Energy Stored in Above-Ground Biomass Fractions and Model Trees of the Main Coniferous Woody Plants
, Danica Krupová 1, Michal Pástor 1, Marcel Raček 3 and Ivica Pivková 2
Round 1
Reviewer 1 Report
This study deals with energy stored in the above-ground biomass fractions and in model trees of the main coniferous woody plants (Picea abies (L.) H. Karst., Abies alba Mill., Pinus sylvestris L., Larix decidua Mill.) sampled in 22 forest stands selected in different parts of Slovakia. A total of 43 trees were felled, of which 12 spruces, 11 firs, 10 pines, and 10 larches. Gross and net calorific values were determined in samples of wood, bark, small-wood, twigs and needles. Our results show that these values significantly depend on the tree species, biomass fraction and sampling point on the tree. The energy stored in the model trees calculated on the basis of the volume production taken over from yield tables increases as follows: spruce < fir < pine < larch. Combustion of tree biomass releases a significant amount of a significant greenhouse gas – CO 2 and an important plant nutrient – nitrogen into the atmosphere. The obtained data must be taken into account in the case of economic utilization of energy stored in the fractions of above-ground tree biomass and in whole trees. The achieved data can be used to assess forest ecosystems in terms of the flow of solar energy, its accumulation in the various components of tree biomass and the risk of biomass combustion in relation to the release of greenhouse gases.
However, in my opinion, there some loopholes in the experimental methods , Data description and analysis.
I am listing below my major comments
The whole paper sampled 43 trees from 22 regions. Is the amount of data sufficient and different trees were selected? How to ensure the representativeness of the selected trees
2.1. Data collection and sampling
In this section, most of the trees selected in this paper are mature trees, and the model is built on this basis. Then how to deal with the trees with younger age? In a forest, mature trees are definitely the main part, but how are the younger trees included in the model, and how do you determine the proportion of the younger trees in a forest?
2.1. Data collection and sampling
In this section, the model of the entire tree is built by taking parts from each part of the tree, but the default is the same part of the trunk to build a model, but there should be slight differences between different parts of the trunk.
2.4. Calculation procedures
The model introduction of this part has been passed. Could you give a detailed explanation?
3.1. 3.1. Gross calorific values of biomass fractions of examined woody plants
There is a range of difference coefficients ,will there be error accumulation during model building? This results in a big difference between the energy calculated and the actual tree.
Author Response
Reviewer 1:
Note 1.
The whole paper sampled 43 trees from 22 regions. Is the amount of data sufficient and different trees were selected? How to ensure the representativeness of the selected trees
Answer:
The stands of the examined woody plants were selected so as to more or less proportionally represent all developmental stages and the range of site indices occurring in the territory of Slovakia.
Note 2. – 2.1. Data collection and sampling
In this section, most of the trees selected in this paper are mature trees, and the model is built on this basis. Then how to deal with the trees with younger age? In a forest, mature trees are definitely the main part, but how are the younger trees included in the model, and how do you determine the proportion of the younger trees in a forest?
Answer:
It can be seen from Table 2 that younger trees (stands) aged 30-40 are also present, only completely young trees (stands) are missing. The authors decided to take into account the higher accumulation of solar energy in older stands and higher commercial interest.
Note 3. – 2.1. Data collection and sampling
In this section, the model of the entire tree is built by taking parts from each part of the tree, but the default is the same part of the trunk to build a model, but there should be slight differences between different parts of the trunk.
Answer:
Differences in the energy content of different parts of the tree depend mainly on the tree species and its developmental stage in specific growth conditions predestined by the properties of the soil and climate. As these are not known in advance, they need to be verified by setting fair values.
Note 4. – 2.4. Calculation procedures
The model introduction of this part has been passed. Could you give a detailed explanation?
Answer: A more detailed explanation was added to subchapter 2.4. Calculation procedures.
Note 5. – 3.1. Gross calorific values of biomass fractions of examined woody plants
There is a range of difference coefficients will there be error accumulation during model building? This results in a big difference between the energy calculated and the actual tree.
Answer:
Sampling, laboratory analyses and statistical processing and testing of the obtained values were performed according to the standard procedures described in Chapter 2. Material and methods. During the creation of the model, the accumulation of errors is possible, but also the mutual cancellation of errors, so the result of these opposing processes is difficult to predict.
Reviewer 2 Report
This paper is judged to be an excellent paper in the analysis of characteristics according to the type and composition of trees. The contents analyzed by subdividing the structure(bark, stem, twig etc, are highly useful as basic data in the future. I request the answers below.
- It is necessary to write the purpose of this study a more clearly in the abstract.
- English expression needs to be changed: Does wood mean stem? It is difficult to understand the meaning of small wood. It is necessary to change to a commonly used expression.
