Seven Years on the Ginkgo Farm: Implications of an Elevated CO2 Growth Experiment for Reconstructing paleo-pCO2 from a "Living Fossil"

Ginkgo is a morphologically conservative plant lineage with abundant Mesozoic and Cenozoic fossil leaves that frequently preserve cuticle recording epidermal cell patterns including stomatal features. These attributes have made Ginkgo popular for reconstructing paleo-pCO2, but many potential proxies remain poorly calibrated because of few long-term growth studies of living Ginkgo biloba under elevated pCO2. Here we report effects of growing Ginkgo biloba for up to seven years under CO2 concentrations ranging from 425-1000 ppm. Primary results are:

no change in discrimination against 13C (D13Cleaf) with increasing pCO2, contrary to the expectations of the "C3 plant proxy"1, 2, which reconstructs paleo-pCO2 from the difference between the inferred d13C of the paleo-atmosphere and the d13C of fossil organic matter;

no significant change in stomatal index (SI) or stomatal density (SD) in plants grown at pCO2 >450 ppm, contrary to the expectations of the SI and SD proxies, which rely on such a relationship to infer paleo-pCO2 levels3, 4;

significant increases in leaf mass per area, C:N ratio, and maximum photosynthetic assimilation rate under elevated pCO2.

We also applied a gas exchange model5 to estimate pCO2 of ginkgos grown under known pCO2 and found the estimates were highly variable, consistently below actual pCO2, and that the magnitude of the underestimate increased at higher actual pCO2 levels. This is not unexpected because two major inputs to the gas exchange model, SD and D13Cleaf, did not vary with pCO2.

We conclude that increasing pCO2 does affect the growth and allocation of resources to different tissues in Ginkgo biloba, but find little or no support for the idea that the stomatal features or carbon isotope composition of Ginkgo fossils are reliable proxies for the highly elevated pCO2 levels thought to have existed during warm intervals of earth history. More generally, we urge thorough testing of pCO2 proxy systems in experiments where actual pCO2 is known.

1Schubert, B.A. & Jahren, A.H., 2012. Geochim. Cosmochim. Acta 96: 29-43; 2Schubert, B.A. & Jahren, A.H., 2018. Earth-Science Rev. 177: 637-642; 3Royer, D.L., et al., 2001. Science 292: 2310-2313; 4Barclay, R.S. & Wing, S.L., 2016. Earth Planet. Sci. Let. 439: 158-171; 5Franks, P.J., et al., 2014. Geophys. Res. Let., 41:4685-4694.