Retrieval of Forest Structural Parameters from Terrestrial Laser Scanning: A Romanian Case Study

Round 1

Reviewer 1 Report

General Comments：

Based on the terrestrial laser scanning (TLS) data and filed measurement data of 12 plots, the authors used single and multiple scans as a basis for extracting forest structural parameters, such as number of trees, DBH, and heights. The results showed that multiple scans technique is better than single scan for identifying the number of trees, little bit better for estimating the diameter of tress, and has no advantage for measuring the tree height. The design is good, and the results are reliable. But the current manuscript can’t be accepted for publication because of the following issues.

1.The writing style needs to be improved, and the manuscript should have a language editing. The section “3 Results” is better to be classified into several sub-sections, and discussion needs to be added in the section “4 Conclusions”. It is little difficult to understand the text for me.

2.Between line 152 and 169, the authors used so much words to describe stem shape, but we have not found any relevant results in section 3. Only in the last paragraph, they said “When trying to quantify stems volume at sample plot level through TLS scanning, not even the simplified approach of equivalent to basic shapes (described solely by diameter and height), do not provide good results. A solution might be found in applying the multiple scan approach in a leaf-off state of the forest sample plot.” If no results can be provided, is it necessary to present the method?

3.Some tables and figures are not well-designed. For example, the figure 1 is difficult to understand, and table 4 shows the evaluation words, not results or data.

 

Special Comments：

Line 19-20: Too simple. How about the materials? Various approaches should be described here.

Line 75-76: I can’t understand the figure 1. Please make it more clear for understanding.

Line 78-79: In general, the unit of diameter is cm, not m.

Line 103-104: In figure 2, the scan positions are not so obvious.

Line 131-132: “was chosen as per Othmani, Piboule, Krebs, & Stolz, 2011” => “was chosen[16]”.

Line 142: “work of Hackenberg, Spiecker, Calders, Disney, & Raumonen, 2015” => ”work of Hackenberg et al.[17]”.

Line 158-159: In the equation (1), what does x mean? It is difficult to understand the equation.

Line 247-248: Why do the authors not give the differences of mean DbH estimates in Table 4, just like the number of trees in Table 3?

Line 281: Discussion should be added.

Line 311-312: “by Xi et a., 2016” => “by Xi et al.[9]”; This sentence “We also observed an  improvement in diameter extraction was also when averages were evaluated” needs to be rewritten.

Line 313: “Saarinen et al., 2017” => “Saarinen et al.[5]”.

Line 330-332: “6. Patents This section is not mandatory, but may be added if there are patents resulting from the work reported in this manuscript” needs to be deleted.

Author Response

Please see the attachment.

Thank you.

Author Response File: Author Response.docx

Reviewer 2 Report

Manuscript Review:

Retrieval of Forest Structural Parameters from 2 Terrestrial Laser Scanning: a Romanian case study

The objective of this study was to compare and contrast the utility of two approaches for collecting terrestrial laser scanner (TLS) data to predict forest census measurements, under differing forest structures and compositions that might be found as typical forest inventory conditions. TLS could potentially save time and reduce the required labor force for collecting forest inventory data, and could potentially reduce human error, and thus the subject of this manuscript is both timely and relevant to forestry given the increasing availability of TLS units, the rapidness of forest change given climate change and increasing forest disturbances, and reductions in staff available in many places to conduct inventories in the field. However, despite these merits of the paper’s focus, missing information and weakly developed sections of this manuscript make it impossible comprehensively evaluate the study or its results. In addition to line by line comments, I offer some general comments for guidance and encourage the authors to resubmit this potentially impactful study when it has been improved.

General comments:

Introduction

The Introduction of this manuscript is too brief and does not adequately describe the need or basis of this work. As someone with experience with this exact subject, I believe that this introduction can be greatly developed and in doing so will increase the relevance of this study and its relationship to other work. Some related work to consider includes but is not limited to:

• Bienert, A., S. Scheller, E. Keane, G. Mullooly, and F. Mohan. "Application of terrestrial laser scanners for the determination of forest inventory parameters." International Archives of Photogrammetry, Remote Sensing and Spatial Information Sciences 36, no. 5 (2006).

