Quantification and Proxy Indicators of the Carbon Pool in Urban Tree Litterfall: A Case Study of Urban Green Spaces in Beijing

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Article # Forests 2783302

General Comments

This is an interesting study with nicely presented results; the article carries useful data on carbon sequestration in an urban ecosystem. However, the Abstract and Introduction chapters are less impressive and need rewriting. Moderate language editing is also required. So, I would suggest a Major revision of this article. 

Title: Ok

Abstract: The abstract needs rewriting. In the first 3-4 lines describe the purpose of this study. In the next 4-5 lines describe briefly the methods used to conduct this study. Next, in 5-6 lines, describe briefly the results of this study with some hard data. In the last 3-4 lines write briefly the conclusion of this study with some projections. Give full form of LAI.

Introduction:

This Chapter too needs rewriting for an unequivocal and sound ecological message. In the first paragraph write/elaborate the purpose of this study. In the second and/or Third paragraph(s) review the pertinent literature directly related to the subject of this study. In the last paragraph, in the light of reviewed literature, build your hypothesis; and in the last, clearly define the objectives of this study in i), ii),  iii) points.

Methods:  Satisfactory; attend the following comments:

1.      Line 96-100; little confusing; kindly rephrase.

2.      Line 102: Start sentence with: The research station occupies…; give reference to Figure 1 in the parenthesis.

3.      Line 104-107: Cite a reference for the climate data.

4.      Line 127: Start the sentence with: The survey circles…; give reference to Figure 3 in the parenthesis.

5.      Line 145: rephrase the sentence to: A plant canopy analyzer (Mode:l Li Cor LAI-2200C; Li Cor Inc., Lincoln, NB, USA) was used…..

6.      Rest is okay.

Results

The results have been presented nicely. The sentences making comparisons of the results of this study with the others’ studies in every section of the Results should be shifted to Discussion.

Discussion

Discussion is too long for the research data; it may be excised to 2/3^rd  or 3/4^th   of present length without losing the essence of these contents.

Conclusion

The conclusion too is long; it may be excised to half of the present length without losing the essence of these contents. Avoid repletion of the results in the conclusion.

References

 

okay

 

Comments on the Quality of English Language

Needs moderate language editing.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Dear Authors,
The manuscript needs improvement as suggested below:
1. The manuscript throughout requires scientific improvement. No proper hypothesis and objectives have been mentioned in the introduction section.
2. Scientific words are used improperly such as carbon word has been unnecessarily in the abstract.
3. Figure 1 has not clearly marked with information.
4. Tables are without statically represented.
5. Materials and methods portion need revision. Several points are not clear to understand.
6. Discussion needs revision. Very few citations in discussions
7. Conclusion is poorly written, concise it and separate it from abstract.
Overall, improve the manuscript

Comments for author File: Comments.pdf

Comments on the Quality of English Language Dear Authors,       Improve the language, write the manuscript scientifically sound.
Regards,

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The article is devoted to the important and current topic of regulation of the carbon cycle by green spaces in urban regions. Written in good and understandable English. The authors have adequately applied diverse and sophisticated methods to estimate the carbon pool in tree litter.

However, the article has one fundamental flaw that requires significant changes. This shortcoming is that the authors do not distinguish between carbon fluxes (C capture/release; C sink/source) and carbon storage (C pool).

The concept of C sequestration includes both C capture and C storage, but in a scientific article it is necessary to clearly distinguish between these two aspects. These are different concepts, they are measured by different indicators and connected to each other in a complex way. Ecosystems with a large C pool can absorb it poorly or even be sources (e.g. drained peat bogs), and vice versa, ecosystems that quickly accumulate C can have a small C pool (e.g. young forests). The role of different pools within ecosystems is also completely different: green foliage absorbs C, while wood, litter and soil store it and release it into the atmosphere after the decomposition of organic matter. Sorry for this didacticism, but I found it useful because the authors confuse the concepts of carbon capture and carbon storage.

The study assesses the carbon pool of tree litter, but the authors repeatedly write about the assessment of C sequestration in tree litter. In a number of cases, they use the terms C sink, C pool, C sequestration, annual sequestration as synonyms, which is fundamentally incorrect.

As a result of mixing these concepts, the authors make a completely incorrect conclusion that “when an urban tree reaches 15 years of age, the cumulative carbon sequestration of litter over its lifetime is about 2.83 times the net carbon sequestration of the living tree” (lines 410-411 and 462-463). The authors did not take the trouble to explain exactly where they got this conclusion, which is not very polite to the readers. However, from the data they provided (lines 405-409), I guessed that the calculations were as follows:

-        C storage in litter = 56,000 tons

-        Total C storage in vegetation = 296,000 tons

-        C storage in litter for 15 years = 56,000 x 15 = 840,000 tons

-        840/296=2.83

If I correctly guessed the authors’ train of thought, then this is a completely wrong conclusion for the following reasons:

- As the authors themselves note (lines394-397), litter in forest ecosystems accumulates for long periods of time and is continuously decomposed by microorganisms. Most of the organic matter is decomposed and re-released back to the atmosphere as CO2. Indeed, litter does not absorb, but releases carbon (only green foliage absorbs carbon). Moreover, C does not accumulate in litter for a long time. After green foliage, it is the most short-lived pool of carbon in forest ecosystems. Wood is a longer-term carbon store, the most long-term is soil.

