Use of Sentinel-2 Satellite Data for Windthrows Monitoring and Delimiting: The Case of “Vaia” Storm in Friuli Venezia Giulia Region (North-Eastern Italy)

Round 1

Reviewer 1 Report

Dear all,

Thank you for the opportunity to read this interesting paper.

The use of satellites to investigate the impact of extreme weather events, which are becoming increasingly frequent due to climate change, is a very important tool for monitoring their consequences and reducing risks. From this perspective, I think the study makes an important contribution to the scientific literature and is worthy of publication.

I think the work is well structured and the aim is scientifically useful. The methodology is described in detail, the results presented and discussed clearly and comprehensively.

For these reasons, I believe the paper is already ready for publication.

Author Response

Reviewer 1

Dear all,

Thank you for the opportunity to read this interesting paper.

The use of satellites to investigate the impact of extreme weather events, which are becoming increasingly frequent due to climate change, is a very important tool for monitoring their consequences and reducing risks. From this perspective, I think the study makes an important contribution to the scientific literature and is worthy of publication.

I think the work is well structured and the aim is scientifically useful. The methodology is described in detail, the results presented and discussed clearly and comprehensively.

For these reasons, I believe the paper is already ready for publication.

Reply:

We would like to thank the reviewer for the positive comment on our manuscript.

Reviewer 2 Report

The submission provides interesting paper (from methodological as well as practical point of view) on mapping coniferous forests damaged by windthrown using Sentinel 2 data. The language of the paper is clear, easy to understand. Some minor issues are as follows:

Title - specify which Sentinel was used:  Use of Sentinel-2 satellite data for windthrows monitoring and delimiting: the case of “Vaia” storm in Friuli 
Venezia Giulia region (North-Eastern Italy)

Materials and methods:
line 141 - abbreviations "l1" and "l2" are a little bit confusing and can be read as 11 and 12. Consider to use capital letters ("L1", "C1", ...)
line 147 - reclassification of aspect. While GRASS GIS users can be familiar with r.slope.aspect and aspect maps produced with East=0 degrees, other 
readers can be not, expecting 0 degrees = North. An explanation should be provided for reclassification rules.  
line 150 - ... satellite images were checked ... - how it was done? visually?
lines 163-164 - this sentece is confusing. Why only one image? In Table 1 there are 5 scenes from 2017-2018!
line 254 - VID abbreviation not explained. VID = vegetation index differencing?

Results:
line 395 - percentage is strange, why 21 and 89%? % of what? 661 and 1884 is 26% and 74% of 2545.
lines 410-412 - delete paragraph

Discussion:
lines 520-548 - It is questionable whether NIR reflectance after windthrown was influenced predominantly by successions or by wood extraction. Huge 
amount of deadwood could influence reflectance substantially even with the biomass of succession plants (growing among stems) comparable to extracted 
sites. Better subtitle of chapter 4.2 would be "4.2 NIR reflectance after windthrow".   

Conclusions:
line 613 - ommit word "However"
line 614 - I suggest to ommit word "fast" as well - this method is fast only in the sense of labor time, but only rarely can be applied faster than field mapping 
(due to the cloud cover).
lines 623-626 - delete paragraph

lines 637-641 - Data availability statement not provided

Author Response

Reviewer 2

The submission provides interesting paper (from methodological as well as practical point of view) on mapping coniferous forests damaged by windthrown using Sentinel 2 data. The language of the paper is clear, easy to understand. Some minor issues are as follows:

Reply: We thank the reviewer for the valuable comments provided. We addressed all the issues raised in the revised manuscript version.

Title - specify which Sentinel was used:  Use of Sentinel-2 satellite data for windthrows monitoring and delimiting: the case of “Vaia” storm in Friuli Venezia Giulia region (North-Eastern Italy)

Reply:

We modified the title according to the Reviewer’s suggestion.

Materials and methods: line 141 - abbreviations "l1" and "l2" are a little bit confusing and can be read as 11 and 12. Consider to use capital letters ("L1", "C1", ...)

Reply:

Amended

line 147 - reclassification of aspect. While GRASS GIS users can be familiar with r.slope.aspect and aspect maps produced with East=0 degrees, other readers can be not, expecting 0 degrees = North. An explanation should be provided for reclassification rules.