 

Methods

The authors only compare young vs. old stands; however, physically the differences that will impact inventory predictions are observed are more likely due to the number of trees per unit area (e.g. clutter and shadowing in lidar datasets due) rather than the stand age itself. In fact, a quick comparison of the data in Figure 3 shows that the difference in %Inc. is linearly related to the field observed number of trees (also note that SY 3 would be an outlier when plotting this). This would be a much more useful way to evaluate how the number of scans may need to increase based on a more relevant and easily measured forest attribute than age (also consider that the number of stems should be pre-existing data for forest census programs and could guide the number of scans required if this approach was to be introduced). Alternatively, since occlusion of the laser is the primary reason more scans are needed when there are more stems, it would be helpful to examine whether basal area was a more robust predictor of the %Inc. than the number of stems. Similarly, this could be expanded upon to highlight how clutter may impact height measurements. This idea of clutter is largely described with data by Liu et al. 2018 in Forests, which you have cited.

Also consider using some additional and fairly standard statistical comparisons to evaluate your data, such as those presented in Liu et al. 2018 or Bienart et al. 2006, and especially ones that describe the types and amounts of error. This is important for future users who may wish to employ this technique for forest inventory work in similar forests.

In general, it would be helpful to add methods that describe the analysis of data. Also see Line by line comments.

Results

See line by line comments. It would be useful to report all of the results for each plot, not just for some plots.

Discussion

The manuscript has no Discussion section. This is a critical and element of a research paper and needs to be included, and is essential for explaining why certain unexpected results may have occurred, how the study could have been improved, and the significance of results. Similarly, it would be beneficial to readers to advise how future research should progress following your work.

Conclusions

Much of the conclusions should be moved to the discussion section and be expanded upon. The conclusions should then reflect the key points and findings of this study in a shorter section.

References

References 18, 19, 22, 23, or 24 appear in the text.

Line by Line Comments:

Line 15: Grammar, “trees” should be “tree’s”

Line 20: Typo, abbreviation for diameter at breast height should be introduced following the full wording of the phrase and should be abbreviated “DBH”

Line 33: I disagree with the use of mean. First, this implies that forest stand characteristics follow a normal distribution, which is not always the case. Second, other distributional parameters may be of interest. Third, this seems to neglect the potential range in completeness of an inventory, which can relate to the potential for nuanced extraction of information from the data.

Line 35-37: Please clarify the link as to how these advances relate to forest inventories.

Line 37-38: Please clarify the link as to how these models are important to TLS or forest inventories.

Line 39: Consider the appropriateness of using such strong words as “guarantee” and “endless [capabilities]” in this context. I strongly suggest revising.

Line 48: Consider waiting to inform the reader about the “two species and various age classes” until the methods section. Also it would be helpful to begin this paragraph by describing what problem you aim to approach with your research.

Line 50: Authors state “the theoretical methodologies” but they have not introduced any methodologies, so it is unclear what they are referring to.

Line 58: “was also” doesn’t seem to make sense here.

Line 58: Consider rewording so that most of this sentence is not in parenthesis

Line 71: This is not a complete sentence.

Line 75: (Figure 1) This figure is of a map and has an interesting/novel appearance; however, a few simple improvements could help it convey its information much more effectively. First, the text used in the map and legend is small and should be increased so that it is easy to read (note that in some cases it is unreadable). Second, it is unclear how the two circular inset maps relate to the three study areas indicated in the coarse scale map. Leader lines or some other reference indicators would be helpful here. Third, three unitless scales seem to be presented in this map. It would be helpful to replace these scales with scales with units that readers could more easily understand and more carefully identify which maps they apply to. Lastly, as someone with substantial mapping experience, I must make the point this map should include two additional elements: a clear Legend label and a north arrow.

Line 78: (Table 1) It would be helpful to include in this table the number of replicates for each condition measured and include the group abbreviations that have been mentioned in the text, for a more parallel relation between the two. Likewise, it would also be helpful to include some measure of variance about the mean (e.g. standard deviation or standard error). Lastly, I suggest revising your stand density metric to something that is easier to interpret. Simply listing “1”, and without units, for all stands is uninformative.

Line 81: Please develop description of methods used for creating reference measurements.

