07 Feb 2022
07 Feb 2022
Status: a revised version of this preprint is currently under review for the journal BG.

Consistent responses of vegetation gas exchange to elevated atmospheric CO2 emerge from heuristic and optimization models

Stefano Manzoni1,2, Simone Fatichi3, Xue Feng4,5, Gabriel G. Katul6,7, Danielle Way6,8,9, and Giulia Vico10 Stefano Manzoni et al.
  • 1Department of Physical Geography, Stockholm University, Stockholm, SE-106 91, Sweden
  • 2Bolin Centre for Climate Research, Stockholm University, Stockholm, SE-106 91, Sweden
  • 3Department of Civil and Environmental Engineering, National University of Singapore, Singapore
  • 4Department of Civil, Environmental, and Geo-Engineering, University of Minnesota, Minneapolis, MN 55455, USA
  • 5Saint Anthony Fall Laboratory, University of Minnesota, Minneapolis, MN 55455, USA
  • 6Department of Civil and Environmental Engineering, Duke University, Durham, NC, 27708-0287, USA
  • 7Nicholas School of the Environment, Duke University, Durham, NC, 27708 USA
  • 8Department of Biology, University of Western Ontario, London, Ontario, N6A 5B7, Canada
  • 9Environmental & Climate Sciences Department, Brookhaven National Laboratory, Upton, NY, 11973 USA
  • 10Department of Crop Production Ecology, Swedish University of Agricultural Sciences (SLU), Uppsala, SE-750 07, Sweden

Abstract. Elevated atmospheric CO2 concentration is expected to increase leaf CO2 assimilation rates, thus promoting plant growth and increasing leaf area. It also decreases stomatal conductance, allowing water savings that have been hypothesized to drive large-scale greening, in particular in arid and semiarid climates. However, the increase in leaf area could reduce the ameliorating effect of elevated CO2 concentration on soil water depletion. The net effect of elevated CO2 on leaf- and canopy-level gas exchange thus remains unclear. To address this question, a heuristic model based on the Partitioning of Equilibrium Transpiration and Assimilation (PETA) hypothesis and a model based on stomatal optimization theory are used and their outcomes compared. Predicted relative changes in leaf- and canopy-level gas exchange rates are used as a metric of responses to changes in atmospheric CO2 concentration. Both models predict reductions of leaf-level transpiration rate due to decreased stomatal conductance under elevated CO2, but negligible (PETA) or no (optimization) changes in canopy-level transpiration due to the compensatory effect of increased leaf area. Leaf- and canopy-level CO2 assimilation are predicted to increase, with an amplification of the CO2 fertilization effect at the canopy-level due to the enhanced leaf area. The expected increase in vapor pressure deficit (VPD) under warmer conditions is predicted to decrease the sensitivity of gas exchange to atmospheric CO2 concentration in both models except at growth temperatures lower than the photosynthetic thermal optimum. The consistent predictions by different models that canopy-level transpiration varies little under elevated CO2 due to combined stomatal conductance reduction and leaf area increase highlights the coordination of physiological and morphological characteristics in vegetation to maximize resource use (here water) under altered atmospheric conditions.

Stefano Manzoni et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2022-36', Anonymous Referee #1, 16 Feb 2022
    • AC1: 'Reply on RC1', Stefano Manzoni, 25 Feb 2022
  • RC2: 'Comment on bg-2022-36', Benjamin Stocker, 03 Mar 2022

Stefano Manzoni et al.

Stefano Manzoni et al.


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Short summary
Increasing atmospheric carbon dioxide (CO2) causes leaves to close their stomata (through which water evaporates), but also promote leaf growth. Even if individual leaves save water, how much will be consumed by a whole plant with possibly more leaves? Using two different mathematical models, we show that plant stands that are not very dense and can grow more leaves will benefit from higher CO2 by photosynthesizing more, while adjusting their stomata to consume similar amounts of water.