Remote Sensing-Based Revegetation Assessment at Post-Closure Mine Sites in Canada

Round 1

Reviewer 1 Report (New Reviewer)

General comments.

This study is highly informative and relevant to the field of sustainable mining. The use of Landsat satellite images to evaluate long-term land cover changes at post-closure mine sites in Canada is a valuable contribution to the literature. The findings demonstrate that active reclamation efforts significantly improve vegetation recovery, while passive revegetation typically shows slow progress. The study's quantification of mine site reclamation behavior and sustainable practices can inform evidence-based resource management practices. The topic seems some interesting and is somehow of importance. The manuscript can fit the scope of Sustainability after major revision. My specific comments are as follows.

1.        line 62-65. The article highlights the advantages of remote sensing technology over ground-based surveys in monitoring land cover changes, such as better spatial coverage, less labor, and lower financial costs. However, the article fails to acknowledge the limitations, scale applicability issues, and potential errors associated with remote sensing technology.

2.        The Regrowth Index (RI) analysis method is stated as the novelty of this study. Regrowth Index (RI) analysis for quantified assessment the speed and extent of landscape and vegetation recovery at the target mine sites over time.  However, the simple index including the landscape and vegetation recovery levels is set as input. As known, the ecologically significant data and indicators to illustrate the success and sustainability are sensitively affected by the environmental impact factors. The authors showed give more evidence on the accuracy of the Regrowth Index (RI) by utilizing the simple input variables.

3.         Additionally, the issue mentioned above may be a potential issue with the data and conclusions presented in this article. At small scales, using remote sensing technology alone without combining field investigations and visual interpretation may lead to significant data errors.

4.        In the description of cases such as Wapisiw Lookout and Gateway Hill, it is suggested that the article provide more ecologically significant data and indicators to illustrate the success and sustainability of these projects.

5.        Lines 156-159. Although the site is small in size and the distortion of the satellite image is limited, it is recommended to perform conventional pre-processing and verification before starting the analysis to improve the validity of the data.

6.        Before introducing the NDVI analysis results, it is suggested that the article briefly explain the significance and impact of NDVI trends on environmental reclamation, so that readers can better understand the value of the following data.

7.        It is suggested that the article further explore the differences and reasons for the effectiveness of reclamation in different mining areas, so that readers can better understand the difficulties and challenges of reclamation in different mining areas

Author Response

General comments.

This study is highly informative and relevant to the field of sustainable mining. The use of Landsat satellite images to evaluate long-term land cover changes at post-closure mine sites in Canada is a valuable contribution to the literature. The findings demonstrate that active reclamation efforts significantly improve vegetation recovery, while passive revegetation typically shows slow progress. The study's quantification of mine site reclamation behavior and sustainable practices can inform evidence-based resource management practices. The topic seems some interesting and is somehow of importance. The manuscript can fit the scope of Sustainability after major revision. My specific comments are as follows.

1. line 62-65. The article highlights the advantages of remote sensing technology over ground-based surveys in monitoring land cover changes, such as better spatial coverage, less labor, and lower financial costs. However, the article fails to acknowledge the limitations, scale applicability issues, and potential errors associated with remote sensing technology.

Response: The limitations are now described and discussed in the revised manuscript (Line 397-410): “Notice that the remote sensing-based reclamation evaluation and monitoring was unavoidably impacted by certain limitations. One limitation is that the spatial resolu-tion of Landsat imagery made it difficult to map any finer scale (< 30 m) land cover change. To detect the fine-scale land cover and land changes, the Landsat images need to be downscaled and used in combination with the field survey to improve the accu-racy of remote sensing-based evaluation. Alternatively, very high spatial resolution satellite (such as SPOT and WorldView series) images are recommended for fine-scale biodiversity monitoring, e.g., [21], although they are usually not freely available to the public. Another limitation is that the Landsat imagery detection (optical remote sens-ing technology) is subject to weather conditions, such as cloud cover, which could cause some gaps for long term monitoring (e.g., those as shown in the NDVI sequences in Figure 2 and in [56]). In the future, microwave remote sensing (e.g., Synthetic Aper-ture Radar), which is less susceptible to cloud cover, can be jointly used with the cloud-free Landsat imagery to provide a better temporal coverage.”