Line 89: Please clarify the purpose of quantifying spectral characteristics (e.g. RGB)

Line 94: Please clarify what is meant by “swat”

Line 100: Check spacing and wording at the end of this line.

Line 101: Check capitalization at the beginning of the sentence.

Line 102: Clarify what “one” refers to with a different word.

Line 115: Please describe how the scans were coregistered and justify the approach.

Line 120: Provide citation or justification for the establishment of these tolerances.

Line 123: Please explain how the reprocessing differed from the initial processing.

Line 132: Inconsistent reference formatting.

Line 142: The citation in this line does not seem to conform to the format used elsewhere in this manuscript – please check.

Line 158: (Equation 1) Please define “x” in the text.

Line 169: It would be helpful to define “primitive” for the reader of this journal, who may not have a strong TLS data-processing.

Line 170: This sentence is difficult to read and unclear. Also, it would be beneficial to incorporate this sentence into another paragraph, or develop it as its own paragraph that develops the explanation of the filtering processes. For instance, explaining what tools were used for filtering would be helpful for subsequent users of this new approach. Likewise, how were filtering thresholds chosen?

Line 172: It would be helpful to the reader to clarify the generation and role of the voxel grid.

Line 176: (Equation 2) It would be helpful to the reader to explicitly define x in the text.

Line 186: It would be helpful to readers to explain what a Hough transform is and it’s basis for use.

Line 187: A circle is described as being fitted here but elsewhere (conclusions) this is described as a cylinder. Clarify what you mean. Also a figure or additional details on how this fitting was performed or evaluated would be beneficial to understanding the results generated.

Line 206: (Table 3) This table provides interesting data. However, it is counterintuitive for it to seem possible that the single scan could reveal a greater percentage of trees than multiple scans, namely in the case of SO 3.

Line 241: Please check on the convention for abbreviating the coefficient of determination in this instance and in others (i.e. r²).

Lin 246: (Figure 5) It would be helpful to the reader to clarify in the figure caption whether this figure is describing results of single or multiple scans. Alternatively, it could be helpful to provide both single and multiple-scan results in a figure if possible, especially considering that you note important differences between the two in the text. Also, please check your legend for accuracy. Given the figure, it appears that the regression line is green (the same color as the points), and the light blue line actually refers to a confidence interval or something other than the regression line. In any event, the figure/legend would benefit if the meanings of all components of the plot were labeled. Also, it would be helpful to provide the r² values on each plot, if possible, since they are described in the text as having interesting differences.

Line 248: It would benefit the paper to add this height data to a table that was similar to that used to describe DBH results, or in a table. Likewise, it would be useful for the reader to compare these results to the actual field measured heights.

Line 255: Please provide a better description of the purpose and types of spatial filters used.

Line 264: I believe the word “figure” should be capitalized.

Line 266: (Figure 6) It is unclear to the reader what this figure is illustrating, and what data is included in it. For instance, it the y-axis appears to indicate point cloud heights above sea-level, but why this scale is used is unclear, when elevation was not a component of this study. Likewise it is unclear how this data is a continuous dataset, and appears to be interpolated between points. I also believe there is also a typo and erroneous capitalization in the y-axis label of the graph. Also, please consider removing or making the gridlines on the graphs more consistent. Please clarify the purpose of this figure and how it relates to your results. Lastly, I suggest that “Height” should not be capitalized in your caption.

Line 269: Please clarify what the percentages refer to. Do they suggest the percentage of time that the measurements are correct, or the percentage of the actual height they actually identified, or something different? Also, are these average percentages, and if so what plots are they averages of and would you consider adding a measure of variance about the mean, such as ± 2 standard deviations)

Line 278: (Table 5) It is unclear why results for only some of the plots are included in this table. Likewise, the footnote at the bottom of this table seems redundant. Lastly, please remove the extra zeros in the 10s space in the values (for example those of St.Dev.)

Line 283: Validation has a very distinctive use in remote sensing and statistics.

Line 283-286: This conclusion statement seems to conflict with your finding that older forests and younger forests had differing results. Also consider my suggestions for the Methods section and that the number of trees appears related to the number of trees successfully identified.

Line 288: It is unclear what the text in parenthesis refers to.

