US forests hold new evidence of global warming

The results run counter to trends seen in the tropics. In undisturbed tracts there, tree mortality and replacement both are increasing.

The study adds to a growing body of research that suggests any country with extensive forests – whether tropical, temperate, or boreal – may deserve a place at the table when global climate talks discuss “avoided deforestation” as a tool for reducing greenhouse-gas emissions.

So far, the approach has been seen largely as a way to encourage developing countries with extensive tropical forests to take part in a new global climate treaty. But last September, an international team of scientists led by Sebastiaan Luyssaert at the University of Antwerp in Belgium, published a study showing that globally, old-growth forests outside the tropics actively soak up and store carbon dioxide from the atmosphere far longer than previous estimates had indicated. [Editor's note: The original version misstated the location of the University of Antwerp.]

Indeed, for many of the tree species in the latest study, the likelihood that a tree will survive for another year increases, rather than decreases, with time, notes Jerry Franklin, a forest ecologist at the University of Washington in Seattle.

“One of the things that absolutely should be on the table in any global carbon agreement is the notion of avoided carbon releases,” Dr. Franklin says. “Old forests have very large accumulations of carbon in them.” If these forests are felled, it will lead to significant carbon emissions over the short and medium terms – emissions that can’t be recouped by turning the wood into lumber or by planting replacement trees, he says.

Housing development could be affected
If the trends continue, they have implications for the region’s efforts to adapt to climate change, adds Thomas Veblen, a biogeographer at the University of Colorado at Boulder. Particularly when the underlying die-off is accelerated by insect infestations, wildfire managers will have to rethink their strategies for reducing wildfire risks, including current policies that encourage people to build large housing developments in wildfire-prone areas.

In addition, the changes are making wildlife conservation efforts more difficult to plan.

Often, conservationists set a target by learning what an area was like in the distant past, then they try to re-create that, says Nathan Stephenson, also with the USGS research center in Three Rivers, Calif. “As climate changes and other environmental changes happen, the past may no longer be the best model for the future. We may switch from trying to keep a snapshot of the past to efforts to help guide things into the future while sustaining old forests.”

For Franklin, even that may be too gentle.  “So much of conservation is focused on going back or keeping it as it is,” he says. Faced with a warming climate, “you can’t go home.”

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