Review Reports - Carbon Sequestration as a Driver of Pine Forest Succession on Sandy Alluvium: Quantitative Assessment and Process Modeling

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

General Comment

This manuscript presents a comprehensive study on the biophysical mechanisms driving autogenic successions in pine ecosystems on sandy alluvium. The research is exceptional in its scope, seamlessly integrating long-term field observations, detailed laboratory experiments, and sophisticated process-based modeling to test a compelling central hypothesis: that carbon sequestration is the key driver of self-organization, which progressively improves soil fertility and water retention, thereby facilitating succession from simple to complex forests. The findings have substantial implications for understanding forest carbon dynamics, predicting the outcomes of afforestation/reforestation projects on poor soils, and can help develop strategies for ecosystem restoration. The manuscript is generally well-written and the data robustly support the conclusions.

Main concern:

The title of this study is “Carbon Sequestration as a Driver of Pine Forest Succession on Sandy Alluvium: Quantitative Assessment and Process Modeling”, The main idea is the carbon sequestration is the driver for the succession. However, maybe the succession induced the carbon dynamics. I think authors should considers this causal relationship

Specific Comments

(Table 1 notes): The designations , *** for pairwise comparisons are explained but could be confusing. Consider using clearer labels like "SPF vs. CPF**" directly in the table note for Tukey p-values.

(Table 2): The distinction between "Indirect assessment by CO₂ emissions" and "Direct assessment by incubation" is a crucial result. The discussion (Section 4.1) does an excellent job explaining the reason for the difference (fresh vs. multi-year litter). This is a key strength of the manuscript.

.Minor Language and Typographical Errors:

The manuscript is well-written but would benefit from a final careful proofread by a native English speaker or professional editing service to catch minor grammatical slips and improve flow in a few places. Examples:

Line 99: "scare previous study" should be "scarce previous studies"?

Line 142: "riverbed ridges (dunes) eroded by flood waters" can be changed to "riverbed ridges (dunes) shaped by flood waters" .

Author Response

Dear Reviewer #1, Thank you very much for your positive evaluation of our manuscript and valuable comments on its improvement. We agree with most of your suggestions and comment on them below in the order presented in the review.

General Comment (maybe the succession induced the carbon dynamics….). Of course, you are right. And we are considering here precisely the cause-and-effect relationship. Plant successions, first afforestation of alluvial sand, then the replacement of simple pine forests by complex pine ecosystems change the rate of dynamics of ecosystem organic carbon, its quantity and quality. These rates of dynamics are determined primarily genetically through the synthesis and fall of the foliage and the biochemical composition of organic matter. In simple pine forests they are lower than in complex ecosystems. This is necessary for successful afforestation of sands with low soil fertility and water retention. In complex ecosystems, the rate of organic matter dynamics increases, which allows more carbon to accumulate in biomass and soil, with the more stable humus dominating in the soil rather than detritus. The accumulation of soil carbon creates a feedback in the "bicenosis-soil" system, improving soil fertility and water retention. We call carbon sequestration the "driver" of successions because carbon is the material carrier of the environment-forming function of the biocenosis, primarily the formation (improvement) of the soil. The necessary explanations of this working hypothesis are included in the text on lines 126-130 (Introduction) and 878-881 (Concluding remarks).
Specific Comments. The designations , *** for pairwise comparisons are explained but could be confusing. Thank you, we have taken this remark into account in the new version of the manuscript (removed pairwise from the table note and added "SPF vs. CPF" directly to table 1). The distinction between "Indirect assessment by CO₂ emissions" and "Direct assessment by incubation" is a crucial result. Thank you very much, you also consider this result important in connection with the main hypothesis of self-organization of pine ecosystems on sandy alluvium through regulation of the kinetics of their organic matter transformation. In addition, it is useful in a methodological sense for setting up field experiments to study the kinetics of organic matter transformation in the soil.
Minor Language and Typographical Errors. Thank you very much! We proofread the manuscript together with a native English speaker and tried to correct the errors pointed out by you and a number of other linguistic errors and typos. For ease of perception, all changes in the manuscript are highlighted in blue.

