Exploring Sensitivity of Phenology to Seasonal Climate Differences in Temperate Grasslands of China Based on Normalized Difference Vegetation Index

Round 1

Reviewer 1 Report

The authors, in a manuscript entitled 'Unraveling the spatiotemporal variability of seasonal climate's impact on phenology in China's temperate grasslands,' investigated the inter-annual variability of phenology and spatial variability of grasslands in selected regions of China based on the normalized difference vegetation index (NDVI) and weather data obtained from 2001 to 2020. The relationships between vegetation phenology and climate were evaluated in the time interval between the start of the season (SOS) and the end of the season (EOS). The manuscript's topic is very topical due to the changing parameters of the earth's climate due to its warming.

The introduction of the manuscript and the literature review are good. Other chapters are also good. The authors indicated where and how they obtained information for the study. It was pointed out where grassland locations were selected. In the manuscript, a statistical analysis of the correlation of phenology about seasonal climate was carried out, based on the terms start of the season (SOS) and end of the season (EOS), and as a result of the interpretation of the obtained results, conclusions are presented. I have no negative comments on the conclusions and methods used in the study. I also have no comments on the graphical illustrations and diagrams concerning the results.

Overall, I have no significant comments on the content and scientific content of the manuscript.

One of my minor comments is that the authors should ensure, by making minor corrections to the text, that all abbreviations of terms are preceded by the whole, exact name of the term, which is placed before the abbreviation. Using the abbreviation and the full term name interchangeably in the manuscript is a good idea. In this way, the manuscript's content is more accessible for the reader, who is only sometimes an outstanding expert on the subject matter described.

Author Response

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Reviewer 2 Report

Dear Authors,

I have completed the review of your manuscript, which focuses on the complex relationship between climate variability and vegetation phenology in China's temperate grasslands. Below, I summarize my criticisms and suggestions for further improving the depth and scientific accuracy of your study.

1. Data Sources and Pre-processing: The paper outlines the data sources and preprocessing methods used in the study. While the use of FLUXNET data for site validation and various datasets for climatic data are mentioned, a more detailed description of the methodology for data selection, quality control, and criteria for excluding certain data points could provide more clarity and allow for reproducibility of the study.

2. Statistical Significance and Trend Analysis: The finding that over 89% of pixels showed no significant trend raises questions about the magnitude and ecological relevance of phenological changes in response to climate variability (in section 3.2. The spatiotemporal changes of vegetation phenology). It would be beneficial to provide a more fine interpretation of these results, considering both statistically significant and non-significant trends, and their potential ecological implications. The study uses non-parametric statistical methods for trend analysis and multiple regression models to explore the relationship between phenology and climate variables. So, addressing how auto-correlation and spatial heterogeneity were managed in the dataset could add to the robustness of the analysis.

3. Sensitivity Analysis: While you have identified regional variability in phenological sensitivity to climate variables, a deeper exploration of the underlying ecological and physiological mechanisms driving these differences would greatly enrich the discussion. Additionally, considering other factors such as land-use changes and species composition shifts could provide a more comprehensive understanding of the observed patterns.

4. Data Validation and Cross-Validation: A detailed account of the validation process for NDVI data, including error analysis and the agreement between different datasets, would enhance the credibility and reproducibility of your findings.

Lastly, the manuscript does not include a direct comparison with other studies/research, which could have enriched the context and underlined the highlighted of its findings within the broader field of climate change impacts on vegetation phenology. Including such comparisons could improve the manuscript by highlighting its contributions and identifying potential areas for future research.

Author Response

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Reviewer 3 Report

General comments:

The authors investigate the impact of various factors on SOS and EOS, using only single-factor models based on linear regressions. However, to provide a more comprehensive analysis, it is necessary to employ multiple regressions, factor analysis, and principal component analysis.

There is no test for multicollinearity and the nature of the distribution of the independent variables.

It is not clear on what scientific criteria the choice of the object is based. It is very large, diverse and with opposite trends in SOS and EOS change (as the authors themselves show).

It is unclear how reliable the predictions based on the results are.

Specific comments:

Lines 2-3. The title is unfortunate. It should be stated that the phenology is derived from NDVI. In addition, a strange term in the title is "Unraveling".

Line 35. It is advisable to exclude the term "sensitivity" from the keywords.

Lines 158-171. The results of the methods described are not available in the paper.

Line 199. Fig. 2. Figure (2a) shows that SOS in the south comes much later than in the north. How do you explain this?

Line 199. Fig. 2. Figure (2b) shows that some grasslands start their vegetation period on day 180. That's the end of June. How is this possible?

Line 199. Fig. 2. (2b, 2c, 2e, 2f) What is the point of summarising these data over 20 years for such a large and diverse area? What scientific conclusions can be drawn from this?

Line 199. Fig. 2.  Judging by the figure (2d) some grasslands have EOS on the 45th day of the year. What could be the reason?

Fig. 3-5. Very low value of R² and slope angle. This casts doubt on the results obtained. This means that using a different data preprocessing technique or adding additional data for another year could change the results of the study very dramatically.

Author Response

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Author Response File: Author Response.docx

Reviewer 4 Report

General Evaluation:

The manuscript titled "Unraveling the spatiotemporal variability of seasonal climate's impact on phenology in China's temperate grasslands" provides an insightful analysis of the relationship between seasonal climate and vegetation phenology in temperate regions of China. The study investigates the interannual variability of grassland phenology and its spatial variation for 20 years using satellite-derived data and weather data. The findings shed light on the sensitivity of phenology to various climate factors and highlight the divergent effects of seasonal warming and precipitation on phenology due to differences in local climate conditions.

Areas for Improvement:

Clarity in Methodology: The methodology section could be expanded to provide more clarity on the data processing techniques and statistical analyses employed in the study. This would enhance the reproducibility of the findings.

Discussion of Limitations: The manuscript would benefit from a discussion of the limitations associated with the methodology and data sources used. Addressing these limitations would strengthen the interpretation of the results and provide context for future research directions. It is recommended to add some prospects, such as recent research on using SIF remote sensing for phenology monitoring, which may improve grassland photosynthetic phenology monitoring. i.e.

Tu, Z., Sun, Y., Wu, C., Ding, Z., & Tang, X. (2024). Long-term dynamics of peak photosynthesis timing and environmental controls in the Tibetan Plateau monitored by satellite solar-induced chlorophyll fluorescence. International Journal of Digital Earth, 17(1), 2300311.

Wu, L., Zhang, Y., Zhang, Z., Zhang, X., Wu, Y., & Chen, J. M. (2024). Deriving photosystem-level red chlorophyll fluorescence emission by combining leaf chlorophyll content and canopy far-red solar-induced fluorescence: Possibilities and challenges. Remote Sensing of Environment, 304, 114043.

Anniwaer, N., Li, X., Wang, K., Xu, H., & Hong, S. (2024). Shifts in the trends of vegetation greenness and photosynthesis in different parts of Tibetan Plateau over the past two decades. Agricultural and Forest Meteorology, 345, 109851.

Overall, the manuscript presents valuable findings on the spatiotemporal variability of grassland phenology in response to seasonal climate variations in China's temperate regions. Addressing the aforementioned areas for improvement would further strengthen the manuscript and its contribution to the field of climate change research.

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

The authors have fully implemented the revisions I suggested. As a result, the article has gained a more comprehensive and impactful quality in its current form.
Thank you.

Reviewer 3 Report

Dear authors!

Thank you very much for answering all my questions. Although there is still room for further discussion, I recommend the revised version of the manuscript for publication.