 

2. The Regrowth Index (RI) analysis method is stated as the novelty of this study. Regrowth Index (RI) analysis for quantified assessment the speed and extent of landscape and vegetation recovery at the target mine sites over time.  However, the simple index including the landscape and vegetation recovery levels is set as input. As known, the ecologically significant data and indicators to illustrate the success and sustainability are sensitively affected by the environmental impact factors. The authors showed give more evidence on the accuracy of the Regrowth Index (RI) by utilizing the simple input variables.

Response: It is acknowledged that “the ecologically significant data and indicators to illustrate the success and sustainability are sensitively affected by the environmental impact factors”. In fact, the purpose of using RI was minimizing this type of impacts such as those from changes in weather/climate, hydrology, solar radiation, soil conditions, etc., over the time since those environmental factors are expected to have very similar effects on the disturbed (mine site) and undisturbed (reference) areas, which are cancelled out during the RI calculation. In this study, the accuracy of RI was mainly verified against the well-known RI behavior demonstrated in previous studies.  Studies (e.g., Reference [20]) have demonstrated that the initial RI (prior to the revegetation/regrowth) was negative and then was inclined toward zero as the substantial vegetation re-growth occurred in the disturbed area. The RI trends over the four actively revegetated sites (Gateway Hill, Wapisiw Lookout, Highmont, and Stanrock) agree well with the finding, which provided evidence for the accuracy of the Regrowth Index (RI). Given the lack of field survey, it is not practical to evaluate the accuracy of RI using the ground-based observations in this study. 

This is now clarified in the revised manuscript (Lines  348-354; 201-205).

 

3. In the description of cases such as Wapisiw Lookout and Gateway Hill, it is suggested that the article provide more ecologically significant data and indicators to illustrate the success and sustainability of these projects.

Response: The relevant information is added (Lines 110-112; 118-121)

“During 2009-2010, approximately 65, 000 truckloads of reclamation materials were applied to the surface of the pond. In 2010, over 620,000 trees, shrubs, and grasses were planted over the site [38].”

“The Gateway Hill site was certified as a forested hill area (~104 ha) extending as high as 40 m above the highway north of Fort McMurray [37, 39]. The ecosystems mainly include spruce/aspen forest, jack pine forest, grassland, and wetland [37, 39].”

4. Lines 156-159. Although the site is small in size and the distortion of the satellite image is limited, it is recommended to perform conventional pre-processing and verification before starting the analysis to improve the validity of the data.

 

Response: In this study, all satellite images were taken from the Landsat Collection 2 Level-2 atmospherically corrected product. The conventional pre-processing (radiometric/atmospheric correction and geometric correction) are not needed since these corrections were conducted when producing the Collection 2 Level-2 products (see https://www.usgs.gov/landsat-missions/landsat-collection-2 & https://www.usgs.gov/media/files/landsat-8-9-collection-2-level-2-science-product-guide)

 

5. Before introducing the NDVI analysis results, it is suggested that the article briefly explain the significance and impact of NDVI trends on environmental reclamation, so that readers can better understand the value of the following data.

Response: The information is now provided (Line 210-214).

 

7. It is suggested that the article further explore so that readers can better understand the difficulties and challenges of reclamation in different mining areas

Response: Due to the lack of the field survey and sampling data, it is not possible to conduct a full investigation on the differences and reasons/factors (e.g.  climate, hydrology, land quality, vegetation behavior, etc.) for the effectiveness of reclamation across different mining areas, which is beyond the scope of this study. However, the remote sensing-based assessment of mine site revegetation and ecological restoration presented in this study provided a consistent explanation for the impacts of different revegetation types/activities.  The active reclamation activities improved the vegetation restoration by modifying the climate and hydrology condition (e.g., irrigation), land quality (adding new topsoils), vegetation behavior (reseeding with various tailored plant species), etc. (see Line 363-367). 