Once again, let me thank you on behalf of the entire team of authors for your work in reviewing the manuscript.

12 Sept. 2025,

Sincerely, Prof. Andrey Smagin.

Reviewer 2 Report

Comments and Suggestions for Authors

Forests: MS ID: forests-3856049-peer-review-v2

Title: Carbon Sequestration as a Driver of Pine Forest Succession on Sandy Alluvium: Quantitative Assessment and Process Modeling by Smagin A and others.

General Comments

The manuscript addresses an important and timely question on the mechanisms of carbon sequestration in pine forest successions on sandy alluvium. The integration of long-term field data, laboratory experiments, and nonlinear process modelling makes the work potentially valuable to the international community of forest ecologists, soil scientists, and climate researchers. The paper is comprehensive, well-documented, and methodologically robust. However, several issues regarding clarity, structure, and interpretation need to be addressed before the manuscript can be considered for publication.

Major Comments

Clarity and Length: The manuscript is very long (30+ pages), with dense descriptions. While rich in content, it may overwhelm readers. Therefore, I suggest trimming of certain details without decreasing the scientific content of the MS. For example; Introduction - Objectives; Long list of objectives can be written as: The main objectives of this study were: 1. to quantify the storage of biophilic elements in different soil horizons, litter and phytomass of the ground cover in the succession series of pine and complex coniferous-broadleaf forest ecosystems on river alluvium through their ecotones; 2. to quantify soil organic matter in different successional series through litter input from litterfall and litter decomposition and humification; 3. To quantify the effect of organic matter on the soil water retention capacity in different successional soils; and 4. Develop a nonlinear trigger model of the "biocenosis-soil" dynamical system using MATLAB-6 numerical implementation and application to predict long-term carbon sequestration during autogenic successions of pine ecosystems on sandy alluvium.

Likewise, some sections (e.g., detailed methodology on funnel experiments, equations) could be shortened or moved to Supplementary Materials.

Terminology and Consistency: Terms like “trigger model,” “self-organization,” and “bio-pump” are repeatedly used but not always clearly defined in ecological or modeling terms. A concise conceptual framework figure early in the manuscript would help to understand better.

Literature Context: The study heavily relies on Russian-language monographs [41–55]. While these are valuable, more recent international literature (e.g., on soil-plant feedback, ecosystem modelling, carbon sequestration in temperate/boreal systems) should be integrated to increase accessibility and global relevance.

Model Verification: While HYDRUS and MATLAB simulations are used, the verification against external datasets beyond Russia/Belarus/Ukraine is limited. Testing or at least discussing applicability to other biomes (e.g., North American or Asian pine systems) would strengthen the claims.

Conclusions: The manuscript sometimes presents successional outcomes (pine → broadleaf-conifer mix → oak quasi-climax) as deterministic. Successional trajectories can be more variable depending on disturbance, climate, and management. This should be nuanced.

Some minor comments:

Abstract: Too long and overly technical; should be tightened for clarity and impact.

Figures: While informative, some are overloaded (e.g., Figures 1–3). Simplify or split for clarity.

Language: Generally understandable but contains awkward phrasing and occasional grammar issues (e.g., “fresh eolian-alluvial sandy sediments” → could be simplified).

References: References are updated with recent international works (last 5–7 years).

Overall, the manuscript has strong potential; however, the revisions suggested would be necessary to improve readability, balance the literature, and moderate claims.

Comments on the Quality of English Language

It is broadly alright and I think authors can improve by themself after my comments.

Author Response

Dear Reviewer #2!

Thank you for your positive review and valuable comments aimed at improving our manuscript (MS). We have tried to take them into account in the new version of the MS, as well as to explain our position on those issues and proposals that seemed controversial to us.

