NDVI or PPI: A (Quick) Comparison for Vegetation Dynamics Monitoring in Mountainous Area
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThis study reports a comparative analysis between PPI and NDVI for monitoring vegetation dynamics in mountain (Alpine) ecosystems.
Overall, this is a nice and well written article, but analysis is poor and there is a lack of concreteness and consistency in the results. All the analyses are well-developed from a qualitative point of view, but they are not supported by numbers with quantitative analyses. Moreover, the final goal of the study is not clear. Which is the final application of this study? What are these results useful for?
Point-by-point comments are reported below:
Abstract: lines 30-32, is this a summary of conclusions?
Introduction is well written and clear. However, the aim of the article is not well presented. The reader can understand that in the article a comparison between PPI and NDVI will be reported, but this can be extended. Which are main research questions? On which aspects will the comparison focus most? Why did the authors investigate PPI and NDVI only? Are there any other indices in use in this context? Which application/field will benefit from these results? In my opinion, the authors could add few lines to the Introduction and giving more insights on their research flow.
2.2.1. Satellite data access and processing:
- Did the authors use any cloud cover thresholds while downloading images?
- Why did the authors use images for the years 2018-2022 for PPI and NDVI, but years 2017-2021 for mapping shrublands?
2.2.2. Vegetation cover
- Typo in line 158 “Finaly”
- Shrublands mapping: how many images per year? Are there enough images to cover the whole study area and to compute median? I can imagine that cloud cover could be a limiting factor in that period of the year.
- Line 176: Bayle, et al. (2024), is this reference 47?
- Lines 181-183: is this issue to be considered as a limitation of your study? Non-Ericaceae shrublands extent can be considered neglectable in your study area? Can you provide a rough estimate of non-Ericaceae vs Ericaceae proportion in your study area?
2.2.3. Vegetation indices:
- DVImax and DVIs are site specific values that you calculated, or values extracted from the literature?
- Lines 224-240: In my opinion, this paragraph would fit better in the Introduction than in the methods section.
2.2.4. PPI & NDVI time-series...:
- Lines 242-250: In my opinion, this paragraph is verbose and not needed, as time-series are well-known in the remote sensing field.
- Line 261: any reference for terra R package?
- The authors should uniform the word “time-series” (not time series)
3. Results:
- Figure 4 and 5: to increase readability, the authors could add minor grid lines on the x-axis of bottom charts
- Figure 6: add legend for PPI and NDVI lines
- Lines 329-332: as the authors did not provide any insights on the comparison assessment in the methodology section, better to add few references on Shapiro test and Spearman correlation here.
4.1 Time-series of the PPI and NDVI...
- Lines 353-355: which results support this statement?
- Lines 366-374: I found quite difficult to understand these statements, as the authors did not report any ground truth values. In the study, only qualitative analysis is presented, but quantitative results are missing. Maybe, the authors could provide a "not-quick" comparison.
4.2. Seasonality parameters retrieval:
- Lines 376-380: It is not necessary to explain the acronyms. The authors have already explained them. In addition, what did the authors mean with “consistency”? From my point of view, “consistency” can be gathering similar information from the two indices, and I do not see this (e.g., grasslands NDVI Length 2018 > Length 2022 vs grasslands PPI Length 2018 < Length 2022)
- Regarding snow cover, the authors pointed out many times that NDVI signal can be highly affected by snow cover. However, their findings are extracted from literature review, as in their study there aren’t any clear evidences. Have they compared their results with a snow cover map? Have they considered the possibility to apply a snow probability mask during image pre-processing together with cloud probability mask?
4.3 Limitations:
- Lines 403-411: the authors mentioned here to use Level 2A data to improve the accuracy of this study. First, what did they mean with accuracy? And, did the authors used Level 1-C imagery? It is not clear, because in the methodology this information is not reported.
- Lines 413-415: how can the authors say that? The authors didn’t perform a validation of the shrublands map
4.4 Contributions and perspectives
- Lines 431-441 repetition, already reported in the Introduction
- Lines 449: typo (ii)
- Lines 449-450: the authors recommend “performing pixel-resolution analysis for further investigations”. I wonder which is the relationship between this recommendation and their study. How did they derive this from their results?
Overall, in the discussion section, the authors try to rely too much on literature to explain their findings, but clear evidences of their outcomes are missing.
Author Response
Please find in the attached Word document all our answers to Reviewer #1
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for Authors1. The last paragraph of the Introduction generally outlines the main structure of the study, detailing the primary content included. The Conclusion section contains some elements of discussion and requires adjustment and revision.
2. The study area section should include additional information on the climate, hydrology, soil, and vegetation types of the research area.
3. In Figure 1, the annotation in the upper-right corner should be either completed or the incomplete annotation removed. The scale unit should be in km rather than Km. Additionally, the scale units in Figures 1 and 3 should be consistent.
4. Elevation has already been defined as the height above sea level. Thus, it is unnecessary to emphasize the unit as meters above sea level (m a.s.l.).
5. First-time abbreviations in the manuscript should be spelled out in full. Briefly introduce the content of Figure 2. The term 'high resolution layers' in Figure 2 is unclear and should be specified.
6. Line 138, the autumn period is defined as from September 1st to November 1st. Generally, autumn in the Northern Hemisphere spans from September to November. Please verify if there was a typographical error.
7. Lines 245-249 mention the revisit intervals of Landsat and Sentinel-2 again, which duplicates information presented earlier in the manuscript.
8. In section 3.1, regarding the vegetation cover map, the authors classified shrublands; the classification accuracy needs to be detailed.
9. In Table 1, the item listed as "land cover types" should be corrected to "area."
10. In Table 2, the phenology parameters such as POS (Point of Start) extracted from NDVI and PPI show significant discrepancies. These differences should be discussed in the discussion section.
11. SOS in 2022 from NDVI is recorded as 38 (DOY), which is evidently incorrect. During this time, Europe is in winter, making it highly unlikely for outdoor vegetation to begin growing. The phenological parameter extraction results need to be validated.
12. In the Conclusion section: The statement that PPI had a nearly linear relationship with Leaf Area Index (LAI) is not a finding of this study.
Author Response
Please find in the attached Word document all our answers to Reviewer #2
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsThe manuscript was carefully revised by the authors, also taking into account my previous comments. Befor epublication, I would like to ask to authors to rephrase the final part of the introduction (lines 163-168) by removing question mrks and including indirect speech.
Finally, I would like to encourage the authors to go further and address remained open issues in a following paper.
Author Response
Please find our answers in the attached Word document
Author Response File: Author Response.pdf
