Disentangling the response of forest and grassland energy exchange to heatwaves under idealized land–atmosphere coupling

van Heerwaarden, C. C.; Teuling, A. J.

doi:https://doi.org/10.5194/bg-11-6159-2014

Articles | Volume 11, issue 21

https://doi.org/10.5194/bg-11-6159-2014

© Author(s) 2014. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue:

Climate extremes and biogeochemical cycles in the terrestrial...

https://doi.org/10.5194/bg-11-6159-2014

© Author(s) 2014. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 11, issue 21

Research article

|

13 Nov 2014

Research article |

| 13 Nov 2014

Disentangling the response of forest and grassland energy exchange to heatwaves under idealized land–atmosphere coupling

C. C. van Heerwaarden and A. J. Teuling

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Final revised paper (published on 13 Nov 2014)
Preprint (discussion started on 28 Apr 2014)

Interactive discussion

Status: closed

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

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- Supplement

RC C1405: 'Review on "Disentangling the response of forest and grassland energy exchange to heatwaves..."', Anonymous Referee #1, 05 May 2014
- AC C1438: 'Response to the major poins of the reviewer', Chiel van Heerwaarden, 06 May 2014
  - RC C1590: 'Reply', Anonymous Referee #1, 15 May 2014
SC C2026: 'Reply to anonymous referee #1', Ryan Teuling, 01 Jun 2014
RC C2082: 'Elegant analysis and presentation but based on flawed assumptions', Anonymous Referee #2, 03 Jun 2014
- AC C2124: 'Response to the major poins of the second reviewer', Chiel van Heerwaarden, 05 Jun 2014
AC C2797: 'Final comments', Chiel van Heerwaarden, 25 Jun 2014

Peer-review completion

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

AR by Chiel van Heerwaarden on behalf of the Authors (22 Jul 2014) Author's response Manuscript

ED: Referee Nomination & Report Request started (25 Jul 2014) by Christopher A. Williams

RR by Anonymous Referee #1 (25 Jul 2014)

Suggestions for revision or reasons for rejection

In my review of the earlier version I was very critical about the modeling approach. This have not changes. I still think that to a large degree you are stating the obvious - if you force no stomatal conductance to high VPD, under wet soil and high VPD you'll have lots of LE.
I do find merit in the sensitivity analysis. There are interesting things to learn from the relative quantification of the strength of the effects of each of the common parameters used to represent different plant functional types (with grass and forest as the two extreme ends of the parametric continuum) on LE and the max temperature. I find section 4.3 and figure 6 interesting and well written.
Figure 2 is creating false expectation – it is focused on the dynamic feedback loops between the land surface and atmosphere. However your modeling approach does nothing to quantify the effects and the strength of these feedback loops, other than a very vague and general "residual" term (fig 6f). It should be relatively straight forward to test these by comparing a fully coupled surface-atmosphere model (as you have) with just the surface model results, driven by prescribed observations, or by the model-resolved atmospheric conditions of the other vegetation type, without a dynamic feedback.
I wish you have added to section 4.2 the response surface of a "grass-like" vegetation with stomata response of "forest" (and vis versa, the difference is shown in figure 6, but the response curve itself will be interesting)
Throughout the manuscript, I resent you describing what you study as forest and grass. This is a virtual test of vegetation parameterization. The fact that there is a potential continuum for all parameters (within a range) should be emphasized and then explain that you chose two extreme realizations, representing grass-like and forest-like ecosystem.
I do not agree with the claim that VPD response is represented to some degree by the temperature response. If you plot F(3) and F(4) over the range of temperature and VPD values in your study you will find that each of these stomata response function looks very different. In addition the effect of Ta on VPD is non-linear. I think the comment in L267-277 should be removed. More importantly, the sensitivity study (section 4.3) should be expanded to test the effects of F3 and F4 separately and not combined as done now (Fig 6d). I suspect that you'll find that F3 (VPD) has the major effect while the effects of F4 (Ta) are minor.
Minor comments
eq 3 has an error should be f^-1 and not f (for all the functions. I check the original in Noilhan and Mahfouf 1996). Or, you need to invert the formulation of Ff and f4 (eqs. 4,5).
gD cannot be unitless (Table 1) – the product gD*VPD must be unitless, and VPD has units.
Please provide the units to ALL the variables you use, not only the parameters. For example- the units of VPD (hPa?), Ta (K?) , LE, S_in… should all be listed near the equation that first uses them and should be consistent throughout the paper (some units appeal in the discussion section when you describe the conditions over grass and forest, make sure they are the same as the ones in the formulation).
Fig 3 – what is the meaning of the different colors of the bars? Are these grass/forest?
Fig 6 – the titles of the subsections b-f should be more accurate or removed, and somewhere in the caption you must explain exactly what is the meaning of each panel. For example – f) "residual" - of what? c) roughness – is it only roughness length or also displacement height? d)stomata resistance – this is wrong. rs (see your eq 3) is not what you tested here but only f1*f2*f3*f4 (As I stated above, I wish you would have at least isolated f3 and f4).

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RR by Anonymous Referee #2 (18 Aug 2014)

ED: Reconsider after major revisions (18 Aug 2014) by Christopher A. Williams

Given a lack of consistent support from reviews I have to judge that further revisions are required. Referee #1 has some enduring critiques with which I tend to agree, and offers some practical suggestions for improvement that should be attended to. Showing environmental response surfaces for stomatal resistance would be helpful, as recommended by the reviewer. Revising the paper's phrasing toward grass-like and forest-like extreme parameterizations as hypotheses is advisable as well, in my opinion, emphasizing that the treatments chosen are not necessarily realistic for these PFTs and are used just for sensitivity testing as a sort of heuristic exploration. I agree with the referee that the vpd effect on stomatal resistance cannot be assumed to be encapsulated in a temperature effect on stomatal resistance and that the two are likely to have different effect sizes as well as interact with each other. Suggestions that the temperature effect includes a vpd effect implicitly should be removed and it should be noted that a lack of sensitivity for grass ET to vpd could be a significant weakness in the degree to which the results are realistic for explaining the findings of Teuling et al. 2010 Nature Geoscience. Also noted by R1, it would help to emphasize and clarify that the key dynamic explaining the forest versus grassland contrasting response is concluded to simply be differential stomatal behavior, which directly influences the rate of evapotranspiration, and thus indirectly influences the near surface temperature condition. This is likely to be a largely first-order direct effect, and it seems to me that it would not require a boundary layer model coupled to the land surface model to capture dynamic feedbacks, though this has not been tested. The perceived simplicity of that explanation gets somewhat muddled by the paper's emphasis on boundary layer feedbacks and this should be addressed and clarified. Referee 1 has a good suggestion of how you could quantitatively test this idea using the coupled model results you have compared to what you get with an uncoupled model (just the land surface part with prescribed observations and no dynamic feedback for the environmental forcing). Please consider developing that further. Lastly, referee 1 also identified some errors that need to be corrected, listed under "minor comments", regarding units and equations.

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AR by Chiel van Heerwaarden on behalf of the Authors (24 Sep 2014) Author's response Manuscript

ED: Publish subject to technical corrections (27 Sep 2014) by Christopher A. Williams

Short summary

This study disentangles the response of forest and grassland to heatwaves, to interpret the findings of Teuling et al. (2010), who found systematically higher temperatures over forests than over grasslands in European heatwaves. By means of a study with a simple coupled land–atmosphere model, we show that the increase in stomatal resistance of vegetation under high values of vapor pressure deficit explains most of the differences and that this increase is enhanced by boundary layer feedbacks.