With Deaths of Forests, a Loss of Key Climate Protectors

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While all types of plants absorb carbon dioxide, known as CO2, most of them return it to the atmosphere quickly because their vegetation decays, burns or is eaten. Every year, during the Northern Hemisphere growing season, plants and other organisms inhale some 120 billion tons of carbon from the atmosphere, then exhale nearly the same amount as they decay in the winter.

It is mainly trees that have the ability to lock carbon into long-term storage, and they do so by making wood or transferring carbon into the soil. The wood may stand for centuries inside a living tree, and it is slow to decay even when the tree dies.

But the carbon in wood is vulnerable to rapid release. If a forest burns down, for instance, much of the carbon stored in it will re-enter the atmosphere.

Destruction by fires and insects is a part of the natural history of forests, and in isolation, such events would be no cause for alarm. Indeed, despite the recent problems, the new estimate, published Aug. 19 in the journal Science, suggests that when emissions from the destruction of forests are subtracted from the carbon they absorb, they are, on balance, packing more than a billion tons of carbon into long-term storage every year.

One major reason is that forests, like other types of plants, appear to be responding to the rise of carbon dioxide in the atmosphere by growing more vigorously. The gas is, after all, the main food supply for plants. Scientists have been surprised in recent years to learn that this factor is causing a growth spurt even in mature forests, a finding that overturned decades of ecological dogma.

Climate-change contrarians tend to focus on this “fertilization effect,” hailing it as a boon for forests and the food supply. “The ongoing rise of the air’s CO2 content is causing a great greening of the Earth,” one advocate of this position, Craig D. Idso, said at a contrarian meeting in Washington in July.

Dr. Idso and others assert that this effect is likely to continue for the foreseeable future, ameliorating any negative impacts on plant growth from rising temperatures. More mainstream scientists, while stating that CO2 fertilization is real, are much less certain about the long-term effects, saying that the heat and water stress associated with climate change seem to be making forests vulnerable to insect attack, fires and many other problems.

“Forests take a century to grow to maturity,” said Werner A. Kurz, a Canadian scientist who is a leading expert on forest carbon. “It takes only a single extreme climate event, a single attack by insects, to interrupt that hundred-year uptake of carbon.”

It is possible the recent die-backs will prove transitory — a coincidence, perhaps, that they all occurred at roughly the same time. The more troubling possibility, experts said, is that the die-offs might prove to be the leading edge of a more sweeping change.

“If this were happening in just a few places, it would be easier to deny and write off,” said David A. Cleaves, senior adviser for the United States Forest Service. “But it’s not. It’s happening all over the place. You’ve got to say, gee, what is the common element?”

Tracking an Ebb and Flow

So far, humanity has been lucky. While some forests are starting to release more carbon than they take up, that effect continues to be outweighed by forests that pack carbon away. Whether those healthy forests will predominate over coming decades, or will become sick themselves, is simply unclear.

The other day, deep in a healthy New England thicket of oaks, maples and hemlocks, two young men scrambled around on their hands and knees measuring twigs and sticks that had fallen from the trees.

“What was the diameter on that?” asked Jakob Lindaas, a Harvard student holding a pencil and clipboard.

Leland K. Werden, a researcher at the university, called out a metric measurement, and they moved to the next twig. It was one of thousands they would eventually have to measure as part of an effort to tell how fast the wood, knocked off the trees in an ice storm in 2008, was decaying.

The debris they were cataloging would not have struck a hiker as anything to notice, much less measure, but the Harvard Forest, 3,000 acres near Petersham, Mass., is one of the world’s most intensively studied patches of woods. The work the men were doing will become a small contribution toward solving one of the biggest accounting problems of modern science.