and a big reduction in the content of stored carbohydrates,” he said. But “which of these mechanisms dominates is still challenging us because they seem very interdependent.”

“We have insights into how trees die,” McDowell said, “but we are far from capturing just how big the problem is going to get. Only by understanding the cascade of steps that lead to tree mortality can we make accurate predictions into the future.”

It’s a mystery that scientists have yet to fully explain. Why do some trees survive while others die? Now, though, a plant physiologist in New Mexico is trying to determine a way to help predict the where the next massive forest die-offs will occur.

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