Ash content: In general, biomass is measured in ash according to ASTM standards. Why did you measure in ISO standards?
* Personally, I am most interested in the accumulation of CO2 and N in this paper. A more detailed analysis and explanation is needed. Carbon neutrality is a global issue and is considered to be used as an important factor in determining carbon dioxide application.
Author Response
Reviewer 2:
Abstract
Line 21: redundancy of the word “significant”. I would suggest using an alternative word for improved readability
Introduction
Line 35: I would suggest spelling out the authors whom the reference corresponds to and moving the reference number ([3]) to the end of the sentences, e.g., Davis et at. (2021) state that the long-term… plots [3]. I would recommend checking the author guidelines for specific indications to the journal style.
Line 42: Similar observation than Line 35
Line 52: Is there a missing reference here, or do these statements still correspond to reference [5]?
Answer: We accept the reviewer's comments. The text of the manuscript has been modified.
Line 59: Could you please expand briefly on how the heating value might be used as an indicator of the cellulose content? Line 55 states that the heating value correlates best with lignin content with cellulose being more relevant for the inner bark. Please, clarify.
Answer: The sentence quoted by the reviewer does not sufficiently describe the complexity of the issue, so we delete it from the text of the article.
Line 60 – 61: Suggested rewording, “The most abundant data on biomass properties is found in the research around wood density, which is usually associated with wood mechanical properties”.
Answer: We accept the wording of the reformulated sentence.
Line 73: Similar observation than Line 72 (please, review this across the whole manuscript). Suggest changing “states” for “presents”.
Answer: We accept the reviewer's comment.
I would suggest restructuring the introduction so that the gap this research is trying to fill is presented in the last paragraph before the hypothesis and objectives. That way it will be clear to the reader what the novelty of the research is, and how this work addresses the issues identified in the existing literature and helps advance the knowledge on the topic. I would also suggest explicitly stating the fundamentals/observations used to formulate the hypothesis, before presenting the hypothesis. Please, see below I suggested table of content for the paragraph between Lines 89 – 97:
- Gap in literature
- Working hypothesis
- Objectives
- Overall implications
Answer: We accept the reviewer's proposal. The text was added from line 89 onwards.
Materials and methods
A good complement to the manuscript could be adding a diagram to exemplify the sampling described in paragraph 121 – 128 with pictures of the different sections (if available).
Answer: Unfortunately, the material needed to create the diagram is not available in sufficient quantity and quality.
Line 166 – 167: Please, review the sentence.
At a tree diameter of 40 cm and a height of 30 m, it reaches e.g. the fresh weight of spruce needles approximately 90 kg and of pine 35 kg.
Answer: We modified the sentence.
Line 169: Please, briefly state why spruce models were taken also for fir.
Formula 1: I wonder if it is correct to have m0*CHtwne within the summation, considering that this is only really one term. Or did you mean to right:
[Summation (vi * rho I * CHi) + m0*CHtwne ] * 10E-3
Please, review.
Answer:
The models were taken from a source (31) which lists models only for spruce, pine and beech. Due to the fact that fir has similar number of needle year-classes as spruce, we used the spruce model for it. We modified the equation (1) according to the reviewer's proposal. Thank you for the alert.
Results
Figure 2. Please, indicate what the letters “a” and “b” mean on the charts.
Answer: The explanatory text was added into caption of Fig. 2.
Table 3. Could you please provide an example in the footnote of how to use the different letters to interpret the data shown in the Table? This will be helpful to help guide the reader. Also, review the decimal points in the results shown in the table. Most of the values could be shown with only one decimal point, e.g., Picea abies bark = 20.3 +- 0.5
Answer: The explanatory text is below Table 3 in form of note. The values in Table 3 are now given only to one decimal place.
Table 4. Please, review decimal positions in the results shown in Table 4. Typically, the standard deviation can be rounded up to one decimal point, and the average can be reported as having the same number of decimal points as the standard deviation. For example,
Picea abies, small-wood: 50.6 +- 0.2 instead of 50.55 +- 0.21
Picea abies, bark 2, average: 18.8 +- 0.3 instead of 18.764 +- 0.283
Answer: The decimal positions in the results shown in Table 4 were adjusted according to the reviewer's proposal. The meaning of the letters has been added below Table 4 in the form of a note.
Line 278: Section 3.3 Please indicate in the Experimental section how the data in Table 5 and Figure 3 were generated.
Answer 278: Information on data generation in Table 5 and Figure 3 was added to the Experimental section.
Figure 4. The units shown in the Y-axis do not correspond to those described in paragraph 308 – 317. Please, review. Also, please indicate the meaning of the letters “a” and “b” and how the energy density was calculated.