Reply:

As suggested, an explanation on GRASS GIS r.slope.aspect function reclassification rule was provided as follows: “In “r.slope.aspect”, the aspect output raster map indicates the direction that slopes are facing counterclockwise from East: 90° is North, 180° is West, 270° is South, 360° is East.”

line 150 - ... satellite images were checked ... - how it was done? visually?

Reply:

In order to select sites with Picea abies and Abies alba forest cover, therefore without deciduous species as Fagus sylvatica which might have influenced indices response, Sentinel-2 pre-storm images of the autumn season, when broadleaves colour change is evident (i.e. October month), were visually checked. The sentence was rearranged and details were added to clarify the employed methodology as follows (L 148 of the revised version): “The FVG 2017 Habitat Map [20] was used to select forest areas dominated by Picea abies and Abies alba. To avoid areas including deciduous species (i.e. Fagus sylvatica), pre-storm Sentinel-2 images acquired during the autumn season were used to visually check for the eventual presence of spots with colours typical of senescent leaves of deciduous species.”

lines 163-164 - this sentence is confusing. Why only one image? In Table 1 there are 5 scenes from 2017-2018!

Reply:

We agree with the reviewer, the sentence was misleading and thus modified as follows (L163 of the revised version):

“Sentinel-2 images availability during late autumn and winter seasons was strongly reduced because of cloud cover, shadows, and snow presence. Accordingly, for the months immediately after the storm (from November to March), it was possible to retrieve only one image in November (15/11/2018) and one in March (05/03/2019)”.

line 254 - VID abbreviation not explained. VID = vegetation index differencing?

Reply:

VID abbreviation refers to Vegetation Index Differencing, and we clearly indicated this in the abstract (L27 of the revised version) and in line 226 of the revised version of the manuscript.

Results:
line 395 - percentage is strange, why 21 and 89%? % of what? 661 and 1884 is 26% and 74% of 2545.

Reply:

The reviewer is correct, as the percentage was erroneously reported in the text. The correct numbers are 26% for 661 ha and 74% for 1884 with respect to the total of 2545 ha for NDWI VID estimations. The percentage numbers at in the text were corrected accordingly (L395 of the revised version).

lines 410-412 - delete paragraph

Reply:

Amended

Discussion:
lines 520-548 - It is questionable whether NIR reflectance after windthrown was influenced predominantly by successions or by wood extraction. Huge amount of deadwood could influence reflectance substantially even with the biomass of succession plants (growing among stems) comparable to extracted sites. Better subtitle of chapter 4.2 would be "4.2 NIR reflectance after windthrow".   

Reply:

We agree with the reviewer: Indeed, It is beyond the aims of this study to assess whether the cause of NIR reflectance change after the windthrown is attributable to secondary successions rather to wood extraction. Chapter 4.2 discussed not only post storm NIR changes, suggesting the establishment of secondary successions, but also other dynamics of vegetation indices occurred in the short period after the storm, namely the post-storm increase in indices variability and the indices values drop in L1 and L2 after batch cleaning. For these reasons, we modified the subheading of section 4.2 in “Spectral reflectance of vegetation after windthrow”

Conclusions:
line 613 - ommit word "However"

Reply:

Amended

line 614 - I suggest to ommit word "fast" as well - this method is fast only in the sense of labor time, but only rarely can be applied faster than field mapping (due to the cloud cover).

Reply:

Amended. We agree with the reviewer that the meaning of fast, when referred to NDWI8A VID method, could be misleading since, although the computation is rather fast but as correctly pointed out by the Reviewer, most of the time clouds prevent satellite multispectral images acquisition.

lines 623-626 - delete paragraph

Reply:

Amended

lines 637-641 - Data availability statement not provided

Reply:

We thank the reviewer for the report, data availability statement is now provided in the appropriate section according to MDPI polices as follows: “Sentinel-2 data presented in this study are openly available at Copernicus Open Access Hub https://scihub.copernicus.eu/. The digital elevation model (DEM) of Friuli Venezia Region is available at: https://cgiarcsi.community/data/srtm-90m-digital-elevation-database-v4-1/. Windthrows areas delimited by the Friuli Venezia Giulia Region Forest Service are available upon request.”