Variable tree rooting strategies are key for modelling the distribution, productivity and evapotranspiration of  tropical evergreen forests

Sakschewski, Boris; von Bloh, Werner; Drüke, Markus; Sörensson, Anna Amelia; Ruscica, Romina; Langerwisch, Fanny; Billing, Maik; Bereswill, Sarah; Hirota, Marina; Oliveira, Rafael Silva; Heinke, Jens; Thonicke, Kirsten

doi:https://doi.org/10.5194/bg-18-4091-2021

Articles | Volume 18, issue 13

https://doi.org/10.5194/bg-18-4091-2021

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

https://doi.org/10.5194/bg-18-4091-2021

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

Articles | Volume 18, issue 13

Research article

|

12 Jul 2021

Research article |

| 12 Jul 2021

Variable tree rooting strategies are key for modelling the distribution, productivity and evapotranspiration of tropical evergreen forests

Boris Sakschewski, Werner von Bloh, Markus Drüke, Anna Amelia Sörensson, Romina Ruscica, Fanny Langerwisch, Maik Billing, Sarah Bereswill, Marina Hirota, Rafael Silva Oliveira, Jens Heinke, and Kirsten Thonicke

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Final revised paper (published on 12 Jul 2021)
Preprint (discussion started on 27 Mar 2020)
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 on "Variable tree rooting strategies improve tropical productivity and evapotranspiration in a dynamic global vegetation model"', Anonymous Referee #1, 28 Apr 2020
- AC1: 'Response to referee 1', Boris Sakschewski, 03 Jul 2020
RC2: 'Review of Sakschewski et al "Variable tree rooting strategies improve tropical productivity and evapotranspiration in a dynamic global vegetation model"', Daniel Falster, 23 May 2020
- AC2: 'Response to referee 2', Boris Sakschewski, 03 Jul 2020

Peer-review completion

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

ED: Reconsider after major revisions (09 Jul 2020) by Martin De Kauwe

AR by Boris Sakschewski on behalf of the Authors (03 Sep 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (17 Sep 2020) by Martin De Kauwe

RR by Anonymous Referee #1 (24 Sep 2020)

RR by Daniel Falster (13 Oct 2020)

Suggestions for revision or reasons for rejection

I reviewed a previous version of this paper, which introduces a scheme for variable rooting depths into the LPJmL4.0 DGVM. While generally enthusiastic about the idea, I found the previous version too long and unfocussed, which prevented any deep engagement and understanding. Both reviewers felt similarly and requested a more focussed narrative.

While the revised paper is a little clearer and has benefited from some minor improvements, my major criticisms have only been superficially addressed. The authors have clarified their goals only a little by adding a few lines of text in the introduction. Otherwise the introduction and methods seems almost identical, and we still have 12 figures, most with multiple panels. The authors really need to do more to streamline the paper and tell a convincing story about the biology of the system.

The way it currently reads, the main motivation is to introduce variable rooting strategies. But this is the solution. The motivation should be to explain some phenomenon: I'd suggest focussing on the inability of previous models to explain the wide distribution of evergreen types in the Amazon. While the authors do mention this in paragraph two, this material seems to come second to the goal of modelling variable rooting depths, and then gets lost amongst the many plots and attention given to biomass and ET.

The significance of the problem relating to evergreens vs deciduous doesn't really hit until we reach Figure 10, which shows just how bad the standard LPJ model does in predicting the distribution of evergreen vs deciduous types. Perhaps these panels could be introduced in the introduction, to motivate the study? Then variable rooting depth introduced as a potential solution?

After reading the paper several times, I still can't say why variable rooting depth leads the LPJmL4.0-VR version to predict wider distribution of evergreens. I see that it does, but don't understand why.