Answer: The units shown in Fig. 4. were changed to GJ m-3. We apologize for the mistake. The energy densities of the biomass fractions were calculated based on their basic densities (lines 161 - 162) and gross calorific values (lines 135 – 139). The meaning of the letters "a" and "b" is clear from text added into caption of Fig. 4.
Line 346: kg missing after 1.99, i.e., 1.99 kg N.
Table 7: One decimal figure is enough for representing percentages.
Answer 346: We accept the reviewer's comment
Line 340 – 341: Is there a reference for such statement or was it a calculated value? If the point is to say that emissions of 2731 – 3182 kg of CO2 are already considered detrimental, this statement could be introduced after your calculated CO2 values (Lines 341 – 343).
Explanation: Amount of CO2 released into the atmosphere after burning the above-ground biomass of model trees (DBH 60 cm, h 30 m) was calculated based on the amount of carbon in the dry matter of model trees (744 - 867 kg C, Tab. 7) and of a coefficient 3.67 (of the atomic weight ratio of CO2 and C).
Lines 340 – 341: Despite releasing CO2, such emissions can be regarded as biogenic carbon and are advantageous to emissions from fossil fuels.
Explanation: We agree with the opponent's opinion. However, we also wanted to point out the problem of excessive CO2 release due to uncontrollable combustion and transformation, e.g. of Brazilian or Indonesian forests to the agricultural land, or as a result of controlled or prescribed burning of forests, e.g. in North America.
Discussion
I would suggest presenting a summary table with the values obtained from the literature and the values obtained in this work to support the discussion. This will make it easier for the reader to quickly see how results from this work compare with different references and have the potential of increasing the number of references of this work.
Please review decimal points here, and in general throughout the manuscript.
Answer:
We believe that the summary table with the values obtained from the literature and the values obtained in this work would be very extensive and time consuming, difficult to implement in a given time horizon. The decimal point we checked throughout the manuscript.
Reviewer 3 Report
General overview
Based on experimental data, this research models the energy stored in different trees species considering calorific values of their individual biomass fractions. The findings are conveyed in a very clear way and are supported and contrasted against published literature. I would like to congratulate the authors and encourage them to continue with such labor.
Below I provide some comments that could help improve minor details regarding clarity in the manuscript throughout the different sections.
My main observation follows:
In general, the manuscript conveys a “negative” connotation towards CO2 emissions from combusting biomass. In reality, such emissions can be regarded as biogenic carbon, and represent an advantage towards emissions from fossil fuels and the main reason why burning biomass is considered a better alternative than non-renewable sources (although there is some debate around the topic). The latter aspect is not presented nor discussed in the manuscript. However, it is critical since it is what makes this research especially relevant nowadays in the context of the bio-economy. In that sense, I would suggest reformulating some sections (introduction, results, discussion, and conclusion) of the manuscript to take advantage of this aspect and strengthen the research.
Abstract
Line 21: redundancy of the word “significant”. I would suggest using an alternative word for improved readability
Introduction
Line 35: I would suggest spelling out the authors whom the reference corresponds to and moving the reference number ([3]) to the end of the sentences, e.g., Davis et at. (2021) state that the long-term… plots [3]. I would recommend checking the author guidelines for specific indications to the journal style.
Line 42: Similar observation than Line 35
Line 52: Is there a missing reference here, or do these statements still correspond to reference [5]?
Line 59: Could you please expand briefly on how the heating value might be used as an indicator of the cellulose content? Line 55 states that the heating value correlates best with lignin content with cellulose being more relevant for the inner bark. Please, clarify.
Line 60 – 61: Suggested rewording, “The most abundant data on biomass properties is found in the research around wood density, which is usually associated with wood mechanical properties”.
Line 73: Similar observation than Line 72 (please, review this across the whole manuscript). Suggest changing “states” for “presents”.
I would suggest restructuring the introduction so that the gap this research is trying to fill is presented in the last paragraph before the hypothesis and objectives. That way it will be clear to the reader what the novelty of the research is, and how this work addresses the issues identified in the existing literature and helps advance the knowledge on the topic. I would also suggest explicitly stating the fundamentals/observations used to formulate the hypothesis, before presenting the hypothesis. Please, see below I suggested table of content for the paragraph between Lines 89 – 97:
- Gap in literature
- Working hypothesis
- Objectives
- Overall implications
Materials and methods
A good complement to the manuscript could be adding a diagram to exemplify the sampling described in paragraph 121 – 128 with pictures of the different sections (if available).
Line 166 – 167: Please, review the sentence.
Line 169: Please, briefly state why spruce models were taken also for fir.
Formula 1: I wonder if it is correct to have m0*CHtwne within the summation, considering that this is only really one term. Or did you mean to right:
[Summation (vi * rho I * CHi) + m0*CHtwne ] * 10E-3
Please, review.
Results
Figure 2. Please, indicate what the letters “a” and “b” mean on the charts.
Table 3. Could you please provide an example in the footnote of how to use the different letters to interpret the data shown in the Table? This will be helpful to help guide the reader. Also, review the decimal points in the results shown in the table. Most of the values could be shown with only one decimal point, e.g., Picea abies bark = 20.3 +- 0.5
Table 4. Please, review decimal positions in the results shown in Table 4. Typically, the standard deviation can be rounded up to one decimal point, and the average can be reported as having the same number of decimal points as the standard deviation. For example,
Picea abies, small-wood: 50.6 +- 0.2 instead of 50.55 +- 0.21
Picea abies, bark 2, average: 18.8 +- 0.3 instead of 18.764 +- 0.283
Please, explain the meaning of the letters in Table 4.
Line 278: Section 3.3 Please indicate in the Experimental section how the data in Table 5 and Figure 3 were generated.
Figure 4. The units shown in the Y-axis do not correspond to those described in paragraph 308 – 317. Please, review. Also, please indicate the meaning of the letters “a” and “b” and how the energy density was calculated.
Line 346: kg missing after 1.99, i.e., 1.99 kg N.
Table 7: One decimal figure is enough for representing percentages.
Line 340 – 341: Is there a reference for such statement or was it a calculated value? If the point is to say that emissions of 2731 – 3182 kg of CO2 are already considered detrimental, this statement could be introduced after your calculated CO2 values (Lines 341 – 343).
Lines 340 – 341: Despite releasing CO2, such emissions can be regarded as biogenic carbon and are advantageous to emissions from fossil fuels.
Discussion
I would suggest presenting a summary table with the values obtained from the literature and the values obtained in this work to support the discussion. This will make it easier for the reader to quickly see how results from this work compare with different references and have the potential of increasing the number of references of this work.
Please review decimal points here, and in general throughout the manuscript.
Conclusions
Very well written.
Author Response
Reviewer 3:
Note 1:
It is necessary to write the purpose of this study a more clearly in the abstract.
Answer:
The text specifying the purpose of the study has been added to the article (line 92 and more)
Note 2:
English expression needs to be changed: Does wood mean stem? It is difficult to understand the meaning of small wood. It is necessary to change to a commonly used expression.
Answer:
In forestry, the terms small-wood, top-wood and branch-wood (with or without a hyphen) are equivalent. Unfortunately, these terms are defined only in general. For this reason, the term was precisely defined in the Materials and Methods chapter (lines 125-126).
Note 3:
Ash content: In general, biomass is measured in ash according to ASTM standards. Why did you measure in ISO standards?
Answer:
We used the International Organization for Standardization (ISO) standard, which is the world's largest developer of voluntary international standards with 165 national members. ASTM International standards are in accordance with procedures and standards adopted or approved by the American Society for Testing and Materials.
Note 4:
* Personally, I am most interested in the accumulation of CO2 and N in this paper. A more detailed analysis and explanation is needed. Carbon neutrality is a global issue and is considered to be used as an important factor in determining carbon dioxide application.
Answer:
The text specifying this issue has been added to the end of Chapter 1. Introduction.
Round 2
Reviewer 1 Report
I think it can be accepted.
Author Response
Editor-in-chief
Sustainability
Dear Editor
I am sending you second version of the paper submitted under the Manuscript ID: sustainability-1430031, supplemented according to the comments of the academic editor.
At the same time, we would like to thank the academic editor for his suggestions, which helped us to improve the quality of this article.
Kind regards, Margita Kuklová, corresponding authorInstitute of Forest Ecology of the Slovak Academy of Sciences, Štúrova 2, 960 53 Zvolen, Slovak Republic Zvolen, November 5, 2021
Answers to academic editor:
Note 1.
This work is inserted in a special issue so you can consider works proposed in order to reply to this point https://www.mdpi.com/topics/actions_for_bioenergy_and_biofuels: energy stored is necessary to reach sustainability? what are the compromises? what are the weaknesses? what is the role of circularity
Answer:
The text was added to the introductory chapter of the article.
Note 2.
Within SI we have proposed an editorial that speaks of sustainable hand.
https://doi.org/10.3390/en14185661. Are you agreed with this approach?
Answer:
This approach is essential if, by a concerted effort, we want to contribute to stop the process of destabilizing natural ecosystems and, consequently, of the society due to climate change, which we most likely initiated ourselves.
Note 3.
In the last section I don't see main issues and future direction of research
Answer:
The text has been added to the last chapter of the article.
Note 4.
Finally, what is the role of mix of renewables in your study?"
Answer:
The text has been added to the last chapter of the article.
Author Response File: 
Author Response.pdf
