Response of CO<sub>2</sub> and H<sub>2</sub>O fluxes in a mountainous tropical rainforest in equatorial Indonesia to El Niño events

Olchev, A.; Ibrom, A.; Panferov, O.; Gushchina, D.; Kreilein, H.; Popov, V.; Propastin, P.; June, T.; Rauf, A.; Gravenhorst, G.; Knohl, A.

doi:https://doi.org/10.5194/bg-12-6655-2015

Articles | Volume 12, issue 22

https://doi.org/10.5194/bg-12-6655-2015

© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/bg-12-6655-2015

© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 12, issue 22

Research article

|

24 Nov 2015

Research article |

| 24 Nov 2015

Response of CO₂ and H₂O fluxes in a mountainous tropical rainforest in equatorial Indonesia to El Niño events

A. Olchev, A. Ibrom, O. Panferov, D. Gushchina, H. Kreilein, V. Popov, P. Propastin, T. June, A. Rauf, G. Gravenhorst, and A. Knohl

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Final revised paper (published on 24 Nov 2015)
Preprint (discussion started on 16 Mar 2015)

Interactive discussion

Status: closed

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

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- Supplement

RC C1168: 'Referee comment', Anonymous Referee #1, 13 Apr 2015
- AC C1761: 'Response to anonymous reviewer (REV1)', Alexander Olchev, 27 Apr 2015
RC C1476: 'Response of CO2 and H2O fluxes of a mountainous tropical rain forest in equatorial Indonesia to El Nino events', Anonymous Referee #2, 20 Apr 2015
- AC C2067: 'Response to anonymous reviewer 2', Alexander Olchev, 11 May 2015
EC C2349: 'Comments on bg-2015-52', Paul Stoy, 21 May 2015

Peer-review completion

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

AR by Alexander Olchev on behalf of the Authors (19 Jun 2015) Author's response Manuscript

ED: Referee Nomination & Report Request started (07 Jul 2015) by Paul Stoy

RR by Anonymous Referee #1 (12 Jul 2015)

RR by Anonymous Referee #2 (10 Aug 2015)

Suggestions for revision or reasons for rejection

The authors responded in detail to most of the reviewers’ criticisms and improved their manuscript substantially. Their answers to the questions about flux corrections and gap filling (incl. wet periods) are sufficient and show that I did indeed misunderstand some of the original descriptions of the site characteristics and the data analysis. These are much clearer now. The research site may indeed be not that different from typical “Fluxnet” site conditions, although this is only a weak argument since the criteria to enter “Fluxnet” are not very strict.

The new Fig.6 (showing data from turbulent midday periods only) is particularly helpful and convincing with respect to the validity of the statements about deviations from mean fluxes in relation to ENSO. The changes that have been made by the authors so far can bring the ms closer towards a defensible version, provided the earlier criticism about the general validity of the data (in terms of the magnitude of the annual NEE means) will be taken seriously and answered appropriately in a new revised version.

The three following major points need yet to be tackled:

1) Although I acknowledge the fact that the authors now explicitly limit their conclusions to monthly anomalies, their answer to my question about the absolute magnitude of NEE (as shown in Fig. 2) is insufficient. In their response letter, the authors compare GPP with that of other tropical forests (with which it agrees) and argue that RE must be lower than in those other forests due to the lower temperatures prevailing in the mountains and, as a consequence, NEE must be very large. Well, a continuous net uptake at a record-high average rate of nearly 1 kg C m-2a-1 must correspond to a steady and sustainable carbon accumulation in either wood or litter or soil (or in several components). How could this be possible and where do the authors think that the carbon goes? Can they provide any independent validation, e.g. through measurements of wood or biomass increment or soil carbon stock? This I would consider as a pre-requisite for defending the NEE rates shown in the ms. If there are no such data available yet, the authors may consider asking foresters for advice on how to conduct such measurements at a remote site without too much effort. The reason why I am insisting on this point is that the results shown in Fig. 2 would have a huge impact if it really was true that old-growth forests are such extremely strong carbon sinks.

2) Filtering with ustar > 0.25 m/s does not remove the doubts about the soil CO2 efflux data, either, since it is not clear, given the nevertheless sloping terrain and the maximum tree height of 36 m, whether any eddies at all penetrate to the ground. It is still possible that the CO2 efflux from the soil remains always decoupled from the eddy flux above the canopy, with most of the soil CO2 flowing down the slope. Are there any CO2 chamber measurements of soil respiration that could be shown for comparison or any turbulence data from within the canopy? Alternatively, are there any other studies that show such data in tall tropical forests and that could be cited?

3) Last, but not least, I am irritated by the fact that the authors, in some places, did in fact not change the manuscript text according to what they announced in the response letter. Examples:
“In our revised manuscript, we will describe the gap-filling approach and the model in more detail.” They did not. The respective text (last 9 lines of section 2.3) was left completely unchanged. (Only the preceding sentences about the ustar filtering were added.)
“The discussion about data uncertainty will be added to the revised version of the manuscript.” It wasn’t! No error margins at all are presented in the text, just a few standard deviations for some multi-year records. Whether this was simply forgotten or deliberately omitted I cannot judge, but in any case the authors should keep the promises they made in their response letter.

In conclusion, I recommend that BG reconsiders the ms following a second revision that takes the points listed above into account.

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ED: Reconsider after major revisions (19 Aug 2015) by Paul Stoy

AR by Alexander Olchev on behalf of the Authors (30 Sep 2015) Author's response Manuscript

ED: Referee Nomination & Report Request started (17 Oct 2015) by Paul Stoy

RR by Anonymous Referee #2 (06 Nov 2015)

ED: Publish as is (06 Nov 2015) by Paul Stoy

Short summary

The time series analysis of the main meteorological parameters and components of CO2 and H2O fluxes showed a high evapotranspiration (ET) and gross primary production (GPP) sensitivity of the tropical rainforest to meteorological variations caused by El Niño-Southern Oscillation (ENSO) events. Incoming solar radiation is the main governing factor that is responsible for ET and GPP variability. Changes in precipitation due to moderate ENSO events did not have any notable effect on ET and GPP.

Response of CO2 and H2O fluxes in a mountainous tropical rainforest in equatorial Indonesia to El Niño events

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Peer-review completion

Response of CO₂ and H₂O fluxes in a mountainous tropical rainforest in equatorial Indonesia to El Niño events