Aboveground Biomass Inversion Using DTM-Independent Crown Metrics from UAV Stereoscopic Imagery in the Greater and Lesser Khingan Mountains

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The article is devoted to the problem of assessing the AGB of trees using stereoscopic images the help of digital terrain model. The topic is undoubtedly relevant, but some of the results presented in the manuscript are questionable. The article also needs further work in terms of formatting. Below are some comments that arose during the reading of the manuscript:

1. The title needs to specify the geographical location. Which area or region in the north-east is meant?

2. L. 29 Please explain the abbreviations DSM and CHM when they are first mentioned in the text.

3. L. 35 R2 =0.26? Apparently, the correlation is too weak for birch to use these indicators to estimate AGB?

4. The units of measurement RMSE are unclear, as mass must be consistent with volume or area.

5. L. 77-78. In the vast majority of cases, the use of LiDAR UAVs can measure the height of the ground surface with sufficient accuracy when the measurement settings are correct.

6. L. 104-108. This is a very controversial theory, as crown volume is highly dependent on the projective cover of the tree stand. In dense forests, the crown volume of trees can be significantly less than in sparse forests with the same tree trunk parameters.

7. L. 126. Use the superscript symbol to denote degrees.

8. There are grammatical errors (Table 2. in the word "pin", L. 26 "modle", etc.).

9. Table 4. Again, the units of measurement for RMSE are unclear. Is this an error per square kilometer? Per hectare? Per tree?

10. L. 308-312. ‘It proves that DTM-independent crown metrics can be used to estimate conifer forest AGB, even in cases where TH cannot be accurately measured without DTM assistance.’ This proves that using the TH indicator without additional parameters is also ineffective for estimating AGB. However, regression equations that take into account several stand parameters, including TH, are still more effective than tree crown metrics alone.

11. The article does not follow the journal's formatting rules. The text of manuscript does not use numerical references to the literature, and the sources in the References section are numbered, which makes it difficult to read.

Several questions arose while reading the manuscript:

How can you explain that crown metrics do not work for calculating AGB of birch?

How can your results be used in the case of mixed forests with two or more tree species?

When you calculated linear models for all species, how did account for the contribution of each species' data to the model? Figure 2 shows that there was significantly more data on larch than on other species. It is questionable that the correlation coefficient calculated for all species is greater than for each species individually.

In dense forests, the boundaries of individual tree crowns are usually incorrectly identified during initial processing and require manual refinement, which can be a major problem when processing large numbers of images. How accurately can the boundaries of individual trees be determined?

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The authors of the paper "Biomass inversion using DTM-independent crown metrics 2 from UAV stereoscopic imagery in northeast forested area." available to me have written an interesting article, which thematically fits the journal forests. However, I have identified a lot of room for improvement, especially in the methodological part, which must be corrected by the authors before the paper is worthy of publication.

Apart from that, I am very surprised that the authors did not follow the guidelines of the journal "Forests", which are available to everyone in the Instructions for Authors:

• the literature used in the body text must be numbered consecutively and
• the DOI (if available) of the cited literature must be indicated.

Furthermore, I would strongly recommend that the authors have the entire text of this paper proofread by a native speaker. I found a number of errors related to the content expression, which suggests that parts of the text were created by an untrained AI, e.g.:

• 125-126,
• 127-128,
• 130-131,
• 146

Now for the content details:

Figure 1 should be presented separately in a and b and then fully described in the figure caption. I would also present Figure 1b within the national borders of China to give an international reader a spatial idea.

The description of the tree species examined, beginning in line 133, and their species naming according to Linnaeus' binary system, is misleading or simply incorrect. Pinus sylvestris is by no means the Mongolian pine. Betula platyphylla is not simply a birch either. The authors must, with the assistance of an expert, correct the botanical classification. In addition, all Latin plant names must be written in italics. I also recommend using an abbreviation commonly used in the forestry industry for each tree species and then using this abbreviation in the rest of the paper (including in the tables and graphs).

In line 147, the authors use the abbreviation DOM without explaining it. What does this mean? In soil science, it defines dissolved organic matter.........

Table 2 uses abbreviations that are only explained much later (L. 246); this needs to be corrected. I also suggest the absolutely standard approach of explaining all abbreviations in a table in the footer. I also miss the description of the measurement of TH and BIO values ​​in the Materials and Methods chapter.

