Carbon Dioxide Fertilization Effects Offset the Vegetation GPP Losses of Woodland Ecosystems Due to Surface Ozone Damage in China

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The paper investigates how elevated CO₂ levels can offset the negative effects of increased surface ozone (O₃) on the Gross Primary Production (GPP) of woodland ecosystems in China from 2001–2020. It uses a process-based model to simulate the interactions between climate change, CO₂ fertilization, and ozone pollution, comments can be found below,

1. The abstract contains vague statements such as “Our simulation study provides a new way of thinking…” without specifying what is novel in the model or findings. Please clearly state what is innovative about this study compared to existing ozone-CO₂ interaction modeling efforts.
2. The literature review lacks depth. While multiple citations are included, the discussion doesn’t critically compare previous work or establish a sharp research gap.
3. The BEPS\_O₃ model modifications are described too briefly. Key changes to account for O₃ impacts (e.g., incorporation of PODy, α parameters) are not sufficiently explained.
4. Equations (6)–(8) are central but not contextualized adequately. What are the assumptions of α? How were they calibrated or validated?
5. Validation with only three flux sites over limited years (2009–2010, 2016) is insufficient to support national-scale claims from 2001–2020. Please broaden validation using more diverse sites or include rigorous cross-validation techniques. Justify site selection and address potential biases from limited spatiotemporal coverage.
6. The paper states fixed 2001 CO₂ values for some runs, but does not explain the implications on LAI, which is climate-responsive.

Author Response

We are grateful for the valuable suggestions provided by the reviewers. Regarding the issues you raised, I will address them one by one.

 

1. The abstract contains vague statements such as “Our simulation study provides a new way of thinking…” without specifying what is novel in the model or findings. Please clearly state what is innovative about this study compared to existing ozone-CO₂ interaction modeling efforts.

 

Re：We further strengthened the expression of the innovative points in the article and revised the wording of the abstract.

 

1. The literature review lacks depth. While multiple citations are included, the discussion doesn’t critically compare previous work or establish a sharp research gap.

 

Re：Thank you to the judges for your suggestions. We have revised the summary section of the paper, striving to highlight the significance of the research. Regarding the issue of insufficient analysis depth, due to the limited revision time, only a few changes were made. We have uploaded the revised Word document in the revision mode, so the reviewers can see our specific revision plans in the revision mode.

 

1. The BEPS\_O₃ model modifications are described too briefly. Key changes to account for O₃ impacts (e.g., incorporation of PODy, α parameters) are not sufficiently explained.

 

Re：In the methodology section, we have added some additional content in order to present our modifications to the BEPS model more clearly. By adding equation (9), it explains how O3 and CO2 interact with photosynthesis, and specifically explains how O3 is coupled into the BEPS model.

 

1. Equations (6)–(8) are central but not contextualized adequately. What are the assumptions of α? How were they calibrated or validated?

 

Re：The parameter "α" in Equation 6-8 mainly comes from previous studies. We have elaborated on the method for obtaining "α" in the supplementary materials, and have listed the corresponding "α" values for each type of forest vegetation in a table.

 

1. Validation with only three flux sites over limited years (2009–2010, 2016) is insufficient to support national-scale claims from 2001–2020. Please broaden validation using more diverse sites or include rigorous cross-validation techniques. Justify site selection and address potential biases from limited spatiotemporal coverage.

 

Re：Due to the certain difficulties in data acquisition, the site validation of the model cannot obtain data support on a longer time scale. We have tried to use the two-year observation data from each site for verification, in order to avoid the accidental nature of the data from a single year.

 

1. The paper states fixed 2001 CO₂ values for some runs, but does not explain the implications on LAI, which is climate-responsive.

 

Re: I am extremely grateful to the reviewers for their constructive suggestions. We consider these suggestions to be very valuable. However, due to the time constraints for rework, we are unable to add simulation scenarios with fixed LAI values to eliminate the sensitivity of LAI to climate. Nevertheless, based on the SURD algorithm, we analyzed the sensitivity of CO2, meteorological factors, O3, etc. to LAI. The results show that CO2 has a certain impact on LAI, but this impact does not lead to significant changes in GPP. We hope that through this method, we can indirectly prove that the influence of LAI on the research results of this study is not very significant, thereby supporting the correctness of the research results.