Line 289-291: This explanation is likely true and likely supported by your data, but is not supported by the given results or data. Also this seems to conflict again with your statement in Lines 283-286.

Line 300: Please explain what FieldMap is.

Line 306: Please explain the definition of the symbol preceeding “0.7”

Line 308: Please elaborate upon this statement about the relationship between this work and that of previous studies, and cite these other works.

Line 311-312: This sentence doesn’t make sense.

Line 314: Significance is mentioned but no significance tests were employed to evaluate the importance of scan distance. It would strengthen the paper to do this.

Line 321: Please further develop your comparison and contrasting of the strengths and weaknesses of single and multiple scanning. This could easily be an entire paragraph in your discussion.

Line 330-332: Please remove the text about patents.

Author Response

Please see the attachment.

Thank you.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

General Comments：

The authors have revised the manuscript, and addressed the concerned issues in the response. I think it can be accepted for publication after minor revision and language editing.

 

Special Comments：

Line304-305: All figures in the Table 4 should keep 1 decimal digit. For example, 20=>20.0, 0=>0.0. Same in Table 5, and it is not necessary to keep the values of SD 2 decimal digits.

Line310-311: The x-axis and y-axis need to have the same ranges in Figure5b, just like those in Figure5a. In addition, the following changes are suggested: Figure5a=>Figure5; Figure5b=> Figure6; Figure6=> Figure7; Figure7=> Figure8.

Conclusions: Most of the text in the Conclusions need to be removed to the section of Discussion, for example, from line 388 to line 417.

Author Response

Please see the attachment.

Thank you.

Author Response File: Author Response.docx

Reviewer 2 Report

Review of Revised Version of

Retrieval of Forest Structural Parameters from 2 Terrestrial Laser Scanning: a Romanian case study

I believe the authors have made a substantial effort to address my previous comments to my satisfaction, and only minor revisions are suggested from this point forward. I explain these further in this review along with my attempts at suggestions. I appreciate the author's effort which is reflected in the quality of this revised version.

The Introduction presents a much improved justification for the needs for this research, and the specific aims of this study. The new text is sufficiently addresses my concerns and frames a broad relevance for the work presented, using Romanian forests as a case for study that is relatable to many other regions of the world. I appreciate the careful editing and attention to grammar in this section.

The new versions of Figure 1 and Table 1 have been improved. In Table 1, I would recommend ordering the use of superscripts such that they follow the order that they are used in the table. For example, since “DBH.m.[cm]” uses superscript “1”, “No. replicates” should use superscript “2”.

Equation 1 has improved from the previous version but would be easier to interpret using subscripts to notate height, since it is seems to be used as an index rather than a variable in the case of this equation. For instance: . Please forgive me if I am incorrect about this. Regardless, I also want to point that this equation can be easily simplified because the exponent and square root simplify to an exponent of ½ of n (i.e. the simpler alternative would be ). Lastly, line 198 of the text suggests this equation should solve for radius rather than diameter, so it seems the equation should directly reflect this.

The new Table 4 and Figures 5a and 5b are greatly improved and provide easy to interpret results highlighting the strengths of the method for extracting DBH measurements but not heights.

In my opinion, the Discussion and Conclusions are very close but need some reorganization. Lines 367-373 seem like they would fit better after line 387 in the Conclusions, while lines 388-428 seem like they would be more appropriate at the end of the current discussion section. It would also be useful to advise the reader where future research should focus, based on your results. What about elevating the height of the TLS unit to try to get a better height measurement, for instance, and reducing the residuals? Or comparing results of data collected under dormant and growing season conditions? In any event, these sections are much improved and only require minor additional improvement.

 

Other line by line comments:

Line 214: Figure 3 caption would be improved if the letters in the image were designated in the caption alongside each shape name.

Line 226: I suggest wording with “lidR and lasvoxelize packages for R” as a more standard wording than “R statistics (lidR – voxelize)”

Line 257: Suggest not abbreviating height, since the rest of the text appears to not abbreviate it.

Line 343: Suggest adding units to labels in table

Line 367: This sentence is confusing.

Author Response

Please see the attachment.

Thank you.

Author Response File: Author Response.docx