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Is photosynthetic enhancement sustained through three years of elevated CO2 exposure in 175-year old Quercus robur?

View ORCID ProfileA Gardner, View ORCID ProfileDS Ellsworth, View ORCID ProfileKY Crous, J Pritchard, View ORCID ProfileAR MacKenzie
doi: https://doi.org/10.1101/2020.12.16.416255
A Gardner
1Birmingham Institute of Forest Research, University of Birmingham, Edgbaston, B15 2TT, England
2School of Biological Sciences, University of Birmingham, Edgbaston, B15 2TT, England
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DS Ellsworth
3Hawkesbury Institute for the Environment, Western Sydney University, Locked Bag 1797, Penrith NSW 2751, Australia
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KY Crous
3Hawkesbury Institute for the Environment, Western Sydney University, Locked Bag 1797, Penrith NSW 2751, Australia
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J Pritchard
1Birmingham Institute of Forest Research, University of Birmingham, Edgbaston, B15 2TT, England
2School of Biological Sciences, University of Birmingham, Edgbaston, B15 2TT, England
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AR MacKenzie
1Birmingham Institute of Forest Research, University of Birmingham, Edgbaston, B15 2TT, England
4School of Geography, Earth & Environmental Sciences, University of Birmingham, Edgbaston, B15 2TT, England
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  • For correspondence: a.r.mackenzie@bham.ac.uk
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Abstract

Current carbon cycle models attribute rising atmospheric CO2 as the major driver of the increased terrestrial carbon sink, but with substantial uncertainties. The photosynthetic response of trees to elevated atmospheric CO2 is a necessary step, but not the only one, for sustaining the terrestrial carbon uptake, but can vary diurnally, seasonally and with duration of CO2 exposure. Hence we sought to quantify the photosynthetic response of the canopy-dominant species, Quercus robur, in a mature deciduous forest to elevated CO2 (eCO2) (+150 μmol mol-1 CO2) over the first three years of a long-term free air CO2 enrichment facility at the Birmingham Institute of Forest Research in central England (BIFoR FACE). Over three thousand measurements of leaf gas exchange and related biochemical parameters were conducted in the upper canopy to assess the diurnal and seasonal responses of photosynthesis during the 2nd and 3rd year of eCO2 exposure. Measurements of photosynthetic capacity via biochemical parameters, derived from CO2 response curves, (Vcmax and Jmax) together with leaf nitrogen concentrations from the pre-treatment year to the 3 rd year of eCO2 exposure, were examined. We hypothesized an initial enhancement in light-saturated net photosynthetic rates (Asat) with CO2 enrichment of ≈37% based on theory but also expected photosynthetic capacity would fall over the duration of the study. Over the three-year period, Asat of upper-canopy leaves was 33 ± 8 % higher (mean and standard error) in trees grown in eCO2 compared with ambient CO2 (aCO2), and photosynthetic enhancement decreased with decreasing light. There were no significant effects of CO2 treatment on Vcmax or Jmax, nor leaf nitrogen. Our results suggest that mature Q. robur may exhibit a sustained, positive response to eCO2 without photosynthetic downregulation, suggesting that, with adequate nutrients, there will be sustained enhancement in C assimilated by these mature trees. Further research will be required to understand the location and role of the additionally assimilated carbon.

Competing Interest Statement

The authors have declared no competing interest.

  • Symbols and Abbreviations

    [CO2] CO2
    concentration of the atmosphere
    A
    photosynthesis
    A-Ci
    curve Photosynthetic CO2 response curve
    aCO2
    CO2 at ambient Ca (~405 ppm)
    Anet
    photosynthetic rates.
    Asat
    Light-saturated net photosynthesis
    C
    Carbon
    CAS
    Canopy access system
    Ci
    CO2 concentration of the intercellular leaf space
    eCO2
    CO2 at elevated Ca (+150 ppm ambient)
    FACE
    Free air carbon dioxide enrichment
    Jmax
    Maximal photosynthetic electron transport rate (a proxy for ribulose-1,5-bisphosphate regeneration)
    N
    Nitrogen
    Na
    Area-based foliar Nitrogen
    Nm
    Mass-based foliar Nitrogen
    Q
    photon flux density
    RH
    relative humidity
    T
    temperature
    Tair
    Air temperature
    Tleaf
    Leaf temperature
    SE
    Standard error of the mean
    Vcmax
    Maximal carboxylation rate of Rubisco
    D
    vapour pressure deficit of the atmosphere
    δ13C
    ratio of 13C to 12C stable carbon isotopes
    δ N ratio
    of 15N to 14N stable carbon isotopes
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    Posted July 09, 2021.
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    Is photosynthetic enhancement sustained through three years of elevated CO2 exposure in 175-year old Quercus robur?
    A Gardner, DS Ellsworth, KY Crous, J Pritchard, AR MacKenzie
    bioRxiv 2020.12.16.416255; doi: https://doi.org/10.1101/2020.12.16.416255
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    Is photosynthetic enhancement sustained through three years of elevated CO2 exposure in 175-year old Quercus robur?
    A Gardner, DS Ellsworth, KY Crous, J Pritchard, AR MacKenzie
    bioRxiv 2020.12.16.416255; doi: https://doi.org/10.1101/2020.12.16.416255

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