Ecosystem fluxes of carbonyl sulfide in an old-growth forest: temporal dynamics and responses to diffuse radiation and heat waves

Rastogi, Bharat; Berkelhammer, Max; Wharton, Sonia; Whelan, Mary E.; Meinzer, Frederick C.; Noone, David; Still, Christopher J.

doi:https://doi.org/10.5194/bg-15-7127-2018

Articles | Volume 15, issue 23

https://doi.org/10.5194/bg-15-7127-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/bg-15-7127-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 15, issue 23

Research article

|

30 Nov 2018

Research article |

| 30 Nov 2018

Ecosystem fluxes of carbonyl sulfide in an old-growth forest: temporal dynamics and responses to diffuse radiation and heat waves

Bharat Rastogi, Max Berkelhammer, Sonia Wharton, Mary E. Whelan, Frederick C. Meinzer, David Noone, and Christopher J. Still

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Final revised paper (published on 30 Nov 2018)
Preprint (discussion started on 09 Mar 2018)

Interactive discussion

Status: closed

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

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RC1: 'Good manuscript if methods are correct', Anonymous Referee #1, 02 May 2018
- AC1: 'Response to reviewer', Bharat Rastogi, 02 Aug 2018
- AC3: 'Manuscript containing relevant changes', Bharat Rastogi, 02 Aug 2018
RC2: 'Rastogi et al.'s valuable observations warrant a more careful analysis', Anonymous Referee #2, 05 May 2018
- AC2: 'Response to reviewer', Bharat Rastogi, 02 Aug 2018
- AC4: 'Manuscript containing relevant changes', Bharat Rastogi, 02 Aug 2018

Peer-review completion

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

ED: Reconsider after major revisions (17 Aug 2018) by David Bowling

AR by Bharat Rastogi on behalf of the Authors (14 Sep 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (01 Oct 2018) by David Bowling

RR by Anonymous Referee #2 (16 Oct 2018)

Suggestions for revision or reasons for rejection

GENERAL COMMENTS

The authors have done a nice job improving this manuscript, which is more clear and reads well. I have just a few concerns remaining, noted below. With these addressed, I think the paper will make an important contribution to the literature.

SPECIFIC COMMENTS

(1) Regarding the OCS flux estimation (Section 2.8)…

First, in the context of flux-gradient theory, g is the conventional symbol for conductance and I think it should not be used for “gradient”. Second, I would spell out the theoretical idea a little more accurately in Section 2.8, as it differs significantly from the Commane et al 2015 paper that is cited as its basis. In Commane et al 2015, the flux-gradient approach was applied to turbulent flux; the assumption was that the turbulent conductance (g = F/gradient) for all gases was the same (no normalization for relative diffusivities necessary because turbulent flux is not diffusive), so that the gradients of the two gases (along a wholly above-canopy turbulent path that passed no sources or sinks) and the flux of one could be used to calculate the flux of the other. Here, instead, your gradient is between the inside of the leaf and the canopy-top atmosphere, which is a path that is mostly diffusive (stomata and, partially, the leaf boundary layer) but partly turbulent (the within-canopy airspace and, partially, the leaf boundary layer), and which may include unaccounted-for sources/sinks in the form of storage flux (i.e. changes in the canopy airspace concentrations) and lateral advection. By normalizing by the diffusivity ratio (i.e. treating the whole path as diffusive) and by neglecting storage and advection, you are effectively taking your canopy-top measurements to represent leaf surface measurements and your eddy flux to represent flux through the stomata. That will definitely invoke some error (possibly bias) and may or may not be a sufficient approximation, so I think the paper should discuss these assumptions and approximations that you are making, how they differ from the cited works, and the resulting uncertainty in the results (ideally quantitatively).

In your response to the first-round review, you argued: “In tall canopies such as our site, the portion of canopy that is coupled to the overlying atmosphere changes considerably during the day, and parts of the lower canopy are likely to be always decoupled from the upper canopy as well as above canopy air (Pyles et al., 2004). This has obvious consequences on canopy storage and venting of gases such as CO2 and OCS.” I agree, and this is exactly why taking canopy-top concentrations and fluxes to represent leaf-surface concentrations and fluxes will invoke error. It is also exactly why storage should be accounted for (not neglected, as you argued). If a significant portion of the flux through the stomata is going into (or coming from) changes in canopy concentration, then your flux gradient equations will become significantly inaccurate.

(2) Regarding the response to heat waves (Section 3.5 and lines 462-464)…

This draft is better in that unsupported claims have been removed; however, the trade-off is that there doesn’t seem to be a proper conclusion from the heat wave analysis. The only one offered is in the Conclusion section, which says that "sequential heatwaves lead to suppression in stomatal gas exchange of all three fluxes” — but that is not quite true, as F_H2O is enhanced during the heatwaves. Moreover, the reduction of F_OCS is not so clear for the first two heat waves in Fig. 7, probably just due to the noise. Perhaps a regression plot or even just a simple statistic on heat-wave vs not-heat-wave F_OCS would help support the point that F_OCS, in keeping with GPP and Gc, is suppressed during heat waves.

(3) Regarding the influence of soil moisture…

I still do not see how the data support the inference that soil moisture (rather than a combination of VPD, temperature, and light) is driving seasonal changes in gas exchange, and although this draft backs away from strong statements about the soil moisture influence, it still suggests that soil moisture is a key driver in some places (e.g. abstract lines 28-29: “OCS fluxes tracked changes in soil moisture”, and lines 411-413: “declining soil moisture likely limits gas exchange as the summer progresses, even as canopy conductance can be reasonably high under overcast skies”). Where soil moisture influence is suggested (and it is certainly a possibility that should be mentioned), it should be on par with other plausible explanations. It is probably also a good idea in general to note somewhere in the manuscript that it is not possible using the present data to tell the soil moisture influence apart from other factors. You attempt to disentangle the drivers in the discussion surrounding Fig 6, but I don’t follow the logic. I don’t see the Gc response being any more similar across time periods than the OCS or CO2 responses are, despite the text’s claim. Nor do I see why such an observation, if it were true, would implicate soil moisture as opposed to temperature or seasonal light levels.

TECHNICAL CORRECTIONS

- line 235: Eq. 6 seems inverted. The correct equation is flux = conductance x gradient => conductance = flux/gradient
- lines 278-280: Fig 2 does not show the nighttime OCS flux.
- Fig 2d: color scale should have a label (PAR).
- line 334: do you mean rainy days?
- line 376: “diffused: total” should be “diffuse:total” (no d and no space)

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ED: Publish subject to minor revisions (review by editor) (18 Oct 2018) by David Bowling

AR by Bharat Rastogi on behalf of the Authors (31 Oct 2018) Author's response Manuscript

ED: Publish as is (02 Nov 2018) by David Bowling

Short summary

Carbonyl sulfide (OCS) has gained prominence as an independent tracer for gross primary productivity, which is usually modelled by partitioning net CO₂ fluxes. Here, we present a simple empirical model for estimating ecosystem-scale OCS fluxes for a temperate old-growth forest and find that OCS sink strength scales with independently estimated CO₂ uptake and is sensitive to the the fraction of downwelling diffuse light. We also examine the response of OCS and CO₂ fluxes to sequential heat waves.