Has the Dominant Climatic Driver for the Carbon Budget of Alpine Grassland Shifted from Temperature to Precipitation on the Qinghai–Tibet Plateau?

Round 1

Reviewer 1 Report

   The manuscript titled “Temperature-dependent Carbon Budget of Alpine Grassland Transferred to Precipitation-dominated on the Qinghai–Tibet Plateau” clarifies the changes in the grassland carbon budget of the Qinghai-Tibet Plateau and the transformation of its climatic driving factors, confirms the legacy effect of climate change on the carbon budget and explains the possible mechanism.

Overall, the topic is meaningful and within the journal's scope. The introduction section provides sufficient background and includes all the relevant references. The methods used are appropriate and well-chosen for the case. The work done is well structured and well written, with interesting findings. Which have strong implications, But there are a few issues that should be corrected before publication. (as below).

1.      Lines 14-15. It would be better to put the abbreviations in parentheses. Please check all other similar issues in this manuscript.

2.      Lines 74-75. The purpose/objectives of the study stated here are not very specific, and it is recommended to specify the main purpose according to the content of this research.

3.      Lines 391-393. In the discussion section, the possible reasons for the legacy effects are analyzed, and the analysis results are not shown in the conclusion section.

·         Why do authors have to put “, “ after every equation in the manuscript?

·         Line 181, delete the sentence “There was …”.

·         Line 191, add “on the QTP” at the end of the sentence.

·         Lines 208-209, change “… area, but the ER was …” to “… area but was …”.

·         Line 209, change “grasslands area” to “grassland area”.

·         Line 210, delete “to the ER”.

·         Line 275, change “… the main limiting factors of …” to “… the main factors hindering the formation of …”.

·         Line 282, delete “many”.

·         Line 286, change “ma” after “According to”

·         Lines 365, change “AGQTP” to “AG_QTP in” to “primary”

 

·         Line 295, add “the results of the…..

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

This study estimated the carbon budget of alpine grassland ecosystems on the Qinghai-Tibet Plateau based on remote sensing models, and analyzed their spatiotemporal dynamics and climatic driving mechanisms. The results showed that the carbon flux indicators (GPP, ER, NEP) increased during 2000-2020, but the increasing rate slowed down. The changes of carbon budget were influenced by temperature and precipitation, but the dominant factor changed around 2010. The paper emphasized the dynamic and lasting effects of climatic drivers on carbon fluxes, and provided references for ecosystem carbon prediction and related studies.

 

The following suggestions are proposed for improvement:

1. The meteorological observation stations on the Qinghai-Tibet Plateau are sparse, especially in the western part of the plateau. In this case, there will be uncertainties for the spatial interpolation with a high spatial resolution (1km). The climatic data are the core data of this paper, and the errors of spatial interpolation will have a great impact on the conclusions of the paper. It is suggested to add the content of accuracy verification of meteorological data interpolation, or use the climatic data with the same spatial resolution published by authoritative institutions.

2. The paper compared the simulation results of this paper with those of CMIP6, but their spatial resolutions are different. The spatial resolution of CMIP6 simulation results is relatively large (should be 50km). If the spatial resolutions are different, the persuasiveness of the research results in areas with strong spatial heterogeneity (such as the eastern part of the Qinghai-Tibet Plateau) will be weak. It is suggested not to put the content of comparison with CMIP6 simulation results in section 3, but in the discussion section, and explain the uncertainty of different spatial resolutions for comparative research results.

3. The length of data in this paper is 20 years, and the analysis is based on year as a unit. However, in statistical analysis, the statistical laws obtained from analyzing 20 statistical objects are less persuasive. It is suggested to set up a special part in the discussion section, combined with relevant studies, to focus on discussing the key conclusions drawn from this paper, that is, whether the change of dominant factors around 2010 is robust.

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

The research is focused on the carbon budget of grasslands in the Qinghay-Tibet Plateau (QTP) and considers the changes on mean temperatures and precipitation over 21 years as driving forces that affect three indicators related to carbon budget: net ecosystem productivity (NEP), gross primary productivity (GPP), and ecosystem respiration (ER). From my point of view the research has serious flaws and doesn't meet the standards to be published on Remote Sensing. Here I highlight my comments.

Title. It was difficult to understand the meaning. Please rephrase it.

General comment. Why the text is highlighted in several parts? It gives the impression it is a draft.

Line 14. Please, put the full names out of the parentheses and the abbreviation in parentheses because this is the first time they appear in the abstract.

Line 60. Please, remove the abbreviation “NEP” after “carbon sink” because Net Ecosystem Productivity is not a synonym of carbon sink, but only a measure of carbon sink/source.