Clarity and Length. Thank you for your comment and specific suggestion. We have used your kind suggestion to shorten the Introduction and replaced Objectives directly in your version. Transferring detailed methodology on funnel experiments, equations to the Supplementary Materials is, in our opinion, not entirely correct, since this is a significant methodological development of our approach, hopefully interesting for specialists in the field of soil organic matter dynamics. This is noted, in particular, by the first reviewer of our manuscript. We also introduced abbreviations of the names of pine ecosystems in the text, tables and illustrations of the manuscript and compiled a list of abbreviations.
Terminology and Consistency. Thank you for this comment/suggestion. We have used and published this terminology before (see e.g. [17, 73], but these articles may not be easily accessible. Therefore, we have added terminological explanations at the end of the article in the section "Abbreviations and terminology".
Literature Context. Thank you very much, we understand your position, similar to the position of the Academic Editor. According to his (her) suggestion, we have added to the Supplementary Materials a separate file with tabular data detailing information from Russian-language monograph sources that are not readily available to English-speaking specialists. From our point of view, the MS contains enough references to modern models of forest ecosystems (66-77) with current general information on soil-plant feedback, ecosystem modeling, carbon sequestration in temperate/boreal systems... Therefore, it seems unnecessary to us to expand the already representative (77 sources) list of references, because this, you must agree, contradicts your comment "Clarity and Length".
Model Verification. We fully agree with you that testing the model in relation to other biomes (for example, North American or Asian pine systems) would increase its value. But this is already a subject for a separate new publication, which will become possible if we collect the necessary material from literary sources. Unfortunately, there are not many available works in the world on the successions of pine ecosystems with a synchronous study of the accumulation of organic matter in the soil. Therefore, we were forced to use only Russian monographs for now.
Conclusions. Of course, you are right! We write about this in the discussion of the advantages and disadvantages of the model (lines 853-856 of the new version of MS). However, this is a matter for future research, but in the conclusion remarks we considered it necessary to include the main achievements of the current work.
Some minor comments. Thank you for your comments. Regarding errors and typos (Language), we have proofread the manuscript with a native English speaker. All corrections to the text are highlighted in blue in the new version of the manuscript. The note about the length of the Abstract contradicts the demands of other reviewers to expand it. In addition, the size and structure of the Abstract correspond to the rules of MDPI rules, so without specific suggestions from you, we are not able to change anything in the Abstract. Regarding References we have already explained our position (see point 3). The quality of the Figures has been checked and improved in the new version of the manuscript in accordance with the comments of all reviewers.

We once again express our most sincere gratitude for the review and apologize that not everything could be accepted and corrected, leaving hope for future research in this direction.

With the best wishes from the author's team, Prof. Andrey Smagin.

12 Sep 2025

Reviewer 3 Report

Comments and Suggestions for Authors

A review for forests-385604 entitled “Carbon Sequestration as a Driver of Pine Forest Succession on Sandy Alluvium: Quantitative Assessment and Process Modeling”

General

This study is of significant value, and the data have been interpreted with the requisite caution, thus leading to valid results.

It is recommended that a minor observation be made with respect to the interpretation of the results. It is acknowledged that the objective of the study is to examine the process of carbon sequestration within forest soils. However, it is imperative to acknowledge that the quality of the organic matter is frequently as significant as, if not more so than, the total carbon content sequestered or stored.

Indeed, in certain types of forests, humus accumulates in relatively untransformed forms, not converted into humic substances, but in the form of litter or raw organic matter with strong similarities to the chemical constituents of plant biomass. This organic matter is more easily biodegradable than in other types of humus, where organic matter has been transformed into complex humic-type substances with a chaotic macromolecular structure, highly resistant to biodegradation, in part because they are forming stable organomineral complexes.

As is evident throughout this document, reference is made to the types of humus and its quality. For instance, lines 390 and 732 refer to "mor-type" humus, and line 674 refers to "higher-quality" humus types.

