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Peer-Review Record

Effects of Forest Gaps on Abies faxoniana Rehd. Leaf Litter Mass Loss and Carbon Release along an Elevation Gradient in a Subalpine Forest

Forests 2022, 13(8), 1201; https://doi.org/10.3390/f13081201

by Han Li^1,2, Ting Du², Yulian Chen², Yu Zhang², Yulian Yang¹, Jiaping Yang¹, Qing Dong¹, Li Zhang² and Qinggui Wu^1,*

Reviewer 1: Anonymous

Reviewer 2:

Renato S. Pacaldo

Reviewer 3: Anonymous

Reviewer 4:

Milica Kašanin-Grubin

Reviewer 5:

Shamsollah Ayoubi

Forests 2022, 13(8), 1201; https://doi.org/10.3390/f13081201

Submission received: 17 June 2022 / Revised: 19 July 2022 / Accepted: 26 July 2022 / Published: 29 July 2022

(This article belongs to the Special Issue Carbon and Nutrient Transfer via Above and Belowground Litter in Forests)

Round 1

Reviewer 1 Report

Title: Physical site locations on the earth’s surface are defined by elevation above sea level and not altitude.

Line 50: Conclusion at this point is not supported by the rigour of the literature cited.

Line 22+: Only checking the citations, some of these references are published results from the same site? Hence, the potential to perpetuate the same assumptions with limited biophysical documentation of site processes is high.

Line 90-94: The hypotheses are weak. Altitude is only a number until properly interpreted with biophysical data. Robust biophysical data allows the processes to be determined, which will have applicability to understanding the effects of altitudes at sites other than the current location. The issue is the scientific relevance of this research.

Line 111: ‘Similar slopes’ range from 21 to 34 degrees. Steep mountains can substantial localized effects on air flow; particularly with how cold air fills gaps.

Line 113: Using ‘>’ to find gap sizes is ambiguous. The data in S1 provides a specific range for each gap size.

Line 115-9: The physical attributes of the vegetation in each gap is not given in Table S1, and is an important issue with gaps at two elevations are natural and the other by logging(?). What is the height, size (basal area), and aerial crown cover of the residual forest canopy? What is the amount and size of the 20 year old understory? All these factors can affect ground level biophysical processes. These are also factors that affect the relevance of the conclusions and recommendations.

Line 125: At each elevation, a gap size was replicated three times, and only one bag was retrieved per date? Where within the gap was the plot because it is affected by gap size [28]? Where were the litter bags installed, indirect contact with mineral soil, on top of the existing litter layer?

Line 129: How many depths of litter was temperature measured? Where relative to bags were temperature measured – on top or underneath, or someplace else?

Line 131+: Biophysical processes can not be defined by calendar dates, particularly when elevation is involved. The best example is stage TS, Table 1.

Line 162: Duncan post hoc test is a very liberal test to determine significant differences in an ANOVA. In Table 2, mass loss is dominated by altitude and carbon content and release (aren’t these the reverse of the other) by time. What is the practical significance of the other factors?

Line 179: How was freeze-thaw cycles defined and at what location? What was the range of plus-minus around 0 degrees C defined as a cycle?

Line 195: Figure listing are not in order. Figure 5 should be 2.

Line 188: See line 162.

Line 198: only the gaps at 2998 m was different than the highest location. Gaps at 2998m were not natural gaps. Linking harvested gaps with natural gaps (Line 42) is not appropriate and can not be ignored, particularly when the description of vegetation is also not reported. The maximum difference in mass loss remains only 10 percent after four years, and about 12 percent of mass loss and between about 16 and 20 percent of the carbon loss occurred in the 10 to 40 days (Figure 2) depending on when the first bags were retrieved after placement on the site. Do you have any data how this data depends on the season of initial placement?

Line 219+: By far the largest change occurred between the initial and first OF; it is up to 20 percent of the total. Explanation?

Line 239: After four years, carbon release was not affected by gap size (Figure 4). Within the time period, the differences are dominated by time (Table 2). Quantitatively, the importance of gap size and interactions are small.

Line 279: Mass loss and carbon release can not be linked to non-growing season. How was growing season defined? Was it based on actual data for each altitude and gap? What was the year to year variations? These differences are most obvious for TS temperature data in Table 1. There a major deficiency in biophysical data and in the reporting to the existing data to rely on liberal interpretation of small significant differences reported in the ANOVA.

Line 285: Continuing from the last comment, what is positive accumulated temperature in Table 3? Non-growing season has not been described by other than a calendar date, and elevation is obvious.

Line 281: This reviewer believes that the partial support of hypotheses 1 and 2 are overstated, particularly when differences occurred at gaps at 2998m which was a harvested site and the others to elevation were natural. The data to support a partial interpretation is missing and also undermines the continuation of this part of the discussion such as at Line 314+.

Line 335+: The differences in NGS and NS is not remarkable given the lack of clarity of how they are defined, particularly if only by calendar date. The losses in the first 10-40 days of the 4 years of monitoring is a substantial part of the NGS data, particularly carbon content. Precipitation and soil moisture under the canopy is probably a factor as well. There is also no data that characterizes the canopy of each gap and particularly the harvest only gaps at elevation 2998m.

Line 362: (Concluslion) There are too many confounding biophysical factors, starting with the differences in gap history and specifically, current characteristics of the gaps after 20 years to make meaningful conclusions about these, undefined types of gaps. Hence, these data do not support the need for gap information in models but does justify collection of far more ecological and biophysical data in understanding decomposition processes in gaps.

Author Response

Dear Reviewer,

We are grateful for your constructive comments and suggestions concerning our manuscript (Manuscript ID: forests-1661247), entitled " Seasonal effects of forest gap on Abies faxoniana litter mass loss and carbon release along an altitude gradient in a subalpine forest". Based on these comments and suggestions, we have made careful modifications to the previous version accordingly. We hope the revised version would meet the publishing requirements of Forests. All revisions to the manuscript were marked up using the “Track Changes”. We also provide a point by point response letter to explain the details of the revisions to the manuscript and our responses to your comments.

