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Elevated CO2 does not increase eucalypt forest productivity on a low-phosphorus soil


Rising atmospheric CO2 stimulates photosynthesis and productivity of forests, offsetting CO2 emissions1,2. Elevated CO2 experiments in temperate planted forests yielded 23% increases in productivity3 over the initial years. Whether similar CO2 stimulation occurs in mature evergreen broadleaved forests on low-phosphorus (P) soils is unknown, largely due to lack of experimental evidence4. This knowledge gap creates major uncertainties in future climate projections5,6 as a large part of the tropics is P-limited. Here, we increased atmospheric CO2 concentration in a mature broadleaved evergreen eucalypt forest for three years, in the first large-scale experiment on a P-limited site. We show that tree growth and other aboveground productivity components did not significantly increase in response to elevated CO2 in three years, despite a sustained 19% increase in leaf photosynthesis. Moreover, tree growth in ambient CO2 was strongly P-limited and increased by 35% with added phosphorus. The findings suggest that P availability may potentially constrain CO2-enhanced productivity in P-limited forests; hence, future atmospheric CO2 trajectories may be higher than predicted by some models. As a result, coupled climate–carbon models should incorporate both nitrogen and phosphorus limitations to vegetation productivity7 in estimating future carbon sinks.

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Figure 1: Pattern of leaf net photosynthesis in the canopy over the first three years of elevated CO2.
Figure 2: Aboveground net primary production (ANPP) in a mature Eucalyptus stand and its components across three years of elevated CO2.
Figure 3: Biomass increment of five different size classes of Eucalyptus trees.

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EucFACE was built as an initiative of the Australian Government as part of the Nation-building Economic Stimulus Package, and is supported by the Australian Commonwealth in collaboration with Western Sydney University. A portion of this work was supported by grants from the Australian Research Council (ARC) Discovery grants scheme, particularly DP110105102 and DP160102452. K.Y.C. acknowledges ARC support (DECRA program), and T.E.G. acknowledges the CSIRO and a Marie S. Curie IEF Fellowship. We thank the team of people who have assisted with the canopy sampling, and S. Wohl, C. Barton, V. Kumar, C. McNamara and C. Beattie, who provided technical assistance. We thank C. P. Osborne for comments on an early version of the manuscript. The data are available as a package from Research Data Australia.

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Authors and Affiliations



D.S.E., I.C.A. and B.E.M. designed the eCO2 experiment. D.S.E., K.Y.C. and T.E.G., designed the photosynthesis measurements and carried out and analysed them with J.C. and J.E.D.; K.Y.C., J.C., J.R.P., D.S.E. and A.N.G. did the litterfall collections and measurements. D.S.E., P.B.R., J.R.P., K.Y.C., M.G.T. and B.E.M. did the analyses and statistical tests. D.S.E. and P.B.R. wrote the draft of the paper. All authors contributed to subsequent versions.

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Correspondence to David S. Ellsworth.

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The authors declare no competing financial interests.

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Ellsworth, D., Anderson, I., Crous, K. et al. Elevated CO2 does not increase eucalypt forest productivity on a low-phosphorus soil. Nature Clim Change 7, 279–282 (2017).

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