EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Global trends of tree-ring carbon isotope discrimination under rising atmospheric CO2 and changing climate

Soumaya Belmecheri1, Paul szejner2, David Frank1, Steve Voelker3, and Alienor Lavergne4
Soumaya Belmecheri et al.
  • 1Laboratory of Tree Ring research, University of Arizona, Tucson-AZ, United State of America, (
  • 2School of Natural Resources and the Environment, University of ArizonaTucson-Arizona, United States of America (
  • 3Department of Environmental Biology, Sunny-ESF, Syracuse-NY, United States of America (
  • 4Faculty of Natural Sciences, Imperial College of London, Ascot, United Kingdom (

Rising atmospheric CO2 concentrations are expected to stimulate plant carbon uptake (A) while also reducing transpiration via a decrease in stomatal conductance (gs), resulting in an increase in the intrinsic water use efficiency (iWUE, i.e. the ratio of A to gs). While there is overwhelming evidence of a secular iWUE increase in response to rising CO2 over the 20th-21st century, the magnitude of changes in iWUE reported so far in the literature strongly varies across climatic regions and biomes. Moreover, increasing iWUE has not systematically been translated into tree growth increment at many forested ecosystems, challenging the CO2 fertilization theory. There is thus a need to track down the key physiological and environmental mechanisms driving changes in iWUE.

Here we estimate the carbon isotopic discrimination (Δ13C) - defined as the difference between the stable carbon isotopic compositions (δ13C) measured in atmospheric CO2 and in tree rings – from 147 tree-ring δ13C chronologies to: 1) investigate the physiological responses of woody C3 plants to increasing atmospheric CO2 and, 2) disentangle climate vs CO2 effects on A and gs. We specifically study deviations of tree-ring Δ13C from the predicted Δ13C response to CO2 as reconstructed from a recent meta-analysis of paleo and elevated CO2 data. We identify the following: 1) negative deviations from the expected Δ13C for most records; 2) no apparent deviation from expected Δ13C or; 3) positive deviations, both in a small minority of records ; and 4) an apparent non-linear response with a switch from a more negative Δ13C deviations to a Δ13C-response consistent with predicted CO2-effects. The widespread negative Δ13C  deviations are consistent with gs having been reduced or A having not increased as much as expected for a given CO2-driven stimulation of A. The presented global tree-ring data analyses suggest that a warmer and often drier climate have had a stronger effect on Δ13C compared to that of rising CO2, and a substantial modulation of recent rises in iWUE by climate effects across the globe.

How to cite: Belmecheri, S., szejner, P., Frank, D., Voelker, S., and Lavergne, A.: Global trends of tree-ring carbon isotope discrimination under rising atmospheric CO2 and changing climate, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12240,, 2020


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