The biomass of undisturbed tropical forests has likely increased in the past few decades1,2, probably as a result of accelerated tree growth. Higher CO2 levels are expected to raise plant photosynthetic rates3 and enhance water-use efficiency4, that is, the ratio of carbon assimilation through photosynthesis to water loss through transpiration. However, there is no evidence that these physiological responses do indeed stimulate tree growth in tropical forests. Here we present measurements of stable carbon isotopes and growth rings in the wood of 1,100 trees from Bolivia, Cameroon and Thailand. Measurements of carbon isotope fractions in the wood indicate that intrinsic water-use efficiency in both understorey and canopy trees increased by 30–35% over the past 150 years as atmospheric CO2 concentrations increased. However, we found no evidence for the suggested concurrent acceleration of individual tree growth when analysing the width of growth rings. We conclude that the widespread assumption of a CO2-induced stimulation of tropical tree growth may not be valid.
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We thank the following people for their valuable contribution to this study: A. Nijmeijer, Q. Hakkaart, R. Haasnoot, J. Buijks, A. R. Seas, K. Paredes, A. Mendez, I. Bender, R. Middendorp, T. Voswinkel, R. Peters, E. Louwers, K. Veldhuijzen, W. Berendsen, F. Kelly, W. M. Nanga, D. Zakamdi, M. G. Singha, T. de Vries, A. Uilhoorn, N. van Rabenswaaij, U. Sass-Klaassen, F. Sterck, P. Schippers, S. Kiratiprayoon, A. Murdjoko, the personnel of logging companies Transformation REEF Cameroon and La Chonta Ltd Bolivia, R. Quevedo of the Carrera Forestal UAGRM, M. Tchamba of the Forestry department, University of Dschang and S. Bunyavejchewin of the National Parks Wildlife and Plant Conservation Department, Thailand. We would further like to thank P. J. Baker, C. Körner, W. F. Laurance, M. Scheffer, D. Sheil and S. J. Wright for comments on the manuscript. P.v.d.S., P.G., M.V. and P.Z. were financially supported by the European Research Council (ERC grant no. 242955).
The authors declare no competing financial interests.
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van der Sleen, P., Groenendijk, P., Vlam, M. et al. No growth stimulation of tropical trees by 150 years of CO2 fertilization but water-use efficiency increased. Nature Geosci 8, 24–28 (2015) doi:10.1038/ngeo2313
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