Quantifying energy use efficiency via entropy production: a case study from longleaf pine ecosystems

Wiesner, Susanne; Staudhammer, Christina L.; Stoy, Paul C.; Boring, Lindsay R.; Starr, Gregory

doi:https://doi.org/10.5194/bg-16-1845-2019

Articles | Volume 16, issue 8

https://doi.org/10.5194/bg-16-1845-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/bg-16-1845-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 16, issue 8

Research article

|

30 Apr 2019

Research article |

| 30 Apr 2019

Quantifying energy use efficiency via entropy production: a case study from longleaf pine ecosystems

Susanne Wiesner, Christina L. Staudhammer, Paul C. Stoy, Lindsay R. Boring, and Gregory Starr

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Final revised paper (published on 30 Apr 2019)
Supplement to the final revised paper
Preprint (discussion started on 08 Aug 2018)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

RC1: 'Review', Axel Kleidon, 15 Sep 2018
- AC1: 'Final comments to reviewers', Gregory Starr, 14 Oct 2018
RC2: 'Review of Wiesner et al., 2018', Anonymous Referee #2, 28 Sep 2018
- AC2: 'Final comments to reviewers', Gregory Starr, 14 Oct 2018

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (03 Nov 2018) by Christopher Still

AR by Gregory Starr on behalf of the Authors (15 Dec 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (19 Dec 2018) by Christopher Still

RR by Axel Kleidon (07 Mar 2019)

Suggestions for revision or reasons for rejection

After reviewing the manuscript in the second round, I would like to first acknowledge the extensive effort that the authors put into rewriting the manuscript. Overall it improved. However, I still have substantial objections that prevent me from recommending to accept this manuscript.

My main point of criticism is that the thermodynamic formulation used to evaluate the observations is still seriously flawed. In particular, I found the following three flaws:

1. The entropy production by solar radiation is calculated by Eq. 4.6, and I agree with this formulation. Solar radiation carries the imprints of the solar emission temperature, and is absorbed and produces heat that is associated with the surface temperature, so the expression for entropy production includes the temperatures as it is commonly done. However, later on page 7 the authors use a metric called “Empirical MEP”, where instead of the temperature at which solar radiation is being absorbed they use air temperature. I see no physical justification for using air temperature. Solar radiation is absorbed at the surface, not in the air.  

2. The radiation efficiency they define in eq. 5.4 appears arbitrary to me. In thermodynamics, efficiencies relate to how much meaningful work can be derived out a flux, e.g., for a power plant, how much electricity can be generated out of a heat flux. With entropy fluxes, this actually loses the point. I want to illustrate this with two cases. Imagine case A of a power plant that derives no work out of a flux. Its entropy production by some irreversible processes within the plant is given by the flux and the difference between the inverses of the temperatures at which heat comes in with combustion and goes out of the cooling tower. Case 2 is a power plant that derives the maximum of work out of the flux, which is eventually dissipated into heat. As it turns out, both power plants will actually produce entropy at exactly the same rate, and the only difference is that in case B work is performed, while in case A no work is performed. For such a situation, efficiency has a real meaning, as it allows us to distinguish whether energy is being used to perform work or whether it is wasted. The current paper does not do this kind of analysis, but only focuses on efficiencies based on entropy fluxes. I see no justification for doing so. I do not see a motivation why this should tell us something about why an ecosystem is more or less efficient. 

3. I still disagree with using NEE to infer entropy fluxes. Photosynthesis creates chemical free energy out of sunlight. Hence, the absorption of solar radiation does not quite produce as much entropy as a green plate that has the same albedo as a leaf, because some of the radiation is not turned into heat, but rather ends up in the chemical free energy associated with carbohydrates and oxygen. Only when these carbohydrates are being respired is heat being generated and entropy being produced. When you use the combined flux NEE, you would diagnose negative entropy fluxes during the day, and positive during the night. But no process produces negative entropy. So these fluxes need to be separated to make sense, and the entropy production by solar radiation would need to be adjusted to account for the fraction of radiation that is not converted into heat.  

There is still a sentence on page 8, L1 that claims that “under ideal conditions, an ecosystem maximizes its entropy production when it converts all incoming Rs and Rl into work”. No, all incoming radiation cannot be converted into work as it would violate the second law of thermodynamics. I think I already explained this in my last review.

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ED: Publish subject to minor revisions (review by editor) (18 Mar 2019) by Christopher Still

AR by Gregory Starr on behalf of the Authors (01 Apr 2019) Author's response Manuscript

ED: Publish as is (15 Apr 2019) by Christopher Still

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Short summary

We studied entropy production in longleaf savanna sites with variations in land use legacy, plant diversity, and soil water availability which experienced drought. Sites with greater land use legacy had lower metabolic energy use efficiency, which delayed recovery from drought. Sites with more hardwood captured less solar radiation but more efficiently used absorbed energy. Future management applications could use these methods to quantify energy use efficiency across global ecosystems.