However, even if all the types of humus studied in this work are of the mor type, it is still possible to make some reference to the quality of the humus using simple values such as pH and the C/N ratio. These presumably differ in some forest types compared to others. A decrease in the C/N ratio and an increase in pH are indicative of more advanced stages of soil organic matter evolution. It is reasonable to hypothesise that these values will be correlated with biological activity or CO2 release. Such disparities could have been addressed in the discussion and even in the abstract.

Indeed, it is often posited that in the event of a potential change in environmental conditions, such as higher temperature and humidity, which accelerates the decomposition of organic matter, the effect would be very different in high-maturity humus types compared to raw humus types.

In any case, if the data obtained in this study indicate that there have been no substantial changes in humus maturity or quality between the different formations compared (which would not be unexpected considering the sandy geological substrate), this could be mentioned as a local observation obtained in this study.

Abstract

In order to obtain a valid abstract that can be read in isolation, it is suggested that some additional information be added.

If possible, we would be grateful if you could provide additional information on the soil types (USDA, FAO; not only "sandy soils"), the geological substrate, and the vegetation (described in generic terms as "coniferous, broadleaf…").

It would also be beneficial to include quantitative data, such as the amount of "soil organic carbon" in the forests where carbon storage is being studied.

Throughout the text, there are minor issues that should be addressed. Please use N-dash for the numerical ranges (lines 21, 27, etc.). Please note that this also applies to references in brackets in the text

Line 22: I consider the term 'rudimentary humus horizons' to be somewhat imprecise. It may be preferable to use other terms that refer to maturity, degree of decomposition, type of humus, etc.

Keywords: It is recommended that simple keywords are used and that several keywords are not combined with conjunctions and prepositions.

Introduction

Line 42 (minor): I suggest separating the unit symbol from the carbon symbol, as is done on line 39 of page C.

Lines 112, 218 and 267 (minor): Do not capitalise software names unnecessarily if they are not acronyms. (https://www.pc-progress.com/en/Default.aspx?hydrus-1d)

Material and Methods

Line 137: Why is 'Arkhangelskoye' in quotation marks?

Line 176: In many journals, it is customary to use the abbreviation 'm a.s.l.'

Lines 198 and 204: Use the multiplication sign × instead of the lowercase letter x.

Line 208: 'Atomic absorption' (not 'adsorption').

Line 209: I think the correct term is 'necromass', not 'mortmass'.

Lines 250 and 252, and throughout the text: It appears that units from outside the International System of Units (SI) are being used. This is the case with the ton (SI: Mg). However, SI units such as the petagram were correctly used at the beginning of the article.

Results

Line 307: Caption of Figure 1: Use italics for 'p' (better use a capital 'P') to indicate probability.

Specify what W is.

It seems appropriate to use SI units throughout the article (e.g. Mg instead of t).

In the figure caption, indicate what the error bars represent (e.g. deviations, least significant difference...).

Apart from the general suggestion of being consistent with the units throughout the text and using SI units, it would be convenient to italicise the R² of the determination index in Figure 2. Fix the typo in autumn (in the second and third groups of columns).

Table 1: Please ensure that you use acronyms consistently. In this instance, they appear to be superfluous, particularly in the second group of rows where 'EZ' is used when the full words 'ecotone zones' can be used, as is the case in the rest of the row groups.

Similarly, the acronyms SPF, CPF and EZ are used in the Table footnote, but not explained. These are not necessary since they are not used in the Table.

The same can be said for the legend of Figure 2, where the acronym EZ is used simultaneously with the full words for SPF and CPF. In the interest of clarity and simplicity, acronyms should be avoided in Figures and Tables. However, if they are used, they should be explained in the Figure caption (as opposed to the "EZ" in Figure 2, which is not explained).

Figure 3.

Please consider the previous recommendations regarding the unnecessary use of acronyms in the case of EZ, and the typo in "Simple pine forest".

