What determines the sign of the evapotranspiration response to afforestation in European summer?

Breil, Marcus; Davin, Edouard L.; Rechid, Diana

doi:https://doi.org/10.5194/bg-18-1499-2021

Articles | Volume 18, issue 4

https://doi.org/10.5194/bg-18-1499-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/bg-18-1499-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 18, issue 4

Research article

|

03 Mar 2021

Research article |

| 03 Mar 2021

What determines the sign of the evapotranspiration response to afforestation in European summer?

Marcus Breil, Edouard L. Davin, and Diana Rechid

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Final revised paper (published on 03 Mar 2021)
Preprint (discussion started on 14 Aug 2020)

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'Reviewer comments', Anonymous Referee #1, 10 Sep 2020
- AC1: 'Author comments to Reviewer #1', Marcus Breil, 23 Oct 2020
RC2: 'referee comments', Anonymous Referee #2, 28 Sep 2020
- AC2: 'Author comments to Reviewer #2', Marcus Breil, 23 Oct 2020

Peer-review completion

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

ED: Reconsider after major revisions (13 Nov 2020) by Christopher Still

AR by Marcus Breil on behalf of the Authors (16 Nov 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (17 Nov 2020) by Christopher Still

RR by Anonymous Referee #2 (03 Dec 2020)

Suggestions for revision or reasons for rejection

The authors have done an excellent job at responding to my comments. The VEG3D model is much clearer now. I have a few minor comments that would further strengthen the manuscript, but I leave it up to the authors to decide whether or not to include these revisions.

1. I am a fan of simplified models. In their response to comment 1b, the authors provide a strong defense for simplified models with the statements: "In order to get to the bottom of these fundamental processes, the use of a less complex model can even be beneficial. In such a model, the degrees of freedom are reduced and functional interrelations can consequently be deduced more easily." This is never said in the manuscript. What not acknowledge up front in that VEG3D is a simplified model? This could be added in lines 62-69. Or perhaps the authors do not think VEG3D is simplified?

2. In their response to comment 2a, the authors reference Table 1 of Breil et al. (2020) to justify parameter choices (comparable to those used in other models). Why not say this in the manuscript, too?

3. The revised description of the aerodynamic resistance is good. One minor suggestion is to change "reflected" to "included". A quick reading of the sentence might make readers think that the "transport of water vapor" is a reflection from the surface. What you are really saying is that ra accounts for turbulent transport of water vapor. Also, it is not clear what (a) refers to following cveg (line 91) and (b) following surface roughness (line 105). My reading of the equations is that three vegetation parameters affect ra: LAI, cveg, and z0, but that (citing Goudriaan 1977) z0 is thought to be the most important. If this is the intent, it could be said more clearly.

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RR by Anonymous Referee #1 (09 Dec 2020)

Suggestions for revision or reasons for rejection

I would like to thanks the Authors for replying and resending the paper.
However, at this stage I do have one main comments, which I found needed to the emerging results:
This is with regard to the comment on the differences between observation and model outcomes in the large dry region. In their answer the authors commented that “…While observational data reflect the local differences between forest and grassland transpiration rates, in our simulation setup, large-scale forestation scenarios are applied to analyze the general transpiration responses to forestation in an idealized and isolated way. It is therefore very difficult to assess the model results quantitatively and qualitatively…“.
Then that the: “… eddy covariance flux towers) reflect the local transpiration responses to forestation (Bright et al., 2017), in the CCLM-VEG3D simulation setup, large-scale forestation scenarios are applied to analyze the general transpiration responses to forestation in an idealized and isolated way …”.
And last (which was not much of clear for me) that: “…a physically consistent explanation for this phenomena, in which the evapotranspiration responses are described as an interplay of two factors, namely the reduced vapor pressure deficit in forests facing their evapotranspiration facilitating biogeophysical characteristics. …”
These statements need explanations to what behind the differences and best, if exist, to provide evidences for screen (i.e., air a 2 m) not surface (i.e., skin; Fig. 6) temperature and humidity differences between forest to grass ecosystems in published papers. Since part of the differences between the ecosystems are not included in the model I would even recommend to consider limiting the model to the seasons within the model capability to assess more directly the vegetation performance.
The sentences in lines 374-8 are unclear.

Thanks and be well!

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ED: Publish subject to minor revisions (review by editor) (09 Dec 2020) by Christopher Still

AR by Marcus Breil on behalf of the Authors (14 Dec 2020) Author's response Author's tracked changes Manuscript

ED: Publish as is (18 Jan 2021) by Christopher Still

Short summary

The physical processes behind varying evapotranspiration rates in forests and grasslands in Europe are investigated in a regional model study with idealized afforestation scenarios. The results show that the evapotranspiration response to afforestation depends on the interplay of two counteracting factors: the transpiration facilitating characteristics of a forest and the reduced saturation deficits of forests caused by an increased surface roughness and associated lower surface temperatures.