Reviewer 2 Report

Comments and Suggestions for Authors

·      The LAI input is taken from satellite observations (GLOBMAP), which already reflect some CO₂ and climate-driven vegetation greening. This may confound the attribution of GPP changes in simulation results. Please clarify to what extent LAI variability might bias your interpretation of CO₂ fertilization effects. Could running the model with a static LAI dataset (for sensitivity analysis) help to isolate GPP changes due to atmospheric variables?

·      The α coefficients used to quantify GPP reduction via O₃ exposure are critical. However, the source, calibration process, and uncertainty range of these values are not sufficiently detailed. Please include a table of α values by vegetation type and explain how they were chosen. Were they derived from empirical measurements, literature values, or fitted from site validation?

·      In Abstract, “the multifactor interactions are poorly known” → replace with “remain poorly understood”.

·      Several sites of (CO2) and (O3) are inconsistently formatted; use subscripted notation (CO₂, O₃) throughout the manuscript.

·      Clearer legends and units should accompany figures 2 and 3 on axes (e.g., GPP in gC/m²/year or similar).

·      Figure 5 (SURD analysis) requires further interpretation in the caption—what do the terms “R,” “U,” and “S” mean?

·      Please check for duplicate references (e.g., Ren et al., 2007 is cited twice). Remove redundancy.

·      In conclusion, explain how China should balance CO₂ mitigation efforts with O₃ pollution control.

 

Author Response

We are grateful for the valuable suggestions provided by the reviewers. Regarding the issues you raised, I will address them one by one.

 

1. The LAI input is taken from satellite observations (GLOBMAP), which already reflect some CO₂ and climate-driven vegetation greening. This may confound the attribution of GPP changes in simulation results. Please clarify to what extent LAI variability might bias your interpretation of CO₂ fertilization effects. Could running the model with a static LAI dataset (for sensitivity analysis) help to isolate GPP changes due to atmospheric variables?

 

Re：I am very grateful for the suggestions for improvement provided by the reviewers. We consider this to be very valuable. However, due to the limitations of the rework time, we are unable to add a simulation scenario with a fixed LAI value to eliminate the sensitivity of LAI to the climate. However, based on the SURD algorithm, we analyzed the sensitivity of CO2, meteorological factors, O3, etc. to LAI. The results indicate that CO2 has a certain impact on LAI, but this impact does not lead to a significant change in GPP. We hope that by this approach, we can indirectly prove that the influence of LAI on the research results in this study is not very significant, thereby supporting the correctness of the research findings.

 

 

2. The α coefficients used to quantify GPP reduction via O₃ exposure are critical. However, the source, calibration process, and uncertainty range of these values are not sufficiently detailed. Please include a table of α values by vegetation type and explain how they were chosen. Were they derived from empirical measurements, literature values, or fitted from site validation?

 

Re：The parameter "α" in Equation 6-8 mainly comes from previous studies. We have elaborated on the method for obtaining "α" in the supplementary materials, and have listed the corresponding "α" values for each type of forest vegetation in a table.

 

3. In Abstract, “the multifactor interactions are poorly known” → replace with “remain poorly understood”.

 

Re：Thanks to the reviewers' suggestions, we have revised the abstract as requested.

 

4. Several sites of (CO2) and (O3) are inconsistently formatted; use subscripted notation (CO₂, O₃) throughout the manuscript.

 

Re：Thanks to the reviewers' suggestions, we have checked the subscripts throughout the entire text and corrected this error. We have submitted the revised version of the manuscript. You can view the specific revisions directly in Word.

 

5. Clearer legends and units should accompany figures 2 and 3 on axes (e.g., GPP in gC/m²/year or similar).

 

Re：Since the calculation is based on the percentage loss of GPP, the unit of GPP for each grid point is not provided.