Please ensure that words in legends are consistently capitalised.

Please ensure that a space is left before and after the unit symbol (gram) on lines 446 and 447, for example.

Figure 4: Remember what I said before: use the acronym 'EZ' only when necessary. Make the spelling mistake in 'simple pine forest' in subgraph (d) correct.

Figure 5: The above information also applies to Figure 5. In this case, there is also a typo in subgraphs (a) and (d).

Table 2: Please explain what the parameters k, T_0.5, T_0.95etc. mean in the title or footnote.

I think 'litter' is a better word than 'detritus' (this suggestion works for the whole text).

Line 503: “contain organic compounds resistant to biodegradation“-- Indeed, the reason for this is that these organic compounds are resistant to biodegradation and also inactivate enzymes, thus hindering the biodegradation process. Furthermore, they have an antimicrobial effect that slows the decomposition of organic matter.

Figure 6: Think about some of the ideas that were suggested for the previous figures: Put the statistical parameters in italics, as well as X, R² and n. In the figure caption, explain what the acronyms SPF, CPF and C... mean.

Explain what the "Theta, %" on the X axis means.

Please explain more clearly why the forest type is not specified before the depth is given, as it is in the other cases (CPF, Bhfe (40–70 cm)).

Why are the depths shown in graph (c) in order from shallow to deep, but in graph (b) they are shown from deep to shallow?

Table 3: Look at the previous suggestions and explain any symbols and acronyms used in the footnote.

Put 'R²' in italics.

Please explain what the first group of three rows corresponds to, since the second and third groups of rows specify that they correspond to the SPF and CPF.

Leave a blank space before the unit symbol on line 559 (and throughout the text).

Figure 8: Please look at the suggestions for the previous figures.

Make sure you put the numbers for statistical indices like R² and n (the number of data points) in italics.

Please clearly specify what the graph axes show, i.e. B, X, the storage of phytomass and soil organic matter in Mg/ha.

Use SI units to avoid confusion.

The symbol for hectare is "ha" (and "h" is for hour). Look at the Y-axis title to see this.

The lowercase t is used to show both the y-axis (tons) and the x-axis (time). You can fix this by not including it in the second case and replacing it with "years". However, I suggest changing the first 't' to a different SI unit.

Also explain what IP means.

Figure 9: Please look at the recommendations in the previous figure.

Use SI units.

Please note that the lowercase t is used to indicate both tons and time.

Please explain what the horizontal red line (Xcr=) represents in the footnote or title, and also explain what Xcr means.

Discussion

Line 856: “biocenosis”

References

Please check the format of the references and correct any common small errors, such as punctuation marks in abbreviated journal names.

Please note that some citations, such as #61, may not be cited correctly, as it appears to be a book.

Supplementary data

Please do not abbreviate the species name in Calamagrostis arundinacea

It seems that Galeobdolon luteum Huds. is an older name for the plant known more widely by its modern name, Lamiastrum galeobdolon or Lamium galeobdolon.

Please do not abbreviate the species name in Polytrichum juniperinum Hedw.

Table S3: Please explain all acronyms used: DT, DS, TD, etc.

Also explain the meaning of S_BET, which is written without superscripts in the footnote.

Also use the full words instead of the acronym EZ.

Indicate the units for the particle size distributions, as the column headers indicate size in mm.

Table S4: I believe the correct unit to indicate exchangeable cations is centimoles of charge per kilogram (cmol_c·kg^-1) equivalent to the old unit milliequivalents per 100 grams (meq·100g^-1).

Do not use unnecessarily the acronym EZ

pH: indicate “in water” instead of водн

N%: use “tr” instead of следы

Table S6. Italicize p for probability, R², etc…

Table S5:

Some reviewers recommend against using the hyphen in Tables with numerical data because it is an ambiguous symbol: It is recommended to use whichever option is appropriate for your particular case: i) indicate "not determined" (nd); ii) indicate 0.0 when appropriate; iii) use "tr" or "< #" or another expression indicating that the value is below the detection limits.