----------------------------------------------------------------------------

Point-by-point response letter

Title: Physical site locations on the earth’s surface are defined by elevation above sea level and not altitude.

We replace “altitude” with “elevation” throughout the draft.

Line 50: Conclusion at this point is not supported by the rigour of the literature cited.

We rewrite the conclusion, please check it in the revision.

Thanks, we added new references.

Line 90-94: The hypotheses are weak. Altitude is only a number until properly interpreted with biophysical data. Robust biophysical data allows the processes to be determined, which will have applicability to understanding the effects of altitudes at sites other than the current location. The issue is the scientific relevance of this research.

Thank you for your suggestion, we revised the hypotheses as you suggested, please check in the revision.

Line 111: ‘Similar slopes’ range from 21 to 34 degrees. Steep mountains can substantial localized effects on air flow; particularly with how cold air fills gaps.

The study sites are located in the transitional area between the Eastern Tibetan Plateau and Sichuan Basin, characterized by steep mountains, therefore, we selected these typical forests to conduct this research.

Line 113: Using ‘>’ to find gap sizes is ambiguous. The data in S1 provides a specific range for each gap size.

We replaced‘>’ to ‘≈’ throughout the revised manuscript.

Thanks for your suggestion, we added more basic biophysical information in the revision. We double-checked our original records which were conducted as a gap investigation in the same region for another project, we found that the formation reason of gaps in 3000 m was also caused by natural stem breakage and corrected it in Table S1.

We clarified the site description in the revision, please check up in the second paragraph of 2.1. We set up 3 replicates for each gap size (large, middle, small, control) in the three elevations, respectively, and we actually retrieved 3 bags in each replicate. We add that the litterbags were placed on top of the existing litter layer.

Line 129: How many depths of litter was temperature measured? Where relative to bags were temperature measured – on top or underneath, or someplace else?

The temperature recorders were labelled and placed in the litterbags, we retrieved the recorders and change new ones in the same litterbags at each sampling.

Line 131+: Biophysical processes can not be defined by calendar dates, particularly when elevation is involved. The best example is stage TS, Table 1.

Thank you for your suggestion, we agree that it might not be the best way to define the biophysical processes by the same calendar dates for the three elevations. However, for this observation research, we need to define the same decomposition periods for all the treatments considering their cooperabilities. Besides, we set the sampling dates based on our previous field investigations which were already published (please see the references: He, W.; Wu, F.; Yang, W.; Tan, B.; Zhao, Y.; Wu, Q.; He, M. Lignin Degradation in Foliar Litter of Two Shrub Species from the Gap Center to the Closed Canopy in an Alpine Fir Forest. Ecosystems 2016, 19, 115–128; Li, H.; Wu, F.; Yang, W.; Xu, L.; Ni, X.; He, J.; Tan, B.; Hu, Y. Effects of Forest Gaps on Litter Lignin and Cellulose Dynamics Vary Seasonally in an Alpine Forest. Forests 2016, 7.). We will consider your suggestion on finding a better way to define the periods in our further study.

Thanks for your helpful suggestion. We aggreged that it was not correct to use one-way ANOVA with Duncan post hoc test, we did a wrong description in the original version and clarified the description of statistical analysis in the revision. We replaced the repeated measurements ANOVA with the linear mixed models with bags within gap sizes as random effects (please see 2.2 and Table 2). Besides, other factors you mentioned were the interactional effects on mass loss and carbon dynamics during litter decomposition when considering the elevation, gap size and decomposition time combined, which were necessary to be shown in our opinion.

Line 179: How was freeze-thaw cycles defined and at what location? What was the range of plus-minus around 0 degrees C defined as a cycle?

We described the freeze-thaw cycles in the first paragraph of 2.3. The temperature was obtained from the recorders in the litterbags as we mentioned above, and one freeze-thaw cycle was defined as the temperature increased above 0 °C or decreased below 0 °C for 3 h or more, followed by a decrease below 0 °C or an increase above 0 °C for at least 3 h.

Line 195: Figure listing are not in order. Figure 5 should be 2.

We corrected.

Thanks for your question, we double-checked our original records which were conducted as a gap investigation in the same region for another project, we found that the formation reason of gaps in 3000 m was also caused by natural stem breakage and corrected it in Table S1. We started to incubate the litterbags in late October when most leaves fell on the forest floor naturally and experienced intense leaching loss in the study area. Based on previous studies, there was obvious mass loss (mainly the soluble carbon components in litter) during the non-growing season (winter), especially in the first non-growing season when litter fell on the forest floor and experienced intense leaching loss generated by precipitation or freeze-thaw cycles caused by temperature fluctuation. Accordingly, our data also proved this.

Thanks for your suggestion, we re-analyzed the data with a mixed effects model as suggested, and revised the description.

Thank you for your suggestion. Based on previous studies, there was obvious mass loss during the non-growing season (winter), especially in the first non-growing season when litter fell on the forest floor and experienced intense leaching loss. The growing season was defined from late April to late October, based on the actual data in our previous investigation for the study area. For the deficiency you mentioned, please check our explanation in “Line 131+” above.

Line 285: Continuing from the last comment, what is positive accumulated temperature in Table 3? Non-growing season has not been described by other than a calendar date, and elevation is obvious.

We deleted the ‘positive accumulated temperature’ in Table 3 under the consideration that it was not significantly related to the decomposition. Thank you for your reminder, we revised the description of the sampling period in the second paragraph of 2.2.

Thanks for your suggestion, please check our above response of revising the gap formation reason in 3000 m.

Please see our above responses of revising the definition of NGS, GS and gap formation reason in 3000 m. We set up 3 plots under the closed canopy as the control in each elevation, and the temperature characteristics were shown and analyzed. Besides, we agree that precipitation and soil moisture data would be helpful, we will consider that in our further study.