- Over 15 years, there is no way that 840,000 tons of carbon in litter can accumulate, since the vast majority of C will be released back into the atmosphere, and a small part (not identified by the authors) will go into the soil for long-term storage. And the authors themselves discovered that in the studied area there are no 840,000 tons, but only 56,000! This is С pool that is retained in the litter in a steady state as a result of the balance between the arrival of new litter from the tree crowns, litter decomposition and the release of carbon back into the atmosphere and the transfer of a small part of carbon to the soil. The authors do not take any of this into account in their reasoning.

Thus, I strongly recommend that authors do the following:

1)     everywhere in the text clearly indicate what you are talking about - C absorption or C storage;

2)     indicate everywhere in the text that you studied C pool (storage) in litter, and not C sequestration, since you did not determine the rate of replenishment of C pool with new litter and the rate of its decomposition;

3)     consider the difference in C absorption and storage between the four main C pools in forests (green phytomass, wood, litter and soil) in your reasoning;

4)      correct the Introduction, interpretation of results (Discussion) and Conclusions to take into account the difference between carbon absorption and storage

 

Detailed comments (see also in uploaded PDF file)

Lines 2-3. I highly recommend replacing article title with "Quantification and proxy indicators of carbon pool in urban tree litter: A case…”

Line 14. Urban plantings are usually not natural.

Lines 27-29. You didn’t show anything about annual sequestration. You showed that C pool in litter is 18.9% of total C pool in vegetation.

Lines 36-37. It's better to make the same number of goals.

Lines 54-57. Without a clear division of processes into carbon capture (absorption), storage and release, these phrases are incomprehensible, and later, this approach led the authors to the wrong conclusion (see general comments and the “Discussion” section)

Lines 60-61. It would be nice to mention here that the further fate of the carbon in the collected litter depends on what I do with it next. It may be advisable to move here the listing of methods for using litter from lines 75-79.

Line 80. What are these investigations?

Line 91. Section “Study area” - To facilitate subsequent understanding, it would be useful to mention the region Fifth Ring Road in this section and indicate its size.

Line 114. Section “3.1. Sample Point Selection and Data Collection". In what season was the data collected? Important for deciduous trees

Line 175. Section 3.4. To facilitate subsequent understanding, it would be useful to clearly explain that this method was used for estimation of litter C storage in the region Fifth Ring Road

Line 218 The term “inefficient” seems unfortunate, since the rate of photosynthesis in conifers can be low, but still more efficient under certain conditions.

Lines 247-255 and Figure 6. Why is such a detailed presentation of these results and detailed Fig. 6 necessary if they are practically not used either in the discussion or in the conclusions (with the exception of a one-time brief mention on page 377). If these results are important, they should be explained in a discussion

Line 278. Table 2. Correct the table header. These are not vegetation types, but regression indicators

Line 290. Section 4.3. It is better to mention this region in advance in the section "Study area"

Line 307. How did you estimate C storage for different months? The use of the same ratio coefficients with LAI for different months is incorrect, since in the studied region there are deciduous trees and the amount of litter can fluctuate greatly throughout the year. If you believe that the ratio to LAI is constant throughout the year, please explain this

Line 318. Figure 8 It is better to divide this into several separate Figures

a-e) It would be useful to indicate the scale of the maps

f, g) F and G is better to place in a separate drawing with a single legend

Line 385. Section 5.3. The main conclusion made in this section is wrong. This section should either be completely reformulated or removed.

Lines 391-392. A pool cannot be part of a flow. These are completely different indicators. This is the same as saying that the water in a barrel is part of the stream that flows into it from the tap

Lines 396-397. This phrase needs to be rephrased, since carbon entering the soil cannot in any way contribute the pool in litter.

Lines 405-407. This data is not included in the Results section. How are they obtained?

Lines 407-408. In the previous phrase you were talking about C storage. Here you are talking about annual C sequestration (i.e. C absorption). These are completely different indicators and cannot be compared.

Lines 410-413. This is wrong conclusion (see general comments on the article)

Lines 462-464. The same

Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Dear authors, I appreciate your humility in acceding to my comments on the previous version of this article and your efforts in improving the readability and scientific quality of the article. I congratulate you for a nice presentation and encourage you to continue studying the subject of C sequestration. 

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors Dear Authors,
The manuscript needs minor English editing to correct its grammatical error as suggested in the attached edited manuscript.
Regards,

Comments for author File: Comments.pdf

Comments on the Quality of English Language Dear Authors,
The manuscript requires minor English editing as suggested in the attached edited manuscript.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The authors corrected the incorrect use of error terms in the interpretation of the results. Minor flaws have also been fixed. I am glad that my review was useful for the authors

Author Response

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Author Response File: Author Response.pdf