 

6. Figure 5 (SURD analysis) requires further interpretation in the caption—what do the terms “R,” “U,” and “S” mean?

 

Re: We are very grateful for the reviewer's reminder. We have now provided explanations for the variables such as S, U, and R in both the article and the illustrations.

 

7. Please check for duplicate references (e.g., Ren et al., 2007 is cited twice). Remove redundancy.

 

Re: We checked the references. It was because the previous citations did not have distinct labels that Ren et al.'s two papers from 2007 were involved. We have already adjusted the references.

 

8. In conclusion, explain how China should balance CO₂ mitigation efforts with O₃ pollution control.

 

Re: We fully endorse the suggestions made by the reviewers, as they contribute to enhancing the practicality of the article and better align with the main theme of this journal. We have added a small section to explain the connection between emission reduction and control of air pollution.

Reviewer 3 Report

Comments and Suggestions for Authors

In the paper, the BEPS_O₃ model is used to simulate multifactor interactions including influence of ozone and carbon dioxide on gross primary production in different regions of China. The conducted model experiments allow concluding on the CO₂ offset of the plant damage caused by O₃. The data are divided into two groups for different decades and climate features. In addition, the recently proposed SURD analysis is implemented to identify three types of causality, showing synergistic effects of CO₂ and O₃, at least under moderate conditions.

In my opinion, the study is carried out at the level needed for its publication. It is worth checking consistency between different parts of the manuscript because, for example, eddy covariance data are mentioned in the abstract and acknowledgment, but not in the main text. Nonetheless, I do not assume these disadvantages to be critical, and therefore I propose minor revision. Some other comments, predominantly technical, are described in the attached file.

Comments for author File: Comments.pdf

Author Response

Thank the reviewers for their approval of the content of the manuscript. We are grateful for the revisions suggested by the reviewers. We have reviewed each part of the manuscript that needed modification based on these suggestions and made the necessary adjustments. The manuscript we submitted is the revised version, and the reviewers can see all our revisions.

Reviewer 4 Report

Comments and Suggestions for Authors

Dear Qinyi Wang

Corresponding author

Journal: Sustainability

Manuscript ID: sustainability-3736596

Type of manuscript: Article

Title:

CO2 fertilization effect offset the vegetation GPP losses of woodland ecosystems due to surface O3 damage in China

 

Comments

1. Please delete the abbreviations from the title.
2. Avoid the abbreviations in the abstract.
3. Firstly, the full name and then the abbreviation.
4. In abstract, “The results of multi-scenario simulations indicate that the GPP of woodland ecosystems will increase by 1-5% due to elevated CO2.” Please clarify whether the increase is significant.
5. “2016/2010” please revise.
6. In subsection “Simulation protocol”, please write the word Experiment 1 before (E1) studied
7. Again, please revise the abbreviations such as “SURD” and R, U and S.
8. “This study focuses on demonstrating the causal relationship between O3 and CO2 and GPP. Firstly, obtain the causal relationship analysis of all variables through SURD; Secondly, split into R, U and S relationships; Thirdly, extract the variable combinations with the maximum values in R, U and S respectively to determine whether the ids corresponding to CO2 and O3 exist among them. If ids exist, pass out the values. The causal relationship of the specified variable is presented through filtering, eliminating the complex process of plotting and data filtering.” This paragraph transfers into the goal or aim of the study in the introduction section.
9. The English could be improved to more clearly express the research.

Comments on the Quality of English Language

The English could be improved to more clearly express the research.

Author Response

We are grateful for the valuable suggestions provided by the reviewers. Regarding the issues you raised, I will address them one by one.

 

1. The abstract contains vague statements such as “Our simulation study provides a new way of thinking…” without specifying what is novel in the model or findings. Please clearly state what is innovative about this study compared to existing ozone-CO₂ interaction modeling efforts.

 

Re：We further strengthened the expression of the innovative points in the article and revised the wording of the abstract.

 

1. The literature review lacks depth. While multiple citations are included, the discussion doesn’t critically compare previous work or establish a sharp research gap.

 

Re：Thank you to the judges for your suggestions. We have revised the summary section of the paper, striving to highlight the significance of the research. Regarding the issue of insufficient analysis depth, due to the limited revision time, only a few changes were made. We have uploaded the revised Word document in the revision mode, so the reviewers can see our specific revision plans in the revision mode.