Thank you for your suggestion, we revised the conclusion based on the above revision and our scientific advances in this observation research.

Reviewer 2 Report

This paper attempted to understand the seasonal effects of forest gap on Abies faxoniana litter decomposition as affected by altitudinal gradient. Unfortunately, the current form and substance of this paper need thorough revision. Details of comments and suggestions are presented below:

Title – need to add leaf—LEAF LITTER because this study only investigated leaf litter, not the forest litter consisting of leaves, fruits, twigs, bark, branches, roots

LINE 52-63 – Include literature about the effects of soil and ambient temperature on the litter decomposition

Line 90-94: Hypotheses are vague. Rewrite. I suggest to write the hypothesis as a null hypothesis.

Line 113-114 –Confusing. Three gaps were arranged for each size, three plots under the ambient closed canopy (control)

Line 124 – 10 g of dried litter is very small. Do you have explanation for this?

Line 125 – Place on soil surface, you mean on the top of litter layer?

Line 126 –Not clear. Explain why you have 16 samplings (do you mean you did 16 samplings for four years or you mean 16 subsamples)

Line 129 – Vague. Add some description on the depth of the insertion of temperature probe into the litter layer. In relation to line 125, how does this affect the litter bags you place on the soil surface?

Line 150 – I do not see any difference of this equation with the next equation. What this equation would generate is the percentage Mass Loss (%ML)

Line 151 – I disagree with the use of the term C release. Change or delete this section.

· To quantify the C release, the authors must conduct C analysis in the laboratory to determine the proportion of C in the dry matter. In most cases, C content is assumed as 50% of dry weight. What the authors are trying to quantify in this equation is the change in weight of litter samples from start to end of the observations, and then expressed this change in percentage. Please note that change in weight of litter is NOT equivalent to C release because it does not mean the amount of mass loss of litter is equivalent to the amount of liberated or released C. The C constitutes only a fraction of dry matter (leaf litter).

· I also suggest the authors should indicate this equation as percentage change in weight of litter from t₀ to t_n.

Line 162 – Specify the error terms in testing the hypothesis you mentioned in the introductory section. Also, indicate whether you are conducting normality test and homogeneity of variance test prior to subjecting the data to Analysis of Variance. Also, provide details on how you treat TIME in your analysis.

Line 186 –Add some good discussion about the relationship between temperature and litter decomposition in your introduction. Also, this should be spelled out as one of your desired objectives in this study

Line 188 – be consistent in writing scientific name

Line 190-192 – The explanation in the discussion section seems not very clear on this. The temperature during this period is more or less the same in all plots, less than 10 C. Why in lower altitude closed canopy has higher decomposition rates than in gap. Is this related to litter temperature or ambient temperature?

Line 194 – higher litter mass loss during NGS when your temperature is less than 1 C in all sites? What is going on here? Do you mean decomposition continue even at freezing temperature? This contradicts to the generally accepted idea that microbial activities stop at temperature close to zero.

Line 202: x-axis of figure should NOT be labeled as decomposition stage, but rather SEASONS. It is misleading to say the deep-freezing stage or GS or NGS as decomposition stages. In figure d, 2998 m, 3309 m, and 3598 m are not decomposition stages, but rather altitudinal gradients.

Line 203 – indicate whether the error bar is standard error of the mean (SE) or standard deviation. Also indicate the n values

Line 212 – Table 2: Here the authors indicated F-value and p-value of mass loss, C content, and C release.

· Presenting both F-value and p-value are already redundant because if the authors wanted to show significant differences, the authors can indicate asterisk on the treatment means of the different sources of variation. The most important data to be presented here are not the statistical calculations of F value and p-value, but rather the values of the means of the decomposition rates (grams), which are reflected in Figure 1, but in percent.

· As I mentioned above, I disagree with the use of C content and C release. C content means the proportion of organic C in the dry matter or biomass, usually assumed to be 50%. In C content analysis, the authors must perform C analysis in the laboratory by performing C/N analysis of the litter or other methods of C analysis. What the authors wanted to show in this study is the change in weight of litter from t₀ to t_n, which is not the same as C content.

· C release could be understood as CO₂ emissions in the C cycle, via respiration process. The authors did not measure CO₂ in this study.

· Thus, it is not also correct to include in the table C content because the authors did not perform C analysis of litter in this study.

· This Table also contained the values of interaction effects of altitude x gap, altitude x time, gap x time, altitude x gap x time. In this analysis, Time (series of observations) is a common factor. I am not sure how the authors analyzed the Time factor, but as mentioned under line 162, all factors are analyzed using a one-way ANOVA. I disagree with treating the data using the one-way ANOVA. I suggest the authors must analyze the data using any of the following approaches:

a. When there is no particular pattern of measuring each Experimental Unit (EU) at each period of Time, considered repeated measures as subsamples for each EU;

b. Each EU is measured at each period of time, but not interested in the time effect: treat Time as a blocking variable and analyze the data using the Randomized Complete Block Design (RCBD)

c. Treat TIME as a second qualitative (i.e. discrete) factor in the factorial design. The main interest in this approach is to test if the response variable is different among the different times.

Line 216-227: Review the use of the word C contents, which I disagree as an appropriate term to describe what the authors had been investigated.

Line 236: I am wondering what the authors is trying to show in this figure. Why need to show a relationship between C content and mass loss?

· Obviously, the amount of C decreases with decreasing mass loss. The proportion of C contents in dry matter (leaf litter) could not be changed by decomposition process. The C content will be degraded through decomposition and released back into the air in the form of CO₂.

· If the authors want to figure out the factors that drive the decomposition of leaf litter, they must show the relationship between driving factors (litter temperature, litter quality, precipitation, air temperature) and mass loss, which can be shown in the R-square values.

Line 238-258: review the term C release because I disagree with the use of this term in this manuscript.

Figure 260: Similar comments as above, C release is not an appropriate term here.

