1. Earth and Biosphere Institute, School of Geography, University of Leeds, Leeds LS2 9JT, UK
2. Plant Systematic and Ecology Laboratory, University of Yaounde I, PO Box 047, Yaounde, Cameroon
3. Resource Management Support Centre, Forestry Commission of Ghana, PO Box 1457, Kumasi, Ghana
4. Department of Environmental Management and Toxicology, University of Agriculture, PMB 2240, Abeokuta, Ogun State, Nigeria
5. Bureau Waardenburg bv, PO Box 365, 4100 AJ Culemborg, The Netherlands
6. Institut de Recherche en Ecologie Tropicale (IRET), BP 7847, Libreville, Gabon
7. SI/MAB Biodiversity Program, Smithsonian Institution, Suite 3123, 1100 Jefferson Drive SW, Washington DC 20560, USA
8. Forest Ecology & Management Group, Department of Environmental Sciences, Wageningen University, PO Box 342, NL-6700, The Netherlands
9. Plantlife International, 14 Rollestone Street, Salisbury, Wiltshire SP1 1DX, UK
10. Wildlife Conservation Society–DRC, 1725 Avenue Monjiba, Chanic Building 2nd floor, Ngalinema, BP 240, Kinshasa I, Democratic Republic of Congo
11. Eco-anthropologie et ethnobiologie, Département Hommes, Natures, Sociétés, MNHN, 4 av. du Petit Château, 91 800 Brunoy, France
12. Centre for Ecology, Law and Policy, Environment Department, University of York, York YO10 5DD, UK
13. Environmental Change Institute, School of Geography and the Environment, Oxford University, Oxford OX1 3QY, UK
14. Department of Botany, University of Dar es Salaam, PO Box 35060, Dar es Salaam, Tanzania
15. Centre for International Forestry Research, PO Box 0113 BOCBD, Bogor 16000, Indonesia
16. Department of Plant & Soil Science, Cruickshank Building, School of Biological Sciences, University of Aberdeen, St Machar Drive, Aberdeen AB24 3UU, UK
17. Department of Geography, Museum Building, School of Natural Sciences, Trinity College, University of Dublin, Dublin 2, Republic of Ireland
18. Faculty of Forestry, University of Toronto, 33 Willcocks Street, Toronto, Ontario M5S 3B3, Canada
19. Sommersbergseestr. 291, A-8990 Bad Aussee, Austria
20. Present addresses: Inventory & Monitoring Program, National Park Service, 120 Chatham Lane, Fredericksburg, Virginia 22405, USA (J.A.C.); Institute of Tropical Forest Conservation, Mbarara University of Science and Technology, PO Box 44, Kabale, Uganda (D.S.); Centre for International Forestry Research, PO Box 6596 JKPWB, Jakata 10065, Indonesia (T.S.).

Correspondence to: Simon L. Lewis¹ Correspondence and requests for materials should be addressed to S.L.L. (Email: s.l.lewis@leeds.ac.uk).

The response of terrestrial vegetation to a globally changing environment is central to predictions of future levels of atmospheric carbon dioxide^{1, 2}. The role of tropical forests is critical because they are carbon-dense and highly productive^{3, 4}. Inventory plots across Amazonia show that old-growth forests have increased in carbon storage over recent decades^{5, 6, 7}, but the response of one-third of the world's tropical forests in Africa⁸ is largely unknown owing to an absence of spatially extensive observation networks^{9, 10}. Here we report data from a ten-country network of long-term monitoring plots in African tropical forests. We find that across 79 plots (163 ha) above-ground carbon storage in live trees increased by 0.63 Mg C ha^-1 yr^-1 between 1968 and 2007 (95% confidence interval (CI), 0.22–0.94; mean interval, 1987–96). Extrapolation to unmeasured forest components (live roots, small trees, necromass) and scaling to the continent implies a total increase in carbon storage in African tropical forest trees of 0.34 Pg C yr^-1 (CI, 0.15–0.43). These reported changes in carbon storage are similar to those reported for Amazonian forests per unit area^{6, 7}, providing evidence that increasing carbon storage in old-growth forests is a pan-tropical phenomenon. Indeed, combining all standardized inventory data from this study and from tropical America and Asia^{5, 6, 11} together yields a comparable figure of 0.49 Mg C ha^-1 yr^-1 (n = 156; 562 ha; CI, 0.29–0.66; mean interval, 1987–97). This indicates a carbon sink of 1.3 Pg C yr^-1 (CI, 0.8–1.6) across all tropical forests during recent decades. Taxon-specific analyses of African inventory and other data¹² suggest that widespread changes in resource availability, such as increasing atmospheric carbon dioxide concentrations, may be the cause of the increase in carbon stocks¹³, as some theory¹⁴ and models^{2, 10, 15} predict.

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