Reviewer 2 Report (New Reviewer)

The authors proposed a manuscript titled “Remote Sensing-Based Revegetation Assessment at Post-Closure Mine Sites in Canada”. The authors highlighted the important topic as revegetation at post-closure mine sites is critically important for sustainable mining and natural resource management. Therefore, the manuscript may be suitable for publication after some major changes. I have included necessary comments to obtain the additional information for improvement in the manuscript.

Abstract

Line 15: Add, how many satellite images were downloaded, for which year? for which satellite?

Line 18: “to assess the speed and extent……..”. include the results of the speed analysis for land use and land cover changes.

Line 19: Add quantitative results of the vegetation recovery for mine sites.

Lines 23-24: How this work confirms that that active reclamation and revegetation at post-closure mine sites is critically important in sustainable mining? Nor clear. Revised these sentences considering the results of the study.

Include one or two lines for inter-comparison of multiple mine sites.

Introduction

“However, although significant progress has been made in remote sensing-based assessment of mine site revegetation and ecological restoration in Canada, most of the evaluations focused on only the reclamation and revegetation behavior at a single mine site”.

Include some latest research studies conducted in different parts of the world and well as in the studies regions which shows the reclamation and revegetation behavior at a single mine site using the remote sensing techniques.

Materials and Methods

Line 147: Need to verify the ending date for Landsat 5 and launching date for Landsat 8.

Line 151: Include, how many images were downloaded for each mine sites?, also included for which duration images downloaded.

Line 156: Include more detail about the Python-based preprocessing of the downloaded Landsat images.

Line 172> add classification year, also include detail about the random forest method for classification

Line 183: Add methodology for the for inter-comparison of multiple mine sites, as objectives were set in the introduction section for inter-comparison of multiple mine sites

Results section needs more interpretation as Figures 3, 4, 5, 6, 7, 8, do have not enough discussion.

Figure 3a, Figure 6c, Figures 8a, 8b, 8c not present in the text.

Add conclusions as a separate section and conclusion should flow from the results of the present study.

See Comments

Author Response

The authors proposed a manuscript titled “Remote Sensing-Based Revegetation Assessment at Post-Closure Mine Sites in Canada”. The authors highlighted the important topic as revegetation at post-closure mine sites is critically important for sustainable mining and natural resource management. Therefore, the manuscript may be suitable for publication after some major changes. I have included necessary comments to obtain the additional information for improvement in the manuscript.

Abstract

Line 15: Add, how many satellite images were downloaded, for which year? for which satellite?

Response: The satellite is Landsat, which is provided at Lines 14 and 17.  Approximately 25 to 38 Landsat images between 1984 (or the respective closure year when it is later than 1984) and 2021 were downloaded for each site (Line 163-165). Due to the word count limit (200 words) of abstract, the information cannot be provided in the abstract, but it does not impact the understanding of the key techniques used in th study.  In an abstract, the key methodology should be briefly described without going into excessive detail.

Line 18: “to assess the speed and extent……..”. include the results of the speed analysis for land use and land cover changes.

Response: The study mine sites presented different recovery speeds (see Table 2). Again, since the abstract is limited to no more than 200 words, it is not possible/necessary to include all the detailed analysis results. The key findings (Line 22-23) “A significant vegetation recovery was quantified for the mine sites that have experienced active reclamation activities. In contrast, the post-closure mine area undergoing only the passive revegetation typically demonstrated a slow and minor increase in vegetation over time. The actively revegetated mine sites can typically be restored to a vegetation-cover level that equals or better than the pre-mining situation.” are clearly provided in the abstract, which should be sufficient for understanding the revegetation speed and extent differences across the study sites.

 

 

Line 19: Add quantitative results of the vegetation recovery for mine sites.

 

Response: Please see the reply to the comment above.

 

Lines 23-24: How this work confirms that that active reclamation and revegetation at post-closure mine sites is critically important in sustainable mining? Nor clear. Revised these sentences considering the results of the study.

Response: The reason was clearly provided at Line 23:  “The actively revegetated mine sites can typically be restored to a vegetation cover level that equals or better than the pre-mining situation.”

 

Include one or two lines for inter-comparison of multiple mine sites.

