A Review of Spectral Indices for Mangrove Remote Sensing
Round 1
Reviewer 1 Report
The manuscript has (1) examined and categorizes spectral indices used in publications related to mangrove remote sensing; (2) assessed their applications in the study of mangrove ecosystems; and (3) proposed future directions for the application of additional spectral indices in mangrove remote sensing. This study helps to answer two main specific scientific questions: (1) What spectral indices have been applied and proven effective for mangrove remote sensing? and (2) Which indices are the potential instruments for understanding spatio-temporal patterns in mangrove ecosystems.
The manuscript is well written and quite well for the reader to follow. The authors have introduced an interesting search strategy and the most recent data analysis approach to understand and organize earlier findings of mangrove studies (i.e, Section 2). The authors have also put so much effort into overview of spectral indices used in mangrove remote sensing in (1) the geographical extent of mangrove studies using spectral indices, (2) categorizing the use of spectral indices for mangrove ecosystems, and (3) evaluation of spectral indices applications in mangrove remote sensing (i.e., Section 3 & 4). This detailed information shows a clear picture on what are the things going on, where, what and how are the spectral indices used in mangrove remote sensing. Moreover, the potential of optical spectral indices for mangrove remote sensing for long-term mangrove monitoring with the use of time series-based approaches in relation to driving factors, fusion of images from multiple sensors was also addressed (i.e, Section 5). This kind of deeper analysis/research brings more insight into the use of advanced technology of remote sensing data processing and analyzing for space- and air-borne based mangrove forest monitoring.
To conclude, the manuscript shows a good contribution to the field and would be of interest to many readers who wish to undertake similar research topics. Therefore, I would recommend publishing the work.
Author Response
Dear Reviewer,
We would like to thank the reviewer for carefully reading our paper, providing insightful comments, and accepting our manuscript for publication.
If you need any further information or clarification, please do not hesitate to contact me.
Sincerely,
Author (s)
Reviewer 2 Report
The study entitled "A Review of Spectral Indices for Mangrove Remote Sensing" provides interesting study, recommended for publication incorporating the following suggestion:
In introduction section some sentences need references
Author Response
Dear Reviewer,
We would like to thank the reviewer for carefully reading our paper and for providing insightful comments. We have provided additional information in accordance with your suggestion. Changes in the manuscript have been highlighted in red. Please, find our revised version uploaded to have further information. If you need any further information or clarification, please do not hesitate to contact us.
Author (s)
Reviewer 3 Report
Very well-structured and developed review of the application of remote sensing spectral indices for Mangrove studies.
References are appropriated and the authors were able to systematize the diversitity of available techniques, producing a sound contribution.
Author Response
Dear reviewer,
We would like to thank the reviewer for carefully reading our paper, providing insightful comments, and accepting it for publication.
If you need any further information or clarification, please do not hesitate to contact us.
Sincerely,
Author (s)
Reviewer 4 Report
Thuong V. Tran and colleagues wrote a review article on remote sensing indices used in mangrove studies. Authors focused on multispectral indices published between 1996 and 2021. A general introduction describes mangrove biogeography and ecological services. The introduction ends with two major questions: “what spectral indices have been applied and proven effective for mangrove remote sensing?” and “which indices are the potential instruments for understanding spatiotemporal patterns in mangrove ecosystems”. Their searching strategy is presented in a second main section. They used Web of Science (WoS) and the Scopus platforms with keywords presented in Table 1 and finally found 195 research papers. R-statistical and VOSviewer software packages were used to analyse these references. The third section presents indices. Authors propose a classification of indices as follows (1) Visible and near infrared indices, (2) Visible and red edge indices, (3) Visible indices of airborne systems, (4) Specific indices for mangrove mapping. The fourth section presents the main applications for which these indices have been applied and tries to evaluate their performance. Finally, authors propose future directions.
Major comments
I think the introduction misses a real story line. Authors should use connecting words and sentences between paragraphs.
A paragraph on mangrove spectral characteristics compared with other types of vegetation is missing. I think it would be very valuable for, at least, the discussion section to write some lines and show more figures of spectral signatures.
In my opinion, the two questions are the same question. What do authors mean by “instrument”?
I think that the classification choice has many flaws. First, the paragraph from line 277 to 293 is unclear. Two almost similar categorizations are proposed, it’s confusing. In my opinion, differentiation between aerial and satellite indices is not relevant nowadays. I would have for example classified in (1) indices using visible bands only, (2) indices using visible and infra-red bands, (3) regular indices that are adjusted by non-spectral factors, and (4) indices specifically designed for mangrove studies. Second, there are indices which use visible, red edge, NIR and SWIR together. Hence, they cannot logically enter in the author classification. For example, table 7 shows indices that are not entering in the classification “The indices in the Visible, Near Infrared (NIR), and Shortwave Infrared (SWIR) bands”.
