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Old-growth forests are carbon sinks, not sources
Monday, 15 September 2008 00:00
A new study in the September 11 edition of the journal Nature smashes the myth that young forests absorb more carbon than old-growth. It finds that old-growth forests are net carbon sinks and that cutting them releases up to 40% of their stored carbon.

Here’s the first paragraph from the Nature study, entitled Old-growth forests as global carbon sinks (emphasis ours):

Old-growth forests remove carbon dioxide from the atmosphere at rates that vary with climate and nitrogen deposition. The sequestered carbon dioxide is stored in live woody tissues and slowly decomposing organic matter in litter and soil. Old-growth forests therefore serve as a global carbon dioxide sink, but they are not protected by international treaties, because it is generally thought that ageing forests cease to accumulate carbon. Here we report a search of literature and databases for forest carbon-flux estimates. We find that in forests between 15 and 800 years of age, net ecosystem productivity (the net carbon balance of the forest including soils) is usually positive. Our results demonstrate that old-growth forests can continue to accumulate carbon, contrary to the long-standing view that they are carbon neutral.

Over 30 per cent of the global forest area is unmanaged primary forest, and this area contains the remaining old-growth forests. Half of the primary forests (6 x 108 hectares) are located in the boreal and temperate regions of the Northern Hemisphere. On the basis of our analysis, these forests alone sequester about 1.3 +/- 0.5 gigatonnes of carbon per year. Thus, our findings suggest that 15 per cent of the global forest area, which is currently not considered when offsetting increasing atmospheric carbon dioxide concentrations, provides at least 10 per cent of the global net ecosystem productivity. Old-growth forests accumulate carbon for centuries and contain large quantities of it. We expect, however, that much of this carbon, even soil carbon, will move back to the atmosphere if these forests are disturbed.

And here’s the Editor’s Summary:

Don’t cut into old wood

It has long been assumed that ageing forests cease to accumulate carbon, and become carbon neutral. They are therefore not recognized for ‘forest credits’ in treaties such as the Kyoto Protocol. Now an extensive literature and database search for forest carbon-flux estimates shows that the net carbon balance of ageing forests is usually positive. The findings suggest that old-growth forests can continue to accumulate carbon, and that they contribute at least 10% of global net ecosystem productivity. Much of this carbon, even soil carbon, will move back to the atmosphere if these forests are disturbed, so it would make sense for carbon accounting rules to give credit for leaving old forests intact.

This study comes out as the Ontario government attempts to ascertain the relationship between logging and carbon capture in the province’s forests. Here’s what they say so far:

Planting trees to sequester carbon will be one of the keys to mitigating the impacts of climate change. Forest scientists have determined that large amounts of carbon are also stored in wood products made from trees harvested from our sustainably managed forests. Trees that go into wood and paper products continues to store carbon 100+ years after harvest.

See the MNR’s webpage on climate change: Climate change in Ontario

So if old-growth forests are worth more standing than cut, and if old-growth forests store more carbon than young forests, remind us why we are still cutting Temagami’s old-growth forests?