Response: In this study, the key differences were found between the two groups of mine sites (actively revegetated vs. passively revegetated), which are clearly presented in the abstract. Since the abstract is limited to no more than 200 words, it is not possible/necessary to include the inter-comparisons across the individual sites.

 

Introduction

“However, although significant progress has been made in remote sensing-based assessment of mine site revegetation and ecological restoration in Canada, most of the evaluations focused on only the reclamation and revegetation behavior at a single mine site”.

Include some latest research studies conducted in different parts of the world and well as in the studies regions which shows the reclamation and revegetation behavior at a single mine site using the remote sensing techniques.

Response: The studies for different parts of the world are now added (Line 68-70). The studies in Canadian study regions are provided at line 74. .

Materials and Methods

Line 147: Need to verify the ending date for Landsat 5 and launching date for Landsat 8.

Response: The operation periods for Landsat 5/8 were clearly provided at Line 153-155: “In this study, the multi-temporal remote sensing images collected from the satellites Landsat 5 (operating from March 1984 to January 1993) and Landsat 8 (operating from February 1993 to present) are used to quantify the land cover change and extent of revegetation at the target mine sites.”

 

Line 151: Include, how many images were downloaded for each mine sites?, also included for which duration images downloaded.

Response: The information is now provided (Line 163-165).

 

Line 156: Include more detail about the Python-based preprocessing of the downloaded Landsat images.

Response: The details were provided at Line 165-169: “Eventually, approximately 25 to 38 Landsat images between 1984 (or the respective closure year when it is later than 1984) and 2021 were downloaded for each site.”

 

Line 172> add classification year, also include detail about the random forest method for classification

Response: The classification year information is added (Line 182-184).   The more detail for the random forest method is added (Line 184-192).  “Given a classification year, the Landsat surface reflectance images from June to Sep-tember of the year were composited and used for producing the classification map based on the median synthesis method. Six indices including NDVI, normalized burn ratio (NBR), normalized difference moisture index (NDMI), tasseled cap greenness (TCG), tasseled cap brightness (TCB), and tasseled cap wetness (TCW) were derived from the composite image and then used for the random forest classification. The training samples and validation samples (at a ratio of 7:3) were collected for each cat-egory. The number of trees used for the random forest model is 85, which was deter-mined by hyperparameter tuning.”

Line 183: Add methodology for the for inter-comparison of multiple mine sites, as objectives were set in the introduction section for inter-comparison of multiple mine sites

Response: The information is now provided (Line 206-207).

 

Results section needs more interpretation as Figures 3, 4, 5, 6, 7, 8, do have not enough discussion.

Response: Some interpretations/presentations are appropriately added to the Results section (Line 276, 283-285, 291-292, 314-318). However, note that for each figure/table it is sufficient to clearly indicate the key results (the trends and land cover change for this study) with interpretation that each conveys. In a scientific manuscript, we avoided reiterating everything and each value from a figure or table.

 

Figure 3a, Figure 6c, Figures 8a, 8b, 8c not present in the text.

Response: They are cited in the text now (Lines 276, 300, 315-318).

Add conclusions as a separate section and conclusion should flow from the results of the present study.

Response: The conclusion section is now provided (Line 411-418).

Reviewer 3 Report (New Reviewer)

Review comments:

I have read the manuscript entitled “Remote Sensing-Based Revegetation Assessment at Post-Closure Mine Sites in Canada”. Surely the content of this manuscript has an interest of reading. However, in the manuscript, there are some issues and I am suggesting for a major revision. Following are my observations:

Specific comments:

1. Each figure and table should have [source: ...] written on it, including when it is "own compilation" or "own compilation based on ...". This needs to be completed.

2. The description of the mine is illegible, incomplete and inconsistent. The full range of data for all six mines needs to be presented uniformly (in accordance with Item 3 of the Detailed Notes).

3. The content of Table 1 should be expanded to include information, such as area, type and amount of mining, type and amount of waste, included in the description in the text (lines 96-141). Care should be taken to ensure that this information is standardized for all mines. Information should be presented for all mines according to this range as presented for the Pine Point mine (lines 96-102). The text from lines 96-141 can, with slight modifications, remain as a commentary to the table. Or, at the discretion of the Authors, and according to the reviewer, this would be even better, instead of expanding Table 1, a separate Table 2 could be added with this standardized, numerical information about the mines.