In my opinion, the most interesting part of the paper is the presentation of indices specifically developed for mangrove studies. This part is the central part, originality and main reason of this paper and should be more developed, amphasized and highlighted.
The section 4 begins with “As mentioned in section 3, a variety of spectral indices is used for mangrove ecosystems, but each index has its strengths and limitations”. I didn’t easily find strengths and limitations for each index. The sentence LN326: “selecting a vegetation index for every mangrove application requires determining the factors that influence these indices”. Authors should maybe provide a more thorough analysis of the factors that influence the performance of each index. I also suggest a table to summarize these factors.
I don’t feel comfortable with the repeated use of accuracy % in section 4. Each study may have a different methodology to measure accuracy. Hence, I think it’s irrelevant to compare studies accuracy in that case. Moreover, the accuracy of a study is not only determined by the index used.
Results of the bibliometric analysis are presented in section 5, however this section is dedicated to discussion and future directions. I think these results would be more suitable in one of the previous sections.
Many ideas presented in section 5 have been already presented in previous sections.
Can you develop what are the sensor effects and also the effects of acquisition parameters.
Figure 5: “mangroves” and “mangrove” should be merged together. Why words are organized like that? What does mean the distance between words?
Minor comments
LN48 Authors must precise that they are lignified plant communities, because other not lignified vegetation types can develop in costal zones (salt marshes for example)
LN50 South of Japan instead of North. I found 30.4°N in the latest release of the Global Mangrove Watch.
LN50 32.4°N for Bermuda
LN51 38.85° for South of Australia
LN52 For consistency cite the highest in latitude for each hemisphere or cite for the highest in latitude for each continent.
LN55 Actually it seems to be the number of frosen days in the year that plays on mangrove presence at high latitudes. See Cavanaugh et al. (2014) for instance.
LN56 It's not clear how sea level rise can affect mangrove biophysical properties ... Please define biophysical properties.
LN59 You sort regions by mangrove cover or mangrove species diversity ? These are two different things.
LN60 What side of Africa east or west ? It's important.
LN83 Please see and cite Tomlinson The botany of mangroves for the number of species.
LN83-86 Please don't compare genera and species. Genera are shared among AEP and IWP zones but not species !!
LN85 Avicennia germinans is the white one in this region.
Laguncularia is the grey one. Please correct.
LN94 the best reference is Lee et al. (2014)
LN97 See and cite articles from Science mag, there are many
LN103 cite a more appropriate article from Donato (2011)
LN137 Add image acquisition parameters
LN260 please explain feature scaling
LN263 function instead of service.
LN264 what is "the process" you refer to ? please explain.
LN277 Are satellite and aerial sensors different ? Does this difference justifies different indices ? I am not shure of that.
LN284 It's obvious. You can delete this sentence.
LN286 are your shure of that ? absolutly nobody has worked on vegetation with SWIR band ??
LN353 20 m for red edge bands not 10 m
Table 4 there is PIR band in this table entitled 'The used indices with visible and red-edge bands ' !
LN 374 it depends of countries. In some cpountries UAV flights are not regaulated.
LN393 1 not 9
LN459 to what refers "These" ? NDVI and EVI studies ? It's unclear. If yes, why only focusing on applications which used NDVI and EVI ?
LN581 Authors should clearly highlight here that NDVI is not suitable to proedict high vegetation biomass. I think this section is not enough devlopped. Authors could provide some plots showing the relation between parameters (LAI, AGB, etc.) and indices.
LN635 you have already said that before in the text
LN661 it's already said in section 3
LN802 is the reference well written ?
Author Response
Dear Reviewer,
We would like to thank the reviewer for carefully reading our paper and for providing insightful comments. We have done our best to address your suggestions and concerns. All the changes in the manuscript have been written in red to highlight them. We hope that it is now appropriate for the standards of Remote Sensing. Please see the attached file to have further response. If you need any further information or clarification, please do not hesitate to contact us.
Sincerely,
Author(s)
Author Response File: 
Author Response.pdf
Round 2
Reviewer 4 Report
Authors have conscientiously responded to all of my comments. I found other minor elements to quickly improve :
LN 55-56 "Actually" is repeated. You may delete one.
LN 144 "The highest spectral signature" is not straitforward to understand. You mean the highest refectance.
Section 3.1 Both VNIR and NIR appear together in the text. You may use just one of them all along the paper.
Author Response
Dear reviewer,
We would like to thank the reviewer for carefully reading our paper and for providing insightful comments. We have done our best to address your suggestions and concerns. All the changes in the manuscript have been written using Track Change Mode to highlight them. Please, find the attached file to see further our revision. If you need any further information or clarification, please do not hesitate to contact us.
Sincerely yours,
Author (s)
Author Response File: Author Response.pdf