Line 278-279: The authors mentioned about Pearson correlation analysis, but not supported by data. I am wondering how the authors analyzed the relationship. A scatter plot will be useful here.

Line 279-281: Needs explanation about the implication of this negative correlation. Does this mean an increased freeze-thaw cycle results in reduced decomposition of leaf litter?

Line 282-283: Confusing. What do you mean positive accumulated temperature and mean temperature? I am not sure what this relationship implies. The authors must clearly explain.

Line 284: In relation to the above comments (line 278-279), the authors here must present the regression analysis with r-square values, rather than presenting p-values. They authors must also indicate the mean values of the data they analyzed.

Line 287-288: I think this is not an accurate statement because this study found increased decomposition rates with forest gaps only in 3309 and 3598, but not in lower altitude in which the decomposition rates decreased with forest gaps. This is not also consistent with the first hypothesis.

Line 293-297: The studies cited here are in warm subtropical climate, which is different from the subalpine conditions. Therefore, it is a little bit off to support the findings of this study. Fagus, Quercus and Castanopsis are broad leaves species, which are easily decomposable materials. Moreover, the study of Zhang is about nutrient releases and not about effects of forest gap. Perhaps the authors could provide some information about the nutrient composition of the leaf of the species in this study to relate the Zhang study.

Line 297-301: The authors relate the role of nitrogen mineralization in the decompositions. However, the results section has no information about the nutrient composition of the leaf litter, particularly the Nitrogen. The authors should provide information that leaf litter of Abies is rich in N.

Line 302-302: As I mentioned earlier, the authors should find appropriate term to replace the word carbon releases.

Line 302-306: Confusing statement. I think divergence is not the proper word here. I am wondering why the authors stated that “different selected areas were not enough resulting in that the effects of ambient disturbance”

Line 306-313: Confusing. The authors simply reported what had been observed, but not providing explanation why these occurrences.

Line 314-319: Revise the presentation for easy digest of the ideas.

Line 320-334: The authors must find appropriate term for carbon release, which could be understood as CO2 emissions.

Line 336-360: Needs rewriting to deliver a clear storyline to the readers. The presentation of ideas in this portion is not easy to digest.

Author Response

Dear Reviewer,

We are grateful for your constructive comments and suggestions concerning our manuscript (Manuscript ID: forests-1661247), entitled " Seasonal effects of forest gap on Abies faxoniana litter mass loss and carbon release along an altitude gradient in a subalpine forest". Based on these comments and suggestions, we have made careful modifications to the previous version accordingly. We hope the revised version would meet the publishing requirements of Forests. All revisions to the manuscript were marked up using the “Track Changes”. We also provide a point-by-point response letter to explain the details of the revisions to the manuscript and our responses to your comments.

----------------------------------------------------------------------------

Point-by-point response letter

Title – need to add leaf—LEAF LITTER because this study only investigated leaf litter, not the forest litter consisting of leaves, fruits, twigs, bark, branches, roots

Thanks, we corrected.

LINE 52-63 – Include literature about the effects of soil and ambient temperature on the litter decomposition

We added.

Line 90-94: Hypotheses are vague. Rewrite. I suggest to write the hypothesis as a null hypothesis.

Thanks for your suggestion, we revised the hypotheses.

Line 111-112 – The description is vague. Is the term site same as “block”? Are the three altitudinal gradients present in one site? Please note that all factors being considered in the study must be present in the site. Line 113-114 –Confusing. Three gaps were arranged for each size, three plots under the ambient closed canopy (control)

We clarified the site description in the revision. The three elevational gradients were presented in three sites, we set up three gap plots for each gap size (large, middle, small) and three control plots under the ambient closed canopy at each site. That means we established 36 plots in total.

Line 124 – 10 g of dried litter is very small. Do you have explanation for this?

Yes, as we used the air-dry litter and considered placing the leaves without overlap, the 20 cm × 20 cm litterbag was almost carpeted with the 10 g litter.

Line 125 – Place on soil surface, you mean on the top of litter layer? Line 129 – Vague. Add some description on the depth of the insertion of temperature probe into the litter layer. In relation to line 125, how does this affect the litter bags you place on the soil surface?

We placed the litterbags on top of the existing litter layer. Thank you for your reminder, we replaced “soil surface” with “top of existing litter layer” and added the description of how temperature recorders were placed in the revision.

Line 126 –Not clear. Explain why you have 16 samplings (do you mean you did 16 samplings for four years or you mean 16 subsamples)

We did 16 samplings for four decomposing years. Please check the details described in the next paragraph.

Line 150 – I do not see any difference of this equation with the next equation. What this equation would generate is the percentage Mass Loss (%ML). Line 151 – I disagree with the use of the term C release. Change or delete this section. To quantify the C release, the authors must conduct C analysis in the laboratory to determine the proportion of C in the dry matter. In most cases, C content is assumed as 50% of dry weight. What the authors are trying to quantify in this equation is the change in weight of litter samples from start to end of the observations, and then expressed this change in percentage. Please note that change in weight of litter is NOT equivalent to C release because it does not mean the amount of mass loss of litter is equivalent to the amount of liberated or released C. The C constitutes only a fraction of dry matter (leaf litter). I also suggest the authors should indicate this equation as percentage change in weight of litter from t0 to tn.

Thank you for your questions, we would like to explain the above misunderstanding together. First, we did conduct the carbon content analysis in the laboratory using the dichromate oxidation-ferrous sulfate titration method (please see the description at the end of 2.2). Second, the carbon release was calculated from litter remaining dry mass and analyzed carbon content. Therefore, the first equation was used to generate the percentage of litter mass loss data plotted in Figure1 and 2, while the second equation was used to generate the percentage of litter carbon release data plotted in Figure 2 and 3.

Line 162 – Specify the error terms in testing the hypothesis you mentioned in the introductory section. Also, indicate whether you are conducting normality test and homogeneity of variance test prior to subjecting the data to Analysis of Variance. Also, provide details on how you treat TIME in your analysis.