4. Due to the lack of characteristics of mines in clear tabular form, it is difficult to assess the quality of the presentation of results. When interpreting the results, the authors should also refer to the characteristics presented by the mines in question.

5. The structure of the article needs improvement. The current version of the article lacks Conclusion. The part of the text that is currently in Discussion can be moved to the Conclusion section. However, there should be more conclusions from the research and they should be clearly presented.

6. In the Discussion chapter, much more reference should be made to the international literature on the topic described.

Comments for author File: Comments.pdf

Author Response

I have read the manuscript entitled “Remote Sensing-Based Revegetation Assessment at Post-Closure Mine Sites in Canada”. Surely the content of this manuscript has an interest of reading. However, in the manuscript, there are some issues and I am suggesting for a major revision. Following are my observations:

Specific comments:

1. Each figure and table should have [source: ...] written on it, including when it is "own compilation" or "own compilation based on ...". This needs to be completed.

Response: In the manuscript, all figures and tables are original. None of them was taken from other studies. The suggestion information is not necessary and required for them (as per the requirement for the paper submission).  

2. The description of the mine is illegible, incomplete and inconsistent. The full range of data for all six mines needs to be presented uniformly (in accordance with Item 3 of the Detailed Notes).

Response: The full range of data for all six mines cannot be identical since each site has its own feature information, but the key information (location/site closure year/revegetation type: active or passive/relevant reclamation activities, etc.) is clearly provided for each site. In addition, we avoided repetitive paragraph structures when describing the sites.

 

3. The content of Table 1 should be expanded to include information, such as area, type and amount of mining, type and amount of waste, included in the description in the text (lines 96-141). Care should be taken to ensure that this information is standardized for all mines. Information should be presented for all mines according to this range as presented for the Pine Point mine (lines 96-102). The text from lines 96-141 can, with slight modifications, remain as a commentary to the table. Or, at the discretion of the Authors, and according to the reviewer, this would be even better, instead of expanding Table 1, a separate Table 2 could be added with this standardized, numerical information about the mines.

Response: The suggested changes (conversion from text to table) will not facilitate understanding the main characteristics of each site.  Instead, it will cause a tedious table. In Table 1, the key information (location/site closure year/revegetation type: active or passive) is clearly provided for each site and should be sufficient for understanding the results presented after that. The auxiliary information (area, type and amount of mining, and type and amount of waste) has little impact on understanding the inter-comparisons in this study.  

4. Due to the lack of characteristics of mines in clear tabular form, it is difficult to assess the quality of the presentation of results. When interpreting the results, the authors should also refer to the characteristics presented by the mines in question.

Response:  Again, In Table 1, the key information (location/site closure year/revegetation type: active or passive) is clearly provided for each site and should be sufficient for understanding the results presented. The auxiliary information (area, type and amount of mining, and type and amount of waste) has little impact on understanding the inter-comparisons in this study.

 

We always refer to the site characteristics when interpreting the results, e.g.,

Line 229-231: “Note that although the Wapisiw Lookout TMA became inactive in 1997, the significant reclamation and planting efforts officially started between 2009-2010 [37]. Accordingly, starting from 2009, the NDVI of the site has increased rapidly (Figure 2, red).”

Line 234-240 : “Although the Highmont site had the lowest NDVI (across the six sites) prior to the reclamation, a monotonic and substantial NDVI increasing trend has been detected for the site (Figure 2, yellow and Table 2). This may be attributed to the implementation of an efficient end land use plan at the site [41]. In contrast, the Gateway Hill site had the highest pre-revegetation NDVI across the study sites. This is not surprising since unlike other sites, Gateway Hill was used as an overburden stockpile for mining operations rather than a storage of tailings.”

Line 242-243:  “The Pine Point TMA and the waste rock dumps of the Clinton Creek mine collectively showed the smallest change in NDVI over time. This is consistent with the findings in [13, 46] and reflects the behavior of passive revegetation.”

