Shifted energy fluxes, increased Bowen ratios, and reduced thaw depths linked with drainage-induced changes in permafrost ecosystem structure

Göckede, Mathias; Kittler, Fanny; Kwon, Min Jung; Burjack, Ina; Heimann, Martin; Kolle, Olaf; Zimov, Nikita; Zimov, Sergey

doi:https://doi.org/10.5194/tc-11-2975-2017

Articles | Volume 11, issue 6

https://doi.org/10.5194/tc-11-2975-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue:

Changing Permafrost in the Arctic and its Global Effects in...

https://doi.org/10.5194/tc-11-2975-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 11, issue 6

Research article

|

15 Dec 2017

Research article |

| 15 Dec 2017

Shifted energy fluxes, increased Bowen ratios, and reduced thaw depths linked with drainage-induced changes in permafrost ecosystem structure

Mathias Göckede, Fanny Kittler, Min Jung Kwon, Ina Burjack, Martin Heimann, Olaf Kolle, Nikita Zimov, and Sergey Zimov

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Final revised paper (published on 15 Dec 2017)
Preprint (discussion started on 25 Oct 2016)

Interactive discussion

Status: closed

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

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RC1: 'Comments', Anonymous Referee #1, 14 Dec 2016
- AC1: 'Author response to comments made by reviewer #1', Mathias Goeckede, 21 Feb 2017
RC2: 'Great study, many editorial issues', Anonymous Referee #2, 04 Jan 2017
- AC2: 'author response to comments made by reviewer #2', Mathias Goeckede, 21 Feb 2017

Peer-review completion

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

AR by Mathias Göckede on behalf of the Authors (30 Mar 2017)

ED: Referee Nomination & Report Request started (02 Aug 2017) by Stephan Gruber

RR by Anonymous Referee #1 (30 Aug 2017)

Suggestions for revision or reasons for rejection

The authors have responded to the comments made in the previous review round, partly modified the manuscript, and added a paragraph about measurement uncertainties as appendix.
Despite some improvements, I still see points that should be addressed before publication:

The authors state that the performed quality assessment has slightly changed their results. In total, the difference between the previous and the revised version is on the order of about 4 Watt per square meter which is indeed a minor shift. The authors, however, aim to evaluate flux differences on the order of a few Watt per square meter so that minor shifts might be relevant for some interpretation. For example, the authors have clearly stated in the previous version that atmospheric fluxes are increased at the drained site. This finding was the basis for speculations on positive feedback mechanisms towards permafrost degradation. The new results have significantly changed this point of the study. Even though the trend in the changes of the Bowen-Ratio remains the same, I think it is necessary to address the sensitivity of the results to the performed quality assessment in the manuscript. Furthermore, the title of the manuscript should be changed since the atmospheric heat fluxes are obviously not increased only the flux partition is moderately changed.

p.1, l.22: The conclusion that an increased sensible heat flux might lead to a positive feedback on permafrost degradation is very arbitrary and after correcting the results I do not see any indication in this study that supports this statement. A very similar argument could be used in order to point out an increased latent heat flux as reason for a positive feedback on permafrost degradation. Water vapor strongly changes the radiation balance and is a very potent GHG. I suggest to focus on the results presented in this study as already outlined in the previous review round.

p.3, l.18: Same comment as above. Besides that, the statement is misleading as it reads like that the net heat transfer into the atmosphere is increased. Following the new results this is obviously not true.

p.10, l.31: The fact that the latent heat flux is not consistently decreased at the drained site in both years indicates that changes in the energy partition depend on various factors. Even though the Bowen-Ratio is consistently higher at the drained site, there is a high interannual variability as stated by the authors themselves. Thus, general conclusions based on observations of two years should be made very carefully if differences in energy partition between the sites also strongly depend on synoptic conditions.

p.12, l.11: I think this statement is misleading. Soil hydrology is the only factor modified in this study. How is it possible to identify it as the dominate control factor without testing other cofactors such as atmospheric conditions? It is important to be precises with such statements.

p.15, l.2: Please present an estimate of how much snow could be melted earlier due to differences in soil temperature. The statement that an increased snow depth leads to increased soil temperatures and, thus, to earlier snow melt requires a sound basis.

p.15, l.12: This estimate assumes the closure term to be constant. It might be possible that the two sites feature different closure terms and that the closure terms change with time.

p.15, l.27: Strictly spoken, the results (thaw depth and uppermost soil temperatures) show indication of a reduced soil heat flux.

p.16, l.29: After the quality assessment the total turbulent fluxes are not observed to be higher.

p.17, l.24: I do not see any result in this study justifying speculations on "tipping points". The same is true for speculations on atmosphere-permafrost feedback processes as already remarked above. The authors present nice measurements that demonstrate that the Bowen-Ratio changes due to drainage. Furthermore, they demonstrate that drainage has impacts on thaw depth and the soil thermal regime (at least within the upper decimeters). I strongly recommend that the discussion focuses on the solid results of this study without pushing speculations too far. Without results that clearly show effects such as tipping points and feedback mechanisms the made speculations appear either arbitrary or overstated.

p.19, l.14: What does "multi-disciplinary" mean in this context? Why is this information necessary?

