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Climate of the Past An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/cp-2020-30
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/cp-2020-30
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 10 Mar 2020

Submitted as: research article | 10 Mar 2020

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This preprint is currently under review for the journal CP.

Elevated CO2, increased leaf-level productivity and water-use efficiency during the early Miocene

Tammo Reichgelt1,2, William J. D'Andrea1, Ailín del C. Valdivia-McCarthy1, Bethany R.S. Fox3, Jennifer M. Bannister4, John G. Conran5, William G. Lee6,7, and Daphne E. Lee8 Tammo Reichgelt et al.
  • 1Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York, USA
  • 2Department of Geosciences, University of Connecticut, Storrs, Connecticut, USA
  • 3Department of Biological and Geographical Sciences, University of Huddersfield, Huddersfield, UK
  • 4Department of Botany, University of Otago, Dunedin, New Zealand
  • 5ACEBB & SGC, School of Biological Sciences, The University of Adelaide, Adelaide, Australia
  • 6Landcare Research, Dunedin, New Zealand
  • 7School of Biological Sciences, University of Auckland, Auckland, New Zealand
  • 8Department of Geology, University of Otago, Dunedin, New Zealand

Abstract. Rising atmospheric CO2 is expected to increase global temperatures, plant water-use efficiency, and carbon storage in the terrestrial biosphere. A CO2 fertilization effect on terrestrial vegetation is predicted to cause global greening as the potential ecospace for forests expands. However, leaf-level fertilization effects, such as increased productivity and water-use efficiency, have not been documented from fossil leaves in periods of heightened atmospheric CO2. Leaf gas-exchange rates reconstructed from early Miocene fossils which grew at southern temperate and tropical latitudes, when global average temperatures were 5–6 °C higher than today reveal that atmospheric CO2 was ~ 450–550 ppm. Early Miocene CO2 is similar to projected values for 2040AD, and consistent with Earth System Sensitivity of 3–7 °C to a doubling of CO2. While early Miocene leaves had photosynthetic rates similar to modern plants, southern temperate leaves were more productive than modern due to a longer growing season. This higher productivity was likely mirrored at northern temperate latitudes as well, where a greater availability of landmass would have led to increased carbon storage in forest biomass relative to today. Intrinsic water-use efficiency of both temperate and tropical forest trees was high, toward the upper limit of the range for modern trees, which likely expanded the habitable range in regions that could not support forests with high moisture demands under lower atmospheric CO2. Overall, early Miocene elevated atmospheric CO2 sustained globally higher temperatures and our results reveal the first empirical evidence of concomitant enhanced intrinsic water-use efficiency, indicating a forest fertilization effect.

Tammo Reichgelt et al.

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Short summary
Carbon dioxide (CO2) levels are increasing in the atmosphere. CO2 has a direct fertilization effect on plants, meaning that plants can photosynthesize more and create more biomass under higher atmospheric CO2. This paper outlines the first direct evidence of a carbon fertilization effect on plants in Earth's past from 23 million year old fossil leaves, when CO2 was higher. This allowed the biosphere to extend into areas that are currently too dry or too cold for forests.
Carbon dioxide (CO2) levels are increasing in the atmosphere. CO2 has a direct fertilization...
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