Line 278-281: “Over the past decades, only limited patches of revegetation appeared on the periphery of the TMA (Figure 3c). This is consistent with the passive revegetation behavior [13, 50, 51], where early plant colonization mainly occurred at the perimeter and stems from existing vegetation.”

Line 310-312: “However, the small bare and water surface (tailing) fragments still exist at the present site (Figure 7b). This may be partially due to the existence of infrastructure (i.e., spill-ways) for the surface drainage from the acid generating tailings [44]”.

 

5. The structure of the article needs improvement. The current version of the article lacks Conclusion. The part of the text that is currently in Discussion can be moved to the Conclusion section. However, there should be more conclusions from the research and they should be clearly presented.

Response: The conclusion section is now provided (Line 411-418).

6. In the Discussion chapter, much more reference should be made to the international literature on the topic described.

Response: More references are added in the Discussion (Lines 361, 376, 407).

Round 2

Reviewer 1 Report (New Reviewer)

The authors have made comprehensive revisions to the manuscript according to the reviewer's comments, and has responded to my concerns. It is recommended to be accepted for publication.

Minor editing of English language was required

Author Response

Thanks! We completed the minor editing of English language in this round. 

Reviewer 2 Report (New Reviewer)

Authors have revised the manuscript.

Authors have revised the manuscript.

Author Response

Thank you! We conducted a minor editing of English language in this round. 

Reviewer 3 Report (New Reviewer)

The authors did not comply with some of the comments contained in the first stage of my review. I am still of the opinion that the way of presenting research objects is not appropriate. When it comes to the source of figures, tables or figures, it is not enough to inform the reviewer that it is your own work. The reader of a scientific article deserves such information. However, some corrections have been made to the article. Therefore, I leave the final decision on the publication of the article to the Editor's discretion.

Author Response

Thanks! Please see our explanations for the remaining concerns in the cover letter. 

Author Response File: Author Response.pdf

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

This is a good design and well-written manuscript. I suggest adding a table with the detailed information for each mine site. This way, it will be clear to see which site has the passive revegetation path.

In addition, the description on page 4, line 149, needs to be clarified. B indicates reflectance values, but it says the digital numbers later. Based on Song et al. (2000), it is necessary to convert digital numbers into reflectance if vegetation indices are used. I am unsure if the data has gone through the preprocessing to reflectance values.

Reviewer 2 Report

The major short of this manuscript lies in the innovation. Only one index, NIDV, is not enough to analyze the environmental legacy of post-closure mine sites.

Reviewer 3 Report

This is a an interesting article using remote sensing to evaluate the level of reclamation of 5 different mine site, is some case, extensively used, in the past. the comparison is between sites which have experienced active reclamation activities against only one which have experienced passive reclamation activities. This study adds extra information on what can be done, in terms of reclamation and revegetation, to contrast in active way the aggravating global warming and local climate risks.

The paper is well written and organised. I have only one comment about the presence of only on site which have experienced a passive revegetation. Author's conclusion would have been (statistically) more solid with the presence (at least) of one more site like the Pine Pont Mine. How can we drop possibility that the result from this mine could be an outlier or that the reason for its very slow revegetation is due to biological or geographical reasons?

Reviewer 4 Report

1. It is suggested to supplement the basis for selecting these five Mine Sites.

2. It is recommended to list specific image information, especially the time of the image.

3. It is suggested to refine the classification of land cover types, otherwise it is not conducive to analyzing the reasons for changes.

4. In Wapisiw Lookout TMA, the water area was large when the pit was closed in 1997, but by 2021, a large area of water has been transformed into vegetated land. Please explain the type of original water area and what land use type these water areas have become. This situation also exists in highmont TMA area.

5. Fig. 2 and Fig. 6a seem to be inconsistent. Fig. 2 shows that the NDVI was negative when the pit was closed in 1984, indicating that it should be dominated by water. However, in Fig. 6a, the vegetation distribution area is large and the Non-vegetated distribution area is not small. Please explain this.

6. It is suggested to analyze the effect of vegetation restoration in combination with climate, hydrology, land quality, vegetation behavior and other factors.