Ideas and perspectives: Allocation of carbon from Net Primary Production in models is inconsistent with observations of the age of respired carbon

Sierra, Carlos A.; Ceballos-Núñez, Verónika; Hartmann, Henrik; Herrera-Ramírez, David; Metzler, Holger

doi:https://doi.org/10.5194/egusphere-2022-34

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https://doi.org/10.5194/egusphere-2022-34

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09 Mar 2022

Status: this preprint is open for discussion.

Ideas and perspectives: Allocation of carbon from Net Primary Production in models is inconsistent with observations of the age of respired carbon

Carlos A. Sierra^1,2, Verónika Ceballos-Núñez³, Henrik Hartmann¹, David Herrera-Ramírez¹, and Holger Metzler² Carlos A. Sierra et al.

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¹Max Planck Institute for Biogeochemistry, 07745 Jena, Germany
²Swedish University of Agricultural Sciences, 75651 Uppsala, Sweden
³Leipzig University, 04103 Leipzig, Germany

Received: 08 Mar 2022 – Accepted for review: 09 Mar 2022 – Discussion started: 09 Mar 2022

Abstract. Carbon allocation in vegetation is an important process in the terrestrial carbon cycle; it determines the fate of photo-assimilates and it has an impact on the time carbon spends in the terrestrial biosphere. Although previous studies have highlighted important conceptual issues in the definition and metrics used to assess carbon allocation, very little emphasis has been placed on the distinction between allocation of carbon from gross primary production (GPP) versus allocation from net primary production (NPP). An important number of simulation models and conceptual frameworks are based on the concept that C is allocated from NPP, which implies that C is respired immediately after photosynthetic assimilation. However, empirical work that estimates the age of respired CO₂ from vegetation tissue (foliage, stems, roots) shows that it may take from years to decades to respire previously produced photosynthates. The transit time distribution of carbon in vegetation and ecosystems, a metric that provides an estimate of the age of respired carbon, indicates that vegetation pools respire carbon of a wide range of ages, on timescales that are in conflict with the assumption that autotrophic respiration only consumes recently fixed carbon. In this contribution, we attempt to provide compelling evidence based on recent research on the age of respired carbon and the theory of timescales of carbon in ecosystems, with the aim to promote a change in the predominant paradigm implemented in ecosystem models where carbon allocation is based on NPP. In addition, we highlight some implications for understanding and modeling carbon dynamics in terrestrial ecosystems.

Carlos A. Sierra et al.

Status: open (until 20 Apr 2022)

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RC1: 'Comment on egusphere-2022-34', Yingping Wang, 26 Mar 2022 reply

Review report “Ideas and perspectives: Allocation of carbon from Net Primary Production in models is inconsistent with observations of the age of respired carbon”

This is a well written piece. The authored argued why carbon allocation should be based on GPP rather than NPP, as often done in most land surface or ecosystem models. These models assume zero residence time of respired carbon through autotrophic respiration, which contradicts the field measurements of the age of respired C from plants. However, there are several significant limitations with the proposed approach: (1) ecosystem GPP cannot be measured directly; where NPP can be easily measured in the field and with a large number of observations available globally; (2) autotrophic respiration of individual biomass components are rarely measured, therefore allocation fractions of GPP to individual biomass components often are not available at ecosystem scale; (3) to model the age of respired C in an ecosystem or land surface model using the proposed approach by the authors here, we will need to represent the total carbon in each biomass component using multiple pools with different availability for respiration and their responses to stresses, which will introduce quite a few additional, poorly constrained model parameters, and additional uncertainties in the model. On the other hand, most of the respired carbon is less than one-year old in leaf, and much younger than the mean age of woody carbon in stem. In my view, the authors should provide a more balanced view of the pros and cons of GPP-based and NPP-based approaches for carbon allocations, and their potential limitations.

Some detailed comments

Figure 1. I disagree with the statement that majority of the models assume “a constant proportion of GPP”. I know that ACCESS-ESM, GFDL, NorESM2, BCC are not. Not sure about other models. Because of the pool-size based approach, there is a negative feedback between respiration and pools, therefore the ratio of autotrophic respiration of GPP is rather constant when averaged over a year globally.

L121. CABLE does not assume a fixed fraction of GPP being respired by plants. See Wang et al. (2012), GEOPHYSICAL RESEARCH LETTERS, VOL. 39, L19405, doi:10.1029/2012GL053461.

P9, the last two lines below Figure 3. “the median transit time is 0 yr, because the autotrophic respiration flux, which corresponds to 50% of GPP..”. That statement may be specific to the model of Emanuel et al. (1981) and that model is more than 40-year old! In most land surface models, Ra/GPP is not constant, particularly at daily or seasonal time scale. The median transit time would have to be weighted by the carbon flux, then the median transit time will not be zero!

Ying-Ping Wang

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Citation: https://doi.org/10.5194/egusphere-2022-34-RC1

Carlos A. Sierra et al.

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Short summary

Empirical work that estimates the age of respired CO₂ from vegetation tissue shows that it may take from years to decades to respire previously produced photosynthates. However, many ecosystem models represent respiration processes in a form that cannot reproduce these observations. In this contribution, we attempt to provide compelling evidence based on recent research, with the aim to promote a change in the predominant paradigm implemented in ecosystem models.


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