It would be advisable to delete either 'average' or 'mean', as it appears that there is no requirement to use both of them simultaneously.

Author Response

Dear Reviewer #3,

Thank you very much for your highly professional, thorough review of our manuscript (MS) and valuable, specific suggestions for its improvement. We have tried to take all your comments into account in the new version of the MS, highlighting the changes in blue. Below we provide responses to your comments in the order presented in the review.

General. Observation on the role of organic matter quality. You are absolutely right! In our study, we were able to divide organic matter into only two qualitative groups - plant detritus (litter) and humus. They differ greatly in terms of resistance to biodegradation and turnover rates in the soil (see Table 2, Fig. 9). However, despite the narrowing of the C/N ratio and the increase in pH (see Table S4 in the Supplementary Materials), the stability of humus in the complex coniferous-broadleaf ecosystem was only slightly higher than in the simple pine forest, without statistically significant differences (Table 2). We agree with your point of view that this may be a specific feature of coarse-textured (sandy) soils. The corresponding explanations are given in the "Discussion" (lines 759-767) in the new version of the MS.
Abstract and Keywords. We thank you for your comments and have fixed them in the new version of the MS. We have replaced short dashes with N-dash for the numerical ranges throughout the MS.
Introduction. We thank you and have separated the C designations, and removed the capital letters for Matlab software.
Material and Methods. We thank you for your valuable comments and have made corrections to the Materials and Methods section, as well as other text, figures and tables in the new version of the Manuscript (regarding the SI units Mg instead of t).
Results, Discussion. Thank you very much! We have accepted almost all your comments on numerous, alas, our mistakes, typos in the text, tables and figures and have tried to correct them in the new version of the manuscript. First of all, we used SI units as you advised, as well as italics for statistical indicators. We have redone the figures, correcting typos and using a unified notation style with explanations in the legends. We have replaced "detritus" with "litter" throughout the document. We have changed the capitalise software names for MATLAB, which does not contain abbreviations. We have streamlined the location of soil horizons in the figures, as well as the rows in Table 3, explaining the names of all three groups of rows. We have eliminated confusion between ha and h (hour), tons and t (time); we have provided explanations and/or links to the text for the notations in the tables and figure captions. We have also solved numerous problems with the names of forest types (types of compared forest ecosystems) taking into account the opinion of another reviewer about the abbreviation of the Manuscript, namely, we have used only abbreviations (SPF, EZ, CPF) in the text, tables and figures. Their decoding in accordance with the rules of the journal is given at the first mention, as well as on a separate list of abbreviations at the end of the manuscript. We have corrected the typos and grammatical errors you found and used the notations and signs you suggested ('m a.s.l.', ×, etc.).
References. We have checked the list of references, but could not understand what mistakes we have in the abbreviations of journals and the designation of books as journals. Apparently, different styles are used. Allow us to leave this issue to the discretion of the technical Editor at the final stage of preparing the article.
Supplementary data. Thank you very much! We have removed the abbreviations of Latin names, and also accepted your suggestion regarding the name Lamium galeobdolon. We have explained the abbreviations used. Also in the main manuscript, explanations are given in the section "Materials and Methods", and on a separate list of abbreviations at the end of the MS. We have accepted all your suggestions about SI units, as well as about replacing dashes in tables (both the Manuscript and SM) with the designations '0.0', “tr”and 'nd'. We excluded "mean" and left "average" in the first column of Table S6.

On behalf of the entire team of authors, I would like to once again express my deepest gratitude for one of the most comprehensive and creative reviews in my almost 40-year scientific career. We hope that the new version of the manuscript, prepared taking into account your review and the opinions of two other reviewers, has become better in both form and content.

12 Sept. 2025.

Sincerely yours, Prof. Andrey Smagin.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

MS has been improved considerably as per the comments passed for the review, and it can be published now.