That said, the results suggest it's not only about variable rooting depth. I was interested to see in Figure 10 that that the LPJmL4.0-VR-base model actually goes a considerable way to explaining the wide distribution of evergreens. So there seems to be two changes making a difference: from LPJmL4.0 -> LPJmL4.0-VR-base, then from LPJmL4.0-VR-base -> LPJmL4.0-VR. The authors need to explain the biological mechanism causing LPJmL4.0 and LPJmL4.0-VR-base to differ and perhaps temper their claims about variable rooting accordingly.

Unfortunately, the LPJmL4.0-VR-base model is never really introduced or explained in any detail, so we can only guess why it predicts a wider distribution of evergreens than LPJmL4.0. I also note strong differences between LPJmL4.0 and LPJmL4.0-VR-base in Tables A6 and A7. My naive expectation is that LPJmL4.0 and LPJmL4.0-VR-base should produce near identical results, as they mostly do in Tables A3-A5. Why is this the case, if there's no variance in rooting depth?

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ED: Reconsider after major revisions (14 Oct 2020) by Martin De Kauwe

AR by Boris Sakschewski on behalf of the Authors (02 Dec 2020)

ED: Referee Nomination & Report Request started (04 Dec 2020) by Martin De Kauwe

RR by Anonymous Referee #3 (12 Feb 2021)

Suggestions for revision or reasons for rejection

This study by Sakschewski and colleagues presents an updated version of the DGVM LPJmL that is able to reproduce the observed distribution of tropical vegetation based on representation of competing rooting strategies between evergreen and deciduous trees. It is exciting to see that the sub-model version LPJmL4.0-VR seems to capture spatio-temporal patterns of ecosystem fluxes across South-America based on spatially explicit information on soil depth and thus considering variable rooting depth and differences in relative carbon investment into belowground biomass between coexisting plant species in competition for limiting resources. Nevertheless, in line with the comments provided by foregoing referees some weakness in delivering the findings presented in this study could still be resolved in order to more concisely present respective findings to the reader. For instance, one option would be to use the structure of the sub-sections presented in the discussion section, i.e. 4.1, 4.2, 4.3 to formulate respective hypothesis at the end of the introduction section. For more specific examples of how to improve the presentation of the manuscript please see the following points and some minor suggestions below:

1. As has been criticised by referee #2 some aspects of the manuscript are presented quite lengthy and the language used to explain some of the main findings could still be improved, e.g. to further clarify some of the contrasting findings presented in L59-61 and L61-64.

2. In line with this criticism, the manuscript could be streamlined further by reducing the number of display items that are critical to understand the main findings without presenting the underlying method, which could be moved into the appendix, e.g. by reducing the figures to only Fig. 2/5/10 and moving the rest in the appendix.

3. While I would disagree with the referee's comment that major criticisms have only superficially addressed in the revision, I wonder if the presentation of the manuscript could be made even more explicit by using respective sub-section headers (i.e. 4.1, 4.2, 4.3) in the discussion section for formulating specific hypothesis at the end of the introduction section?

L64: consider rephrasing to “and biomass in fire prone ecosystems”.
L65: please clarify which effects (i.e. on what) you are talking about?
L594: consider rephrasing “this potential treasure”.

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RR by Anonymous Referee #4 (18 Feb 2021)

ED: Publish subject to minor revisions (review by editor) (22 Feb 2021) by Martin De Kauwe

AR by Boris Sakschewski on behalf of the Authors (09 Apr 2021) Author's response Author's tracked changes Manuscript

ED: Publish as is (13 Apr 2021) by Martin De Kauwe

AR by Boris Sakschewski on behalf of the Authors (22 Apr 2021) Manuscript

Short summary

This study shows how local adaptations of tree roots across tropical and sub-tropical South America explain patterns of biome distribution, productivity and evapotranspiration on this continent. By allowing for high diversity of tree rooting strategies in a dynamic global vegetation model (DGVM), we are able to mechanistically explain patterns of mean rooting depth and the effects on ecosystem functions. The approach can advance DGVMs and Earth system models.