1. Department of Biology, University of Antwerp, 2610 Wilrijk, Belgium
2. College of Forestry, Oregon State University, Corvallis, Oregon 97331-5752, USA
3. Max-Planck Institute for Biogeochemistry, 07701 Jena, Germany
4. ETH Zürich, Institute of Plant Sciences, CH-8092 Zürich, Switzerland
5. Laboratoire des Sciences du Climat et de l'Environnement, IPSL-LSCE, CEA-CNRS-UVSQ, 91191 Gif sur Yvette Cedex, France
6. School of GeoSciences, The University of Edinburgh, Edinburgh EH9 3JN, UK

Correspondence to: Sebastiaan Luyssaert^1,2 Correspondence and requests for materials should be addressed to S.L. (Email: sebastiaan.luyssaert@ua.ac.be).

Abstract

Old-growth forests remove carbon dioxide from the atmosphere^{1, 2} at rates that vary with climate and nitrogen deposition³. The sequestered carbon dioxide is stored in live woody tissues and slowly decomposing organic matter in litter and soil⁴. 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^{5, 6}. 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 forests⁷. Half of the primary forests (6  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 productivity⁸. Old-growth forests accumulate carbon for centuries and contain large quantities of it. We expect, however, that much of this carbon, even soil carbon⁹, will move back to the atmosphere if these forests are disturbed.

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