p. 19, l.16: It should be pointed out that temperature changes are limited to the upper most decimeters. The differences in soil temperature are only 0.27K in 64 cm depth. This leads to further questions such as: What is the accuracy of the soil temperature sensors used? How accurate were the soil temperature sensors installed in the ground? How was the soil surface defined? What do these uncertainties mean for the soil temperature comparison (also the observed seasonal differences)?

p.19, l.17: This statement should include the wording "most likely" since the study neither presents data on soil heat capacities, thermal conductivities, nor soil heat fluxes.

p.19, l. 30: Why is there a profound impact on forecasts of the sustainability of Arctic permafrost ecosystems under future climate change? Changes in the atmospheric fluxes below 10% might be judged as rather moderate impact. In particular when the natural spatial heterogeneity of tundra is taken into account.

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RR by Anonymous Referee #3 (05 Sep 2017)

Suggestions for revision or reasons for rejection

This study of a long-term drainage experiment with a focus on energy budget effects is interesting and relevant to ongoing efforts to understand how ecosystem change in the Arctic might feedback on belowground and atmospheric processes. I have a number of comments listed below that I hope the authors will find useful.

Pg 2, line 19 – Improve wording. Perhaps, “In addition to warming, shifts in the water balance in this region are also expected to trigger profound …”
Pg 3, paragraph starting line 3 – Include a reference to the Kittler et al. Biogeosciences paper that reports on the CO2 flux analysis at this study site.
Pg 3, line 14 – Suggest wording change to “ …to quantify several secondary disturbance effects linked to lower water tables, including changes in vegetation community, radiation budget and soil thermal regime.”
Pg 3, lines 18-21 – These last 2 sentences repeat the results summarized in the abstract. I suggest these be rephrased in terms of a hypothesis or leave out entirely.
Pg 4, lines 9 – 12 – This last sentence reviewing Merbold et al. (2009) results is not needed in the methods and it is a repeat of information from the introduction. I suggest it be removed.
Pg 6, Section 3.1.2 – It isn’t clear to me why this analysis/detail on long-term temperature and precipitation is needed in this manuscript which focuses on the results of a manipulation experiment.
Pg 7, line 30 – The method used to calculate aerodynamic roughness needs to be included in the methods.
Pg 9, lines 10-11 – Is it reasonable to attribute the warmer conditions in the dry microsite to deeper snow as a) no snow depths were measured and b) this microsite might be relatively close in space to the wet microsite (as both are within the drained transect)? Could cooler winter temperatures at the wet microsite also or instead be due to greater thermal conductivity promoting heat loss in the saturated and frozen vs. dry soil? Or do you expect the dry microsite surface peat to become saturated in fall and be relatively similar?
Pg 9, line 15 – Were these results from just the 2 microsites at the control and the 2 microsites at the drained site or dry vs. wet microsites at the drained site only as shown in Figure 7?
Pg 9, line 26 – Perhaps reword to “Here, higher soil moisture promotes higher soil temperatures…”
Pg 12, lines 22 – 27 – Unless I missed it, there was no direct data to support a finding that snow was deeper in the drained site with greater shrub cover (which was also not assessed for height as noted on pg 13, line 1). Make sure to be clear that your data is not direct evidence for snow depth effects or include some additional data that helps support this conclusion.
Pg 14, line 13 – Can you speculate on the mechanism leading to this difference in H+LE response to reduced net radiation in 2015?
Pg 15, line 12 – But could this heat storage in water be included in the ground heat flux term if the surface is defined as the water surface?
Pg 15, line 13 – This was an increase of 8%? In other words, more energy may have been going to soil heat fluxes in 2015 at the control site?
Pg 15, line 28 – Any heat transfer impacts in winter?
Pg 16, line 15 – Improve wording.
Pg 17, line 5 – Reword “multiple links with factors” to improve clarity.
Pg 17, line 8 – Suggest revise wording to “…that the capture of drifting snow by shrubs increases snow depth and soil temperatures in winter…”
Pg 19, line 24 – Figure 10 needs to be introduced/discussed before the conclusion and in greater detail. It could instead be removed if preferred.
Pg 26, line 19 – Suggest wording change to “were amended for this study by flags also reflecting overall…”
Pg 27, line 7 – Remove “e.g.”
Pg 34, Fig 4 caption – What does the height of 0 cm reference to?
Pg 40, Fig 10 – What do the green quadrilateral shapes represent?

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ED: Publish subject to minor revisions (Editor review) (13 Sep 2017) by Stephan Gruber

AR by Mathias Göckede on behalf of the Authors (27 Sep 2017) Author's response Manuscript

ED: Publish subject to technical corrections (23 Oct 2017) by Stephan Gruber

AR by Mathias Göckede on behalf of the Authors (24 Oct 2017) Author's response Manuscript

Short summary

Shifts in hydrologic conditions will be a key factor for the sustainability of Arctic ecosystems under future climate change. Using a long-term manipulation experiment, we analyzed how energy exchange processes within a permafrost ecosystem react to sustained dry conditions. Changes in several important ecosystem characteristics lead to reduced evapotranspiration and increased sensible heat fluxes. Heat transfer into the soil was strongly reduced, keeping the permafrost colder.