Introduction section. The introduction doesn’t place the research in a broader context. It is focalized on the QTP without providing a clear explanation of its contribution in the context of climate change, which is a global threat. The aim of the research is not clearly defined and it doesn’t provide hypothesis to be tested. Please, highlight why the research and its aim are important also on a global perspective (this can justify its publication on an international journal) and clarify the tested hypothesis.

Study area section. The description of the study area is insufficient. Please, add information about geology and geomorphology, altitudinal ranges, presence of deserts, rocky or sparsely vegetated areas, perennial snowy areas and other information that can help to understand such a vast area. Since temperature and precipitation are considered, in the manuscript, as the main explanatory variables, more information about temperature and precipitation are needed to describe the study area. Annual average is not enough. At least provide the annual range of temperatures (mean of the coldest month and mean of the warmer month). Please, provide more information about the six types of grasslands involved in the study.

Figure 1. Please, consider that the manuscript, if published, will be available to an international community, but in the map there is a regional scale only. Please, add also a more global scale because not everybody knows the QTP. Provide information on white areas in the legend. I suggest to add hillshading to the map of grassland types, in order to provide information on geomorphology. I also suggest to provide the maps of the distribution of mean temperatures and precipitations in the QTP as a baseline that can help understanding the climate conditions of the study area. Moreover, I suggest to delete the north arrow because it is useless in this case. Add the reference system in the caption, instead (e.g. WGS 84).

Line 98. Please, provide full name of CMIP6 because it is the first time it appears in the main text.

Lines 128-130. The authors didn’t describe the statistical analyses and coefficients adopted to correlate simulated data with EC-based and CMPI6 data. Therefore, it is not possible to me to assess if the statistical analyses are appropriate for the type of data and to address the aims of the study. This is a serious shortcoming.

Line 136. How did you verify the significance? Which statistical analyses did you use? This should be described clearly and thoroughly.

Lines 140-142. Ok, this is visually interpreted on the maps, but you should verify it statistically comparing NEP data among grasslands types and providing the significance of differences.

Lines 149-161. Which method/analyses did you used to study trends and test for significance? This should be provided in the M&M section.

Lines 162-163. This should be described, thoroughly, in the Materials and methods section.

Section 3.3.1. Which method did you used to correlate temperature and precipitation with GPP, ER and NEP? Which correlation coefficient? Did you tested data for normal distribution, linearity, presence of outliers, before choosing the correlation coefficient? All this information is not provided, so it is not possible to me to evaluate the goodness of the analyses and results.

Section 3.3.2. As the previous comment.

Line 296. Wasn’t the window set to 9 years, as you stated in line 163?

Discussion and conclusion sections. From my point of view the comments provided in the discussion and conclusion sections go beyond what the results can support. The authors didn’t provide descriptions of the statistical analyses of data. By the results I suppose the authors used Pearson’s correlation and Linear regression, but there is no trace of tests for assumptions that justify the use of these analyses, and there is no trace of ANOVA, or similar non parametrical analyses, to test for significance. Moreover, correlations cannot be used to state that a variable causes the changes of another variable. The research needs controls and to consider other explanatory variables to be more confident on the results. The study area is very large and there is no stratification based, for example, on grasslands types or spatial heterogeneity of the area. The EC stations used to validate simulated data seem to be distributed mostly on the Alpine meadow areas, therefore there is not a homogeneous distribution among the different areas of grassland types. The temporal period of 21 years is too short and not representative to support the comments provided in the discussion section. The authors should be more cautious discussing the results, especially when talking about the possible relationships between vegetation dynamics and environmental factors. Mean temperature and precipitation are a little part of the environmental factors that can affect vegetation, and the relationships among environmental variables and ecosystems can be very complex, direct and indirect, linear and not linear. This should be taken into account especially when considering such a vast area. Therefore, the authors should discuss the results considering all their limitations and only for what they merely show.

For the reasons exposed in the previous comments, the research has serious shortcomings regarding the methods and statistical analyses. From my point of view, the comments provided in the discussion section are pure imagination because they are not supported by robust statistical analyses.

I’m sorry but I cannot suggest corrections not even as major revision. I would suggest to the authors to resubmit the manuscript after a deep revision of the entire research:

- Define the aim of the research more clearly and realistically;
- Resize the study area basing on a suitable scale of analysis to address the aim and basing on the available data. For example, you can focalize the study only on the Alpine meadow, because the EC stations are distributed mainly in this grassland type;
- Define an appropriate experimental design taking into account controls and other explicative variables;
- Choose suitable and robust statistical tests (e.g. ANOVA or analogous non-parametric analyses, Spearman' rho for non-parametric correlations, Willmott index, CCA etc.);
- Discuss the results considering their limitations and without going beyond what they show;
- Give a more global breath to your research, because at present it is very closed in itself. Climate change is a global threat; therefore, your results should be discussed in a wider context if you choose to publish them in an international journal.