 

1. The BEPS\_O₃ model modifications are described too briefly. Key changes to account for O₃ impacts (e.g., incorporation of PODy, α parameters) are not sufficiently explained.

 

Re：In the methodology section, we have added some additional content in order to present our modifications to the BEPS model more clearly. By adding equation (9), it explains how O3 and CO2 interact with photosynthesis, and specifically explains how O3 is coupled into the BEPS model.

 

1. Equations (6)–(8) are central but not contextualized adequately. What are the assumptions of α? How were they calibrated or validated?

 

Re：The parameter "α" in Equation 6-8 mainly comes from previous studies. We have elaborated on the method for obtaining "α" in the supplementary materials, and have listed the corresponding "α" values for each type of forest vegetation in a table.

 

1. Validation with only three flux sites over limited years (2009–2010, 2016) is insufficient to support national-scale claims from 2001–2020. Please broaden validation using more diverse sites or include rigorous cross-validation techniques. Justify site selection and address potential biases from limited spatiotemporal coverage.

 

Re：Due to the certain difficulties in data acquisition, the site validation of the model cannot obtain data support on a longer time scale. We have tried to use the two-year observation data from each site for verification, in order to avoid the accidental nature of the data from a single year.

 

1. The paper states fixed 2001 CO₂ values for some runs, but does not explain the implications on LAI, which is climate-responsive.

 

Re：I am very grateful for the suggestions for improvement provided by the reviewers. We consider this to be very valuable. However, due to the limitations of the rework time, we are unable to add a simulation scenario with a fixed LAI value to eliminate the sensitivity of LAI to the climate. However, based on the SURD algorithm, we analyzed the sensitivity of CO2, meteorological factors, O3, etc. to LAI. The results indicate that CO2 has a certain impact on LAI, but this impact does not lead to a significant change in GPP. We hope that by this approach, we can indirectly prove that the influence of LAI on the research results in this study is not very significant, thereby supporting the correctness of the research findings.

 

 

 

We are grateful for the valuable suggestions provided by the reviewers. Regarding the issues you raised, I will address them one by one.

 

1. The LAI input is taken from satellite observations (GLOBMAP), which already reflect some CO₂ and climate-driven vegetation greening. This may confound the attribution of GPP changes in simulation results. Please clarify to what extent LAI variability might bias your interpretation of CO₂ fertilization effects. Could running the model with a static LAI dataset (for sensitivity analysis) help to isolate GPP changes due to atmospheric variables?

 

Re：I am very grateful for the suggestions for improvement provided by the reviewers. We consider this to be very valuable. However, due to the limitations of the rework time, we are unable to add a simulation scenario with a fixed LAI value to eliminate the sensitivity of LAI to the climate. However, based on the SURD algorithm, we analyzed the sensitivity of CO2, meteorological factors, O3, etc. to LAI. The results indicate that CO2 has a certain impact on LAI, but this impact does not lead to a significant change in GPP. We hope that by this approach, we can indirectly prove that the influence of LAI on the research results in this study is not very significant, thereby supporting the correctness of the research findings.

 

 

2. The α coefficients used to quantify GPP reduction via O₃ exposure are critical. However, the source, calibration process, and uncertainty range of these values are not sufficiently detailed. Please include a table of α values by vegetation type and explain how they were chosen. Were they derived from empirical measurements, literature values, or fitted from site validation?

 

Re：The parameter "α" in Equation 6-8 mainly comes from previous studies. We have elaborated on the method for obtaining "α" in the supplementary materials, and have listed the corresponding "α" values for each type of forest vegetation in a table.

 

3. In Abstract, “the multifactor interactions are poorly known” → replace with “remain poorly understood”.

 

Re：Thanks to the reviewers' suggestions, we have revised the abstract as requested.

 

4. Several sites of (CO2) and (O3) are inconsistently formatted; use subscripted notation (CO₂, O₃) throughout the manuscript.

