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Diminishing CO2-driven gains in water-use efficiency of global forests

Abstract

There is broad consensus that, via changes in stomatal conductance, plants moderate the exchanges of water and carbon between the biosphere and atmosphere, playing a major role in global hydroclimate. Tree rings record atmospheric CO2 concentration (ca) and its isotopic composition (13C/12C)—mediated by stomatal and photosynthetic influences—that can be expressed in terms of intrinsic water-use efficiency (W). Here, we compile a global W dataset based on 422 tree-ring isotope series and report that W increased with ca over the twentieth century, but the rates of increase (dW/dca) declined by half. Angiosperms contributed more than gymnosperms to the slowdown, and in recent decades, dW/dca for angiosperms was close to zero. dW/dca varies widely across climatic regions and reflects pauses in emissions during the Great Depression and after World War II. There is strong spatial variability in climate forcing via an increasing W, which is weakening globally with time.

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Fig. 1: Geographical locations of 422 tree-ring isotope series that contain data from the period 1851–2015.
Fig. 2: dW/dca based on 422 tree-ring isotope series from the period 1851–2015.
Fig. 3: Changes in sensitivity of W to changes in ca over time.
Fig. 4: Patterns in dW/dca.

Data availability

The data for W can be accessed via the Zenodo repository (https://doi.org/10.5281/zenodo.3693240). Source data for Fig. 1 are provided with the paper.

Code availability

The code used in processing data can be accessed via the Zenodo repository (https://doi.org/10.5281/zenodo.3693240).

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Acknowledgements

We thank the authors who provided additional information that allowed us to include their data in this work. We thank A. Barlow for help with digitizing the isotope series and M. Gharun for help with climate data. M.A.A. and T.L.T. thank the Australian Research Council for general support. M.A.A. thanks an anonymous donor for funding of this study. T.N.B. acknowledges support from the National Science Foundation (grant no. 1557906) and the USDA National Institute of Food and Agriculture (Hatch project 1016439).

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Authors and Affiliations

Authors

Contributions

M.A.A., T.L.T. and T.N.B. developed the original ideas included in this paper. T.L.T. screened the literature, led the extraction of data and characterized the dataset. T.N.B. interrogated the dataset for analysis and prepared the modelling. T.L.T. analysed the data and prepared the figures and tables. M.A.A. wrote the paper with T.L.T. and T.N.B.

Corresponding author

Correspondence to Mark A. Adams.

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Competing interests

The authors declare no competing interests.

Additional information

Peer review information Nature Climate Change thanks Katrin Fleischer and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data

Extended Data Fig. 1 Relationship of W to sample year.

For all 422 tree-ring isotope series containing data from the period 1851–2015, W was calculated using Equation 3.

Extended Data Fig. 2 Effects of methods of calculation of W.

a, Relationship between dW/dca calculated using Equation 2 with that calculated using Equation 3 (based on 13C/12C ratios of wood tissue for all series shown in Extended Data Fig. 1. b, Example of the difference in calculated W if either Equations 2 or 3 are applied to isotope data as extracted from Loader et al.42.

Extended Data Fig. 3 Descriptive statistics for the global dataset of rates of change in W with ca (dW/dca).

Data are as shown for Extended Data Fig. 1. a, Frequency of dW/dca. b, Frequency of R2 values for relationships between W and ca. (c) Frequency of P-values for relationships between W and ca.

Extended Data Fig. 4 Effects of climatic variables on rates of change in W with ca (dW/dca).

Linear regressions of dW/dca and climatic variables: mean annual precipitation, MAP; potential evapotranspiration, PET; mean annual temperature, MAT; vapour pressure; and radiation. (ae) All data as shown in Extended Data Fig. 1. (fo) Angiosperms (n = 147) and gymnosperms (n = 275) considered separately.

Supplementary information

Supplementary Information

Supplementary Tables 1 and 2 and references (data sources).

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Adams, M.A., Buckley, T.N. & Turnbull, T.L. Diminishing CO2-driven gains in water-use efficiency of global forests. Nat. Clim. Chang. 10, 466–471 (2020). https://doi.org/10.1038/s41558-020-0747-7

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