Author Response

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Author Response File: Author Response.docx

Reviewer 4 Report

I recommend the manuscript be revised and resubmitted for review. The manuscript needs an extensive edit to improve English grammar and the scientific writing style.  The research appears to consist of an extensive modeling effort for alpine grasslands in the Tibet Plateau, but the quality of the research is lost in the poor scientific writing to convey the researcher's efforts.  Much of the writing contain qualitative versus scientific quantitative language.  For example, in the Abstract, results are reported as: 

" Our results showed GPP, ER, and NEP all increased during 200-2020 with substantial spatiotemporal variability, but their increase rates slowed down."

"Increased", by how much?   "Increase rates slowed down", by how much?

The manuscript demonstrates poor scientific writing style throughout, with much of the research couched in qualitative terms rather than the appropriate quantitative numerical and statistical descriptions of the data.

As another example, in lines 52-54.  This sentence could be edited to read,

"The uncertainty associated with estimating regional carbon budgets in the QTP is confounded by the regional mosaic of vegetation, soils, and temperature and precipitation patterns; insufficient ground-based data for model calibration and validation of GPP, ER, and NEP; and the accelerated rate of climate change driven alterations of temporal and spatial temperature and precipitation patterns ."

In line 56, "a process-based ecosystem model", what model?  State the model.  In line 57, "a light use efficiency model", what model? State the model.

I am happy to review the science after the manuscript has been revised to improve the English and scientific style.

 

 

 

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Agree with the author's revision of the manuscript. The revised manuscript has reached the level that can be published.

Author Response

Dear Reviewer,

Thank you for taking the time to review our manuscript. We appreciate your support of our revised manuscript and are glad to hear that you believe it has reached the level that can be published.

Best regards,

Authors of the manuscript

Reviewer 3 Report

Dear Authors, I carefully read the responses and the changes made in the manuscript. I appreciate the effort made to improve the research. Many of the issues that I highlighted have been solved. However, I still have doubts about the reliability of the results.

My first doubt is about the simulated data validation. The correlation with CMIP6 data has been moved to the discussion section because of the resolution incongruity and the high level of uncertainty of CMIP6 data. Regarding EC stations, they are mainly distributed in alpine meadows, therefore, they can validate most likely the simulated data for this grassland type. Moreover, it is still not explained in the M&M section which index has been used to test the agreement between simulated vs EC data.

Second. In the study area description, you stated that six grasslands types were involved. But in the ANOVA described in Response 11, the grasslands were divided into two categories: meadows and steppe. What is the rationale of this categorization? Are alpine meadows really similar to temperate meadows, and temperate steppe similar to desert steppe in terms of plant communities, functional traits, productivity, species richness, etc.? Please, consider also Mann-Witney U test instead of ANOVA to compare two groups, if data were not normally distributed.

Third. In Response 16 you stated that the “use of partial correlation analysis allowed to control for other explanatory variables during the correlation analysis”. Which are these explanatory variables? Moreover, you stated that the “study focused on spatial heterogeneity mainly in terms of region and orientation (e.g., different climate zones), rather than different grassland types”. Therefore, for me it is still not clear why you considered six types of grasslands if you wanted to compare spatial heterogeneity. In this case, in the M&M section you should explain that the experimental design is focused on a type of spatial heterogeneity and introduce the different types of areas. Probably grasslands types reflect different climate zones, therefore unify temperate and desert steppe, or alpine and temperate meadows should be avoided if the focus is on climate zones. The design of the research should be better clarified.

My conclusions. I think the manuscript has been significantly improved after the first revision, but the aspects above should be clarified. We are talking about a study area that is almost 2% of the world’s land area. As you stated, it is known as the Third Pole of the Earth and it is the largest and highest plateau in the world. Changes in ecosystem processes in this area can affect the global climate. For these reasons, the research should be very very careful. Affirm that the drivers changed from temperature to precipitation is a serious statement if it is related to an area like this. Therefore, the results should not leave doubts about methods. In this case there are a number of weaknesses, that you also recognized, about data availability and reliability (e.g. distribution of EC sites, CMIP6 data, only 21 years, absence of stratified experimental design, absence of other explanatory variables and controls). The uncertainty is also showed by the controversial literature. Therefore, I consider the manuscript can comply for publication on Remote Sensing after clarifying the points above and if the results in the abstract, discussion and conclusion sections (not just in the Uncertainties and Limitations section) are made less categorical. Maybe, also transforming the title into a question could help to achieve a proper level of prudence (e.g., “Is The Dominant Climatic Driver on the Carbon Budget of Alpine Grassland changed From Temperature to Precipitation on the Qinghai–Tibet Plateau?”).

Author Response

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Author Response File: Author Response.pdf