 

Re：Thanks to the reviewers' suggestions, we have checked the subscripts throughout the entire text and corrected this error. We have submitted the revised version of the manuscript. You can view the specific revisions directly in Word.

 

5. Clearer legends and units should accompany figures 2 and 3 on axes (e.g., GPP in gC/m²/year or similar).

 

Re：Since the calculation is based on the percentage loss of GPP, the unit of GPP for each grid point is not provided.

 

6. Figure 5 (SURD analysis) requires further interpretation in the caption—what do the terms “R,” “U,” and “S” mean?

 

Re: We are very grateful for the reviewer's reminder. We have now provided explanations for the variables such as S, U, and R in both the article and the illustrations.

 

7. Please check for duplicate references (e.g., Ren et al., 2007 is cited twice). Remove redundancy.

 

Re: We checked the references. It was because the previous citations did not have distinct labels that Ren et al.'s two papers from 2007 were involved. We have already adjusted the references.

 

8. In conclusion, explain how China should balance CO₂ mitigation efforts with O₃ pollution control.

 

Re: We fully endorse the suggestions made by the reviewers, as they contribute to enhancing the practicality of the article and better align with the main theme of this journal. We have added a small section to explain the connection between emission reduction and control of air pollution.

 

 

Thank the reviewers for their approval of the content of the manuscript. We are grateful for the revisions suggested by the reviewers. We have reviewed each part of the manuscript that needed modification based on these suggestions and made the necessary adjustments. The manuscript we submitted is the revised version, and the reviewers can see all our revisions.

 

 

 

We are grateful for the valuable suggestions provided by the reviewers. Regarding the issues you raised, I will address them one by one.

 

 

1. Please delete the abbreviations from the title.

 

Re: Thanks to the reviewers' suggestions, we have made the necessary adjustments to the manuscript as requested.

 

2. Avoid the abbreviations in the abstract.

 

Re: Thanks to the reviewers' suggestions, we have made the necessary adjustments to the manuscript as requested. Thank you for the reviewers' suggestions. We have made the necessary adjustments to the manuscript as requested. All the abbreviations in the abstract have been revised except for "CO2" and "O3".

 

3. Firstly, the full name and then the abbreviation.

 

Re: Thanks to the reviewers' suggestions, we have made the necessary adjustments to the manuscript as requested.

 

4. In abstract, “The results of multi-scenario simulations indicate that the GPP of woodland ecosystems will increase by 1-5% due to elevated CO2.” Please clarify whether the increase is significant.

 

Re: In the SURD analysis, we separately calculated the impact of CO2 on GPP, as can be seen in Figure 6d. This analysis should be able to demonstrate a significant positive correlation between carbon dioxide and the growth of GPP.

 

5. “2016/2010” please revise.

 

Re: Thanks to the reviewers' suggestions, we have made adjustments to the expression of the years.

 

6. In subsection “Simulation protocol”, please write the word Experiment 1 before (E1) studied

 

Re: Thanks to the reviewers' suggestions, we have made adjustments to the abbreviated form.

 

7. Again, please revise the abbreviations such as “SURD” and R, U and S.

 

Re: Thank you for the suggestions made by the reviewers. We carefully checked all the abbreviations and made adjustments to them.

 

8. “This study focuses on demonstrating the causal relationship between O3 and CO2 and GPP. Firstly, obtain the causal relationship analysis of all variables through SURD; Secondly, split into R, U and S relationships; Thirdly, extract the variable combinations with the maximum values in R, U and S respectively to determine whether the ids corresponding to CO2 and O3 exist among them. If ids exist, pass out the values. The causal relationship of the specified variable is presented through filtering, eliminating the complex process of plotting and data filtering.” This paragraph transfers into the goal or aim of the study in the introduction section.

 

Re: We are grateful for the reviewer's suggestions on the structure of the manuscript. We have revised the content and structure of the manuscript, and have submitted the revised version in Word format. The reviewer can see all our revisions in the manuscript.

 

9. The English could be improved to more clearly express the research.

 

Re: Due to the short review period, we hope to further utilize MDPI's services to refine the language of the manuscript after submitting it.

 

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors conducted the reviews effectively.