Thanks for your helpful suggestion. We added that “All data was conducted normality test and homogeneity of variance test prior to analysis of variance.” in the last paragraph of 2.2. Besides, we did a wrong description in the original version and clarified the description of statistical analysis in the revision. We replaced the repeated measurements ANOVA with the linear mixed models (please see 2.2 and Table 2), in order to test the effects of gap size, elevation, decomposition time and their interaction on litter mass loss, carbon content and release, and bags within gap sizes were set as random effects.

Thanks for your helpful suggestions, we revised the objectives and discussion part.

Line 188 – be consistent in writing scientific name

Thanks, we corrected.

We added more explanation of differences generated in different elevations in the revised discussion as you suggested.

Yes, based on previous studies, there was obvious mass loss during the non-growing season (winter), especially on the first non-growing season when litter fell on the forest floor and experienced intense leaching loss. Additionally, the isolation protection of snow coverage could maintain high microbial biomass and enzyme activity to help litter decomposition even when the atmospheric temperature was low. Accordingly, our results also proved the litter loss in winter. (Please check the references: Qiqian Wu. Season-dependent effect of snow depth on soil microbial biomass and enzyme activity in a temperate forest in Northeast China. Catena, 2020, 195,104760; Xin Wu, Gong Lu, Jingjing Zhao, Hqiqiang Zhu. Litter decomposition, microbial community dynamics and their relationships under seasonal snow cover. Ecological Engineering, 2021, 159, 106089.)

We corrected it as suggested.

Line 203 – indicate whether the error bar is standard error of the mean (SE) or standard deviation. Also indicate the n values

We added.

Line 212 – Table 2: Here the authors indicated F-value and p-value of mass loss, C content, and C release.

Presenting both F-value and p-value are already redundant because if the authors wanted to show significant differences, the authors can indicate asterisk on the treatment means of the different sources of variation. The most important data to be presented here are not the statistical calculations of F value and p-value, but rather the values of the means of the decomposition rates (grams), which are reflected in Figure 1, but in percent.· This Table also contained the values of interaction effects of altitude x gap, altitude x time, gap x time, altitude x gap x time. In this analysis, Time (series of observations) is a common factor. I am not sure how the authors analyzed the Time factor, but as mentioned under line 162, all factors are analyzed using a one-way ANOVA. I disagree with treating the data using the one-way ANOVA. I suggest the authors must analyze the data using any of the following approaches:

When there is no particular pattern of measuring each Experimental Unit (EU) at each period of Time, considered repeated measures as subsamples for each EU;
Each EU is measured at each period of time, but not interested in the time effect: treat Time as a blocking variable and analyze the data using the Randomized Complete Block Design (RCBD)
Treat TIME as a second qualitative (i.e. discrete) factor in the factorial design. The main interest in this approach is to test if the response variable is different among the different times.

Thanks for your helpful suggestion. We aggreged that it was not correct to use one-way ANOVA, we did a wrong description in the original version and clarified the description of statistical analysis in the revision. We replaced the repeated measurements ANOVA with the linear mixed models (please see 2.2 and Table 2), in order to test the effects of gap size, elevation, decomposition time and their interaction on litter mass loss, carbon content and release, and bags within gap sizes were set as random effects.

As I mentioned above, I disagree with the use of C content and C release. C content means the proportion of organic C in the dry matter or biomass, usually assumed to be 50%. In C content analysis, the authors must perform C analysis in the laboratory by performing C/N analysis of the litter or other methods of C analysis. What the authors wanted to show in this study is the change in weight of litter from t0 to tn, which is not the same as C content.·C release could be understood as CO2 emissions in the C cycle, via respiration process. The authors did not measure CO2 in this study.·Thus, it is not also correct to include in the table C content because the authors did not perform C analysis of litter in this study.

Line 216-227: Review the use of the word C contents, which I disagree as an appropriate term to describe what the authors had been investigated.

Thanks, please check our above responses to the litter carbon analysis.

Line 236: I am wondering what the authors is trying to show in this figure. Why need to show a relationship between C content and mass loss?

Obviously, the amount of C decreases with decreasing mass loss. The proportion of C contents in dry matter (leaf litter) could not be changed by decomposition process. The C content will be degraded through decomposition and released back into the air in the form of CO2.
If the authors want to figure out the factors that drive the decomposition of leaf litter, they must show the relationship between driving factors (litter temperature, litter quality, precipitation, air temperature) and mass loss, which can be shown in the R-square values.

Thank you for your question. There were two reasons for us to conduct this figure, one was to see if there was a significant linear relationship between litter carbon content and remaining mass; the other one was to see if there was a significant trend for carbon content among gap sizes and elevations at the same litter remaining mass (which stands for litter decomposition degree).

Line 238-258: review the term C release because I disagree with the use of this term in this manuscript.

Figure 260: Similar comments as above, C release is not an appropriate term here.

Line 302-302: As I mentioned earlier, the authors should find appropriate term to replace the word carbon releases.

Line 320-334: The authors must find appropriate term for carbon release, which could be understood as CO2 emissions.

Thanks, but we would rather keep the C release according to the responses we mentioned above.

Line 278-279: The authors mentioned about Pearson correlation analysis, but not supported by data. I am wondering how the authors analyzed the relationship. A scatter plot will be useful here.

Thanks, another reviewer also mentioned this unappropriated Pearson correlation analysis, which we replaced with Spearman as suggested. Please check the revision.

Line 279-281: Needs explanation about the implication of this negative correlation. Does this mean an increased freeze-thaw cycle results in reduced decomposition of leaf litter?

Line 282-283: Confusing. What do you mean positive accumulated temperature and mean temperature? I am not sure what this relationship implies. The authors must clearly explain.

We deleted the ‘positive accumulated temperature’ in Table 3 under the consideration that it was no significant related to the decomposition. The Spearman correlation results indicated that how litter mass loss and carbon release differences generated by gap sizes were related to the temperature characteristics (freeze-thaw cycles and mean temperature). For example, in the non-growing season, higher litter mass loss and carbon release in the forest gaps were significantly related to reduced freeze-thaw cycles.

Thanks，for the Spearman correlation analysis, the values shown in table 3 were correlation coefficient, and the bold coefficient with asterisk indicated the significant P-value, with *P < 0.05, **P < 0.001. Besides, we used duplicate data to present the analysis.

We agree with your suggestion and revised the description.

We revised the description.

We revised as you suggested.

We deleted the statement.

Line 306-313: Confusing. The authors simply reported what had been observed, but not providing explanation why these occurrences. Line 314-319: Revise the presentation for easy digest of the ideas. Line 314-319: Revise the presentation for easy digest of the ideas.

Thank you for your comments, we revised the description throughout the discussion part as suggested.

Reviewer 3 Report

Seasonal effects of forest gap on Abies faxoniana litter mass loss and carbon release along an altitude gradient in a subalpine forest

Here authors carried out a four-year litterbag experiment to assess litter mass loss and carbon releases in a Abies faxoniana forest in China. In addition, they explore how the gap size, elevation belts, and time affect the above-mentioned variables. Overall, the manuscript has valuable information particularly testing the forest response across different elevations, however, I detected a few shortcomings, and some points must be clarified. Although the methods seem appropriate, the analysis on different time spacing between measurements (e.g., the last two years) can be problematic when testing differences over time using classic approaches such as ANOVA repeated measurements. For instance, other methods such as linear mixed models can deal with different time spacing coupled with the inclusion of clustering groups (bags nested within gaps) and random effects. So, I suggest the authors look into this. Furthermore, the Discussion must be improved in two aspects. First, I encourage the authors to subdivide the discussion according to the goal/hypothesis, since it is hard to follow all the aspects in one single section. Concerning this, it is also convenient to include the Figures or Tables when the results are been discussed. This is helpful for the readers. The second point is that discussion must be improved, especially in the fact of the divergent response of the first elevation belt in some metrics (e.g. decomposition constant). Finally, I suggest the authors generalize the elevation belts (e.g. 3000, 3300, and 3600 masl). Therefore, I encourage the authors to improve some parts of the manuscript and clarify the key message. Comments are attached.

Abstract

L19. Please see McVicar and Körner (2013) on the use of altitude/elevation terms in ecological studies. Furthermore, I suggest making the elevation belts more general (3000, 3300, and 3600 m asl). I do not find relevant such precision in the proposed categories

McVicar, T. R., & Körner, C. (2013). On the use of elevation, altitude, and height in the ecological and climatological literature. Oecologia, 171(2), 335-337. doi:10.1007/s00442-012-2416-7

L20. I am not a botanist but some sources find Abies faxoniana as a synonym of Abies fargesii var. faxoniana (Rehder & E.H.Wilson) Tang S.Liu. Please review it.

L24. Please add the months included in each category

Introduction

L42. Please move “human deforestation” to the end of the list. Please add some natural disturbances such as fires or pest events.

L44. Please add “temperature” or “radiation”.

L45. I think the authors can do a better job summarizing the literature abort gap size and litter decomposition rather than only mentioning two manuscripts

L57. Can the authors add examples of the soil biological indicators?

L62. Please add “size” after the gap

L66. Driven mainly by soil temperature and moisture

L67. “forest fap” – “forest gap”

After reading the Supplement I suggest changing the “>” symbol for “≈” which makes more sense since the first one implies ranges.

L87-90. Maybe it is my impression, but I think that your goal was more related to seeing how gap sizes can affect litter decomposition and carbon cycling along elevation gradients and time.

L91. Please add “sizes” after “gap” or make clear that you are expecting a stronger response with bigger gaps

L93. The (ii) it is too general, what would happen to higher or lower elevations?

Materials and methods

L108. Reference

L111. Please add the slope and aspect within the parenthesis. Replace with A. faxoniana

L118. 20 years? So, the gaps have not changed during that time? Please clarify

L131. The growing season is for A. faxoniana, right?

L142. What is the negative accumulated temperature?

L148. During a day/week/month?

L158. Please add the meaning of “a” and “e” for the readers who are not familiar with the equation

L169. Not sure if the Pearson correlation is appropriate considering that some variables especially the freeze-thaw cycles are probably not normal. Maybe Spearman could be a good option

Results

L177. Can the authors add a reference value for temperature?

L180. Same idea here, what was the mean frequency of the cycles?

L181. Add (November to March)

Table 1. It would be interesting to know the soil water content throughout the year. Can the authors add some information about it? Probably, it is strongly related to Freeze-thaw cycles.

Year = Average?

Can the authors add letters for multiple comparisons within stages? For example, how different is the temperature between large gap and closed canopy within OF at some elevation?

L188. Change to A. faxoniana

L195. Figure 5?

Figure 1. I think that the label of the x-axis and y-axis in subplot “d” are wrong.

Table 2. The authors did not mention anything about mixed effects models, which is different from ANOVA (repeated measurements). So please clarify this point. For instance, my suggestion is to use linear mixed models since allowing the clustering in groups (bags nested within gap sizes) and deals more efficiently with time measurements. For example, it allows the use of different time spacing between measurements which in fact, it is something that happened here.

Change to A. faxoniana

The significant factors for mass loss are not in bold.

Figure 2. It is not supposed that a lower carbon content would be expected across time within the same gap size? For example, why did the carbon content increase for the last year within large gaps at 3000 masl?

Table 3. Can the authors explain what is the positive accumulated temperature?

Discussion

I suggest the authors subdivide the discussion accordingly to the results or the goals of the study. Furthermore, mentioning the Figure or Table where the results are been discussed is useful for the readers.

L287. “forest fap” – “forest gap”

L293. Add a space after [20]

L302. Add (Figure 5) after “size gradient”

L302 - 306. This could be confirmed by the analysis that I mentioned in Table 1. Looking for real differences among climatic variables along the altitude can help to discuss why in some cases there are no differences despite the gap size or the elevation.

L315. 2298 m site. But relabel to 2300 masl

L319. Please add the following Table or Figure

L337. Please add the following Table or Figure

L338. Please add “.”

L360. For instance, I did not read anything about the decomposition constant and the divergent trend among elevations (Figure 1).

Conclusion

L361. I encourage the authors to conclude something related to the null difference among forest gap sizes at all elevations at the end of the experiment, so the first two years are crucial especially the non-growing seasons.

Author Response

Dear Reviewer,

We are grateful for your constructive comments and suggestions concerning our manuscript (Manuscript ID: forests-1661247), entitled " Seasonal effects of forest gap on Abies faxoniana litter mass loss and carbon release along an altitude gradient in a subalpine forest". Based on these comments and suggestions, we have made careful modifications to the previous version accordingly. We hope the revised version would meet the publishing requirements of Forests. All revisions to the manuscript were marked up using the “Track Changes”. We also provide a point-by-point response letter to explain the details of the revisions to the manuscript and our responses to your comments.

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Point-by-point response letter

Here authors carried out a four-year litterbag experiment to assess litter mass loss and carbon releases in a Abies faxoniana forest in China. In addition, they explore how the gap size, elevation belts, and time affect the above-mentioned variables. Overall, the manuscript has valuable information particularly testing the forest response across different elevations, however, I detected a few shortcomings, and some points must be clarified. Although the methods seem appropriate, the analysis on different time spacing between measurements (e.g., the last two years) can be problematic when testing differences over time using classic approaches such as ANOVA repeated measurements. For instance, other methods such as linear mixed models can deal with different time spacing coupled with the inclusion of clustering groups (bags nested within gaps) and random effects. So, I suggest the authors look into this. Furthermore, the Discussion must be improved in two aspects. First, I encourage the authors to subdivide the discussion according to the goal/hypothesis, since it is hard to follow all the aspects in one single section. Concerning this, it is also convenient to include the Figures or Tables when the results are been discussed. This is helpful for the readers. The second point is that discussion must be improved, especially in the fact of the divergent response of the first elevation belt in some metrics (e.g. decomposition constant). Finally, I suggest the authors generalize the elevation belts (e.g. 3000, 3300, and 3600 masl). Therefore, I encourage the authors to improve some parts of the manuscript and clarify the key message. Comments are attached.

Thank you for your detailed comments and suggestions, it is so helpful for us to improve our manuscript. The three points you mentioned have been clarified in the revision, and the responses point-by-point were listed as followed:

Abstract

McVicar, T. R., & Körner, C. (2013). On the use of elevation, altitude, and height in the ecological and climatological literature. Oecologia, 171(2), 335-337. doi:10.1007/s00442-012-2416-7

We replaced “altitude” with “elevation”, and revised the elevation belts to 3000, 3300, 3600 m throughout the manuscript as suggested.

L20. I am not a botanist but some sources find Abies faxoniana as a synonym of Abies fargesii var. faxoniana (Rehder & E.H.Wilson) Tang S.Liu. Please review it.

Thanks, we double-checked and revised the species name to Abies faxoniana Rehd.

L24. Please add the months included in each category

We added.

Introduction

L42. Please move “human deforestation” to the end of the list. Please add some natural disturbances such as fires or pest events.

We added.

L44. Please add “temperature” or “radiation”.

We added.

L45. I think the authors can do a better job summarizing the literature abort gap size and litter decomposition rather than only mentioning two manuscripts

We revised the description by summarizing more references.

L57. Can the authors add examples of the soil biological indicators?

We revised as suggested.

L62. Please add “size” after the gap

We added.

L66. Driven mainly by soil temperature and moisture

We added.

L67. “forest fap” – “forest gap”

We corrected.

L84. Ok, but what happened to gaps ranging from 0 to 40 m2? Are they included? Could the authors make clear the categories? My impression is that large gaps are > 250 m2, middle (125 – 250 m2), and small (40 -125 m2). After reading the Supplement I suggest changing the “>” symbol for “≈” which makes more sense since the first one implies ranges.

Thanks, we replace the “>” with “≈” as you suggested.

L87-90. Maybe it is my impression, but I think that your goal was more related to seeing how gap sizes can affect litter decomposition and carbon cycling along elevation gradients and time.

L91. Please add “sizes” after “gap” or make clear that you are expecting a stronger response with bigger gaps

L93. The (ii) it is too general, what would happen to higher or lower elevations?

Thanks, we clarified the objectives and hypotheses as suggested in the revision.

Materials and methods

L108. Reference

We added.

L111. Please add the slope and aspect within the parenthesis. Replace with A. faxoniana

We added.

L118. 20 years? So, the gaps have not changed during that time? Please clarify

The study area is located in a national nature reserve region, so the gaps could not be disturbed by human activities and remain stable starting with their formation. We double-check the investigation records and revised them to “25 years”, we mentioned “25 years” here to indicate that all the gaps experienced quite a long time to develop microenvironmental differences within the gaps compared to the closed canopy.

L131. The growing season is for A. faxoniana, right?

Yes, the growing season is for A. faxoniana in the study area.

L142. What is the negative accumulated temperature?

We deleted the “negative accumulated temperature”, it is not supposed to be in the manuscript.

L148. During a day/week/month?

Based on our every 2 h temperature data, we calculated the total freeze-thaw cycles between every two samplings, then divided by days to get the “Frequency of freeze-thaw cycles (time·d^-1)” per day during each decomposition period.

L158. Please add the meaning of “a” and “e” for the readers who are not familiar with the equation

We added the meanings and revised the equation.

L169. Not sure if the Pearson correlation is appropriate considering that some variables especially the freeze-thaw cycles are probably not normal. Maybe Spearman could be a good option

We replaced Pearson with Spearman as suggested.

Results

L177. Can the authors add a reference value for temperature?

We revised the description.

L180. Same idea here, what was the mean frequency of the cycles?

As we divided the total freeze-thaw cycles by days during each litter decomposition period, we use “frequency” to indicate cycles per day.

L181. Add (November to March)

We added.

Table 1. It would be interesting to know the soil water content throughout the year. Can the authors add some information about it? Probably, it is strongly related to Freeze-thaw cycles.

Thank you for your suggestion, but we did not collect the soil water content data, we will consider that in the further study as you suggested.

Year = Average?

Yes, “Year” here means average. However, we deleted it because we thought it was not necessary to analyze the average year data here.

Can the authors add letters for multiple comparisons within stages? For example, how different is the temperature between large gap and closed canopy within OF at some elevation?

Thank you for your suggestion, but we only got one group of temperature data so we cannot analyze the differences.

L188. Change to A. faxoniana

We corrected.

L195. Figure 5?

We corrected it in Figure 2.

Figure 1. I think that the label of the x-axis and y-axis in subplot “d” are wrong.

We revised the labels of the x-axis and y-axis in figure 1 d.

Thanks for your helpful suggestion. We replaced the repeated measurements ANOVA with the linear mixed models with bags within gap sizes as random effects.

Change to A. faxoniana

We corrected.

The significant factors for mass loss are not in bold.

We corrected.

A possible explanation is that the carbon content means the percentage of carbon components existing in the litter, not the net carbon remaining mass, thus there would be slight fluctuation during the four years of decomposition.

Table 3. Can the authors explain what is the positive accumulated temperature?

We deleted the ‘positive accumulated temperature’ in Table 3 under the consideration that it was not significantly related to the decomposition.

Discussion

Thanks for your helpful advice, we revised the discussion substantially and labelled the Figure and Table as suggested.

L287. “forest fap” – “forest gap”

We corrected.

L293. Add a space after [20]

We added.

L302. Add (Figure 5) after “size gradient”

We added.

Thanks, we revised the description as suggested.

L315. 2298 m site. But relabel to 2300 masl

We corrected.

L319. Please add the following Table or Figure

We added.

L337. Please add the following Table or Figure

We added.

L338. Please add “.”

We added.

L360. For instance, I did not read anything about the decomposition constant and the divergent trend among elevations (Figure 1).

Thanks, we added the decomposition constant in the revised discussion.

Conclusion

Thanks for your helpful suggestion, we revised the conclusion.

Reviewer 4 Report

Dear authors,

Thank you very much for submitting the manuscript entitled “Seasonal effects of forest gap on Abies faxoniana litter mass loss and carbon release along an altitude gradient in a subal-pine forest” to the journal Forests.

The manuscript has a lot of typos, please read carefully the entire text and correct this. I am not a native English speaker, but would highly recommend that a proficient English speaker reads the manuscript and corrects is.

You write about seasonal changes, but in the site description the seasonal climate differences are not presented is sufficent detail. Please explain better the climate conditions. Also, the site description is not clear. It would be very useful if you provided a map, and some site photos.

Besides the need for editing, the results are presented in clear manner. However, I would suggest that the novelty and significance of this research is explained more clearly. As not written, the novelty is not clear. Paper reads more like a survey report which lacks essential focus, rather than a scientific article. A more meaningful, and in-depth discussions should be added. Generally, there are still spaces to improve the readability of the text.

Author Response

Dear Reviewer,

We are grateful for your constructive comments and suggestions concerning our manuscript (Manuscript ID: forests-1661247), entitled " Seasonal effects of forest gap on Abies faxoniana litter mass loss and carbon release along an altitude gradient in a subalpine forest". Based on these comments and suggestions, we have made careful modifications to the previous version accordingly. We hope the revised version would meet the publishing requirements of Forests. All revisions to the manuscript were marked up using the “Track Changes”. We also provide a point-by-point response letter to explain the details of the revisions to the manuscript and our responses to your comments.

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Point-by-point response letter

Thank you for your suggestion, we have carefully revised the typos and invited a native English-speaking colleague to check the entire text.

Thanks, we added the climate conditions and site description, as well as the detailed information on how the study sites were set up in the revision. Besides, we also made the sampling dates related to seasons clarified.

Thanks for your helpful suggestions, we revised the whole manuscript substantially, focusing on the issues you mentioned about the novelty, in-depth discussion and improving the readability of the text.

Reviewer 5 Report

Dear Editor in Chief of Forest

As the reviewer of the manuscript entitled as ‘Seasonal effects of forest gap on Abies faxoniana litter mass 2 loss and carbon release along an altitude gradient in a subal- 3 pine forest, I went through the manuscript and found that it has merits to publish in an international Journal. It suffers from minor shortcomings that I have highlighted on the attached PDF manuscript file. In overall, I am recommending for publication after minor revision.

Sincerely Yours .

Comments for author File: Comments.pdf

Author Response

Dear Reviewer,

We are grateful for your constructive comments and suggestions concerning our manuscript (Manuscript ID: forests-1661247), entitled " Seasonal effects of forest gap on Abies faxoniana litter mass loss and carbon release along an altitude gradient in a subalpine forest". Based on these comments and suggestions, we have made careful modifications to the previous version accordingly. We hope the revised version would meet the publishing requirements of Forests. All revisions to the manuscript were marked up using the “Track Changes”. We also provide a point-by-point response letter to explain the details of the revisions to the manuscript and our responses to your comments.

----------------------------------------------------------------------------

Point-by-point response letter

Line 38. see and cite to:

Tajik, S., Ayoubi, S., Khajehali, J., & Shataee, S. (2019). Effects of tree species composition on soil properties and invertebrates in a deciduous forest. Arabian Journal of Geosciences, 12(11), 368.

We added it to the reference list.

Line 93. provide the objectives here

Thanks, We added.

Line 114-118. provide some information about geology and soil classification

We added the geology and soil information of the study sites as you suggested.