The caption of Figure 2 must be correctly described. The basic principle is that a figure, along with its corresponding caption, must be self-explanatory! All sub-figures must be lowercase, and then described. Since this figure is very large (and therefore confusing), I recommend that authors separate it: the individual tree species in one figure and the combined figures in the next.

Comments on the Quality of English Language

I would strongly recommend that the authors have the entire text of this paper proofread by a native speaker. I found a number of errors related to the content expression, which suggests that parts of the text were created by an untrained AI, e.g.:

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Dear authors

The study introduces five DTM-independent crown metrics derived from DSM and CHM data obtained using UAV stereoscopic imagery: crown volume (CV), crown surface area (CSA), crown cross-sectional area (CCSA), crown length (CL), and the ratio of crown surface area to crown volume (CSA/CV).

However, several aspects of the manuscript require clarification and improvement. The use of acronyms throughout the text should be carefully checked and standardized.

In the abstract, a concluding sentence summarizing the main findings and implications of the study should be added to strengthen the overall message.

In the section describing the study area, the geographical coordinates should be expressed in degrees using the sexagesimal system. Figure 1 also requires significant improvement: the right-hand maps are missing scales, legends, and north arrows. The subfigures should be clearly labeled as (a), (b), and (c), and the figure caption should describe each subfigure accordingly. Additionally, the middle image does not appear to be a true-color image—please verify this.

The manuscript needs a thorough English language revision. Many sentences are overly long and difficult to follow; shortening and restructuring them would greatly improve clarity and readability.

Latin names of species should be presented in italic, following standard scientific conventions.

Some terms and concepts require further clarification. Please explain what “DOM” stands for, and describe what a “DBH tape” is—does it refer to a specific measurement instrument? Equation 1 is not cited in the text; it appears to represent an allometric equation, but its derivation or source is not specified. Please include appropriate references.

In Table 2, it is unclear how the parameter ranges were defined—this should be explained in the text. To improve clarity, I suggest including a flowchart illustrating the methodology used in the study.

In Section 3.2, some details—such as parameter choices and values—should be justified. In line 221, the term “this paper” is ambiguous; please clarify whether it refers to the current study or another reference.

Equations 8 and 9 are not cited in the text, and the preceding paragraph (lines 254–258) is confusing and should be rewritten for clarity. In Table 8, please clarify why Greek letters (e.g., β) are used instead of the variable acronyms. Consistency in notation is recommended.

Finally, the conclusion section should not contain references, as this section should focus on summarizing the main findings and implications of the study.

Overall, the study addresses a relevant topic, but substantial revisions are required to improve clarity, structure, and scientific rigor. Therefore, I recommend that the manuscript be revised substantially (major revision).

Comments for author File: Comments.pdf

Comments on the Quality of English Language

The manuscript needs a thorough English language revision. Many sentences are overly long and difficult to follow; shortening and restructuring them would greatly improve clarity and readability.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors meticulously followed the reviewers' suggestions, thereby significantly improving the quality of the paper. In my opinion, only the literature references in pages 192 and 195 need to be checked, and the scientific plant names in Figure 3 need to be italicized. However, all of this can be done in collaboration with the editors without having to send the revised version back to the reviewers.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Dear authors

The study introduces five Digital Terrain Model (DTM)-independent crown metrics derived from Digital Surface Model (DSM) and Canopy Height Model (CHM) data obtained using UAV stereoscopic imagery: crown volume (CV), crown surface area (CSA), crown cross-sectional area (CCSA), crown length (CL), and the ratio of crown surface area to crown volume (CSA/CV).

The abstract has been substantially improved, providing clearer objectives and results, and Figure 1 has also been enhanced to better illustrate the study’s workflow. However, there are still a few acronyms that require definition upon first use (see PDF comments), and the local coordinate system should be specified for completeness and reproducibility. In addition, some references contain formatting or citation errors and should be carefully reviewed.

A new flowchart figure was created and added to the manuscript, which is a valuable addition, but it is not cited in the text. The authors should include an appropriate in-text reference to this figure when discussing the methodological framework.

Overall, the study addresses a relevant and timely topic, and the authors have made significant improvements over the previous version. Therefore, I recommend minor revisions to correct these remaining issues before the manuscript can be reconsidered for publication.

Comments for author File: Comments.pdf

Comments on the Quality of English Language

The English was improved.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf