Response of water use efficiency to summer drought in a boreal Scots pine forest in Finland

Gao, Yao; Markkanen, Tiina; Aurela, Mika; Mammarella, Ivan; Thum, Tea; Tsuruta, Aki; Yang, Huiyi; Aalto, Tuula

doi:https://doi.org/10.5194/bg-14-4409-2017

Articles | Volume 14, issue 18

https://doi.org/10.5194/bg-14-4409-2017

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

https://doi.org/10.5194/bg-14-4409-2017

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

Articles | Volume 14, issue 18

Research article

|

29 Sep 2017

Research article |

| 29 Sep 2017

Response of water use efficiency to summer drought in a boreal Scots pine forest in Finland

Yao Gao, Tiina Markkanen, Mika Aurela, Ivan Mammarella, Tea Thum, Aki Tsuruta, Huiyi Yang, and Tuula Aalto

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Final revised paper (published on 29 Sep 2017)
Supplement to the final revised paper
Preprint (discussion started on 10 Jun 2016)
Supplement to the preprint

Interactive discussion

Status: closed

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

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RC1: 'Review', Anonymous Referee #1, 20 Jun 2016
- AC1: 'Reply to Referee 1', Yao Gao, 18 Apr 2017
RC2: 'Comments on “Response of water use efficiency to summer drought in boreal Scots pine forests in Finland” by Yao Gao et al.', Anonymous Referee #3, 09 Oct 2016
- AC2: 'Reply to Referee 2', Yao Gao, 18 Apr 2017
RC3: 'Review', Anonymous Referee #4, 19 Oct 2016
- AC3: 'Reply to Referee 3', Yao Gao, 18 Apr 2017
RC4: 'Review: Response of water use efficiency to summer drought in boreal Scots pine forests in Finland', Anonymous Referee #5, 23 Oct 2016
- AC4: 'Reply to Referee 4', Yao Gao, 18 Apr 2017
RC5: 'Comment on "Response of water use efficiency to summer drought in boreal Scots pine forests in Finland" by Gao et al.', Anonymous Referee #2, 29 Oct 2016
- AC5: 'Reply to Referee 5', Yao Gao, 18 Apr 2017

Peer-review completion

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

ED: Reconsider after major revisions (20 Apr 2017) by Ivonne Trebs

AR by Yao Gao on behalf of the Authors (26 May 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (30 May 2017) by Ivonne Trebs

RR by Anonymous Referee #5 (20 Jun 2017)

Suggestions for revision or reasons for rejection

General comments
The authors did a good job improving over the original submission. They dealt well with most of the comments raised on earlier review. However, the manuscript contains many language errors and I recommend that it carefully be edited by a native English speaker. Some are listed below in the minor comments but the list is by no means exhaustive. Also where graphs share the same y-axis title and y-axis label, the readability of the graphs could be enhanced by using only one (on the left hand side) y-axis title and y-axis label instead of having them for each panel (for example, Figs. 2 and 3).

I recommend accept with minor revision.

Minor revisions to consider:

Lines 71–72: Not sure what message this sentence is meant to convey?

Lines 76–77: ‘The appropriateness of uWUE at daily time scale was also demonstrated (Zhou et al., 2015).’??

Lines 87–88: ??… (3) to evaluate how adequately the JSBACH land surface model (LSM) captures plant responses to changes in environmental variables.??

Line 95: Check LAI in Table 1.

Line 96: ‘+’ is not necessary.

Line 107: ‘ … by excluding …’

Lines 120–121: Move this to line 193 to explain why the first layer was not included in the analysis.

Line 215: Perhaps present some measure of variation (e.g. standard deviation, confidence interval, etc) in addition to averages in the 11-year climatic conditions so that readers could have an idea of the deviations during the drought period.

Lines 222 and 224: Coefficient of determination was used elsewhere; no need to put the numbers in brackets; is three significant digits necessary here?; what is the difference between correlation coefficient and time correlation?.

Line 225: Replace ‘was’ with ‘being’

Line 228: Replace ‘presented’ with ‘observed’

Line 230: Replace ‘is’ with ‘was’

Line 232–233: Please reword the sentence ‘The biggest difference of 1.054 kPa took place on August 5th, the day with highest Ta in August 2006.’

Line 266: Replace ‘environment’ with ‘environmental’

Lines 291–292: Provide units; the SMI values for the IWUE values are swapped??

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RR by Zhen-Ming Ge (23 Jun 2017)

RR by Anonymous Referee #7 (23 Jun 2017)

Suggestions for revision or reasons for rejection

In the first round of the review, the reviewers have addressed some important points (e.g. filtering of the data, focus on one site only, classification of SMI values etc.).
Most of these comments were adequately addressed by the authors, and the revised version of the manuscript has improved in many aspects. The Methods and Results sections are clearly and well written.
However, the discussion is weak and lacks novel aspects. In particular, I do not agree with the conclusion of IWUE being the most appropriate WUE metric, and
I doubt that this question can be answered based on the results of this study.
See more details below.

More detailed comments:

- I think the introduction would be stronger if the authors mentioned the anticipated occurrence of drought in this region (e.g. after line 49). In the current version, this is only mentioned in the conclusions.

- line 74: "simple stomatal model" does not adequately describe the model by Lloyd & Farquhar 1994. I suggest to reformulate this in a way that clarifies that it is based on the optimality theory developed by Cowan & Farquhar 1977.

- I agree not to use layer 1 soil moisture, but the justification "too sensitive to climate variability" is a bit misleading as "climate" implies a longer time scale than what you mean here.

- line 165: replace "JSABCH" with "JSBACH"

- line 193f: not clear how these soil parameters were determined. Please explain shortly.

- line 204f.: I would not introduce IWUE as a measure of surface conductance. This metric accounts for the confounding effects of VPD on ET, but it does not consider that also Gs decreases with increasing VPD (but uWUE does). Further, the derivation of IWUE is based on strong assumptions (e.g. that Ga is infinitely high) that are not justified when this metric is interpreted in terms of Gs. It's ok to use this metric, but the authors should not over-interpret its physiological meaning. I suggest to introduce it simply as a WUE metric that accounts for the effects of VPD on ET. For the same reasons, don't call IWUE "intrinsic WUE" in line 307

- line 207: cite Zhou et al. 2014 again here, who suggested this metric

- An interesting aspect of Figure 1 is that in some years (e.g. 2003, 2009), the simulated and observed soil water dynamics do not agree well compared to the other years. Do the authors have an explanation of why this could be the case?

- It would probably be too messy to show VPD in Figure 1 as well, but it would help to provide the absolute range of VPD observed in the drought period in the text, and not just the deviations compared to the reference period.

- Lines 255f: this sentence is a bit unclear. Isn't it the other way around? I.e. the GPP-ET relationship is more linear in the simulations?

- I do not agree with most parts of section 4.1 (and the corresponding sections of the conclusions). The authors try to identify the most appropriate WUE efficiency measure, but they do not really explain why IWUE is considered more appropriate than the other metrics. Is it just because IWUE increased, whereas uWUE does not show a response to drought? That may be true but how do we know that this is then the more appropriate response? Another aspect that was not mentioned is that uWUE in Fig. 4 has a better correlation than IWUE. Isn't that an indication that uWUE is a better metric describing the water-carbon coupling? In my opinion, this part of the discussion lacks some more fundamental considerations of the metrics applied. For instance, uWUE was developed to overcome some shortcomings of the IWUE metric (i.e. the stomatal response to VPD is taken into account), and in that sense is a more meaningful metric to characterize ecosystem functioning than IWUE.
The weaker VPD response of IWUE should also (or in particular) play a role in drought periods, so I don't follow the argumentation why uWUE should be better suited for long-term studies than for particular drought periods.

- other main points of the discussion are rather weak and repeat outcomes of earlier studies. For example, it is long known that stomatal conductance responds to air humidity. It is also commonly accepted that non-stomatal limitations are needed to adequately simulate the drought response in models. These aspects are long known to the modelling community.

- Line 318: rewrite to "stomatal conductance in JSBACH does not include a response to air humidity" or simply delete the first part of the sentence.

- Figure 2a: Explain the meaning of the dashed circle also in the caption, not only in the text
- Wouldn't it make more sense to present GPP in gC m-2 d-1? This would also be easier to interpet. Up to the authors.
- Table 1: It would make sense to indicate the grain size distribution under "Soil type" in the lower part of the table.

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ED: Reconsider after major revisions (14 Jul 2017) by Ivonne Trebs

AR by Yao Gao on behalf of the Authors (28 Aug 2017) Author's response Manuscript

ED: Publish as is (04 Sep 2017) by Ivonne Trebs

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Short summary

We investigated the response of water use efficiency (WUE) to summer drought in a boreal Scots pine forest (Pinus sylvestris) on the daily time scale mainly using EC flux data from the Hyytiälä (southern Finland) flux site. Simulation results from the JSBACH land surface model were also evaluated against the observed results. The performance of three WUE metrics at the ecosystem level (EWUE, IWUE, and uWUE) during the severe summer drought were studied and showed different results.