Idea that forests are 'carbon sinks' no longer holds

We are used to hearing that a way to tackle climate change is to plant trees and create new forests or restore deforested areas. Conventional wisdom has it that trees and forests are so-called 'carbon sinks', that is, they suck carbon-dioxide - the most problematic greenhouse gas - out of the atmosphere and store it as solid carbon in their branches, trunks and roots.

New research now shows that instead of carbon sinks, some forests emit more carbon than they store. Forests can do little to improve the future climate or to lower the atmosphere's carbon levels. What they can do is make global warming worse.

This is the conclusion of a Canadian and American team of forest scientists that went into the woods in northern Manitoba to measure the carbon cycle of a forest ecosystem. They wanted to measure carbon going into and out of a living forest, to learn how effectively the forest was sucking carbon dioxide out of the atmosphere and storing it.

The results of this scientific work are congruent with research done in other forest types, most notably in tropical forests where the same observation was found: forests contribute more CO2 to the atmosphere than they store. (See FLUXNET, the world-wide network of carbon cycle measurements, with sites on all continents).

The consequences of these scientific results are manifold: forest nations will not be able to enjoy the benefits brought by the United Nations Framework on Convention on Climate Change because forests can no longer be filed as 'carbon sinks'. Re- and afforestation efforts are no longer a certain quick fix to climate change (they do have many other benefits, though), and large fossil fuel burning utilities who now often contribute financially to such efforts to appease their conscience, must rethink their strategies.

Still, the net CO2 contribution of forests is far lower than that of simply burning fossil fuels, so planting new energy trees (either as part of a re- or afforestation effort) to use them as bioenergy feedstocks to be used instead of coal, gas or oil, remains a good strategy to tackle climate change:
biomass :: bioenergy :: biofuels :: energy :: sustainability :: forests :: carbon cycle :: climate change ::

So how exactly did the researchers reach their conclusions? In the 1990s, they chose an area that belongs to the boreal forest the northern forest dominated by black spruce that is Canada's most widespread, and still most untouched forest.

What they found surprised many.

The team made 22,000 hours of intensive measurements of the soil, the surface of the ground, and all the way up through the 120-year-old forest past the canopy to open air. They learned carbon goes both ways. From late May through July, new growth made the spruce forest 'inhale' one to one and a half grams of carbon per square metre of forest per day. In August and September, the hottest, driest period, the rate of carbon dioxide movement fell to about zero.

But in the late summer and fall, the forest 'exhaled' carbon back into the atmosphere at a rate of a little less than one gram per square metre per day, as warmer soil allowed soil bacteria to digest organic matter and release carbon dioxide. This fell to a much lower rate through the winter

Overall, in three of the four years they measured, the forest was putting slightly more carbon into the air than it took out a bad thing, if we want forests to store this material. The fourth year, the balance tilted the other way: The forest sucked out and stored carbon but not a lot of it.

'Forests on average certainly exchange a lot of carbon with the atmosphere,' team leader Steve Wofsy of Harvard University said in an interview. 'So if you want to say: `Do they remove a lot of carbon from the atmosphere?' yeah, sure they do. Do they put back a lot? Sure, they do that, too.'

But what about all the other forests, the southern ones with their maple-beech-oak hardwoods, and their pines and aspens? Aren't they cleaning our air? Unfortunately, said, Bill Schlesinger of Duke University, even these forests are generally in a steady state in terms of carbon production and sequestration.

'And so you can't really count on them as a big sink,' he said.

Yes, he acknowledges, many people do make the claim that forests will counteract our car-driving, coal-burning ways.

'Oil and coal companies love to say that. So do various forest services,' he said. 'It sort of gives them a raison d'etre.'

'But the idea that they're going to combat the rise of CO2 in the atmosphere has, I think, probably been overstated. If you disturb them,' by cutting them down or burning them, 'then they may exacerbate the rise of carbon dioxide.'

This could be disappointing news for many of Canada's political leaders, who have been counting on credits under the Kyoto Protocol for Canada's forest 'sinks'.

If your forests are taking up carbon, the United Nations Framework Convention on Climate Change says, then you don't have to do as much to stop burning coal, oil and gasoline.

Canada has until January 1 to decide whether it wants to include forests as 'sinks' to gain credit for cleaning up the greenhouse. Federal and provincial government experts haven't finished going through the numbers.

But here's a twist.

All that matters in this accounting is whether our forests suck up carbon for a brief period 2008 through 2012, the time covered by the Kyoto deal. Canada could claim a credit if managed forests showed a burst of regrowth, and probably with an aggressive campaign to limit forest fires, during this period. Results would have to be audited during this period. But this type of accounting looks only at short-term, temporary changes, with no regard for the long-term reality.

Federal scientists know the forests are not going to help much. But in the short run they say forests may be able to suck up some carbon that they will release again later, after the accounting period. It's like a boxer purging to get his weight under a limit in time for a fight, with no thought of staying at that weight afterward.

More information:

Steve Wofsey, et al, Seasonality of ecosystem respiration and gross primary
production as derived from FLUXNET measurements [*.pdf], Agricultural and Forest Meteorology 113 (2002) 53–74.

Carol C. Barford,1* Steven C. Wofsy,1dagger Michael L. Goulden,2 J. William Munger,1 Elizabeth Hammond Pyle,1 Shawn P. Urbanski,1 Lucy Hutyra,1 Scott R. Saleska,1 David Fitzjarrald,3 Kathleen Moore3, Factors Controlling Long- and Short-Term Sequestration of Atmospheric CO2 in a Mid-latitude Forest, Science 23 November 2001: Vol. 294. no. 5547, pp. 1688 - 1691 DOI: 10.1126/science.1062962

Ottawa Citizen: Idea that forests are 'carbon sinks' going down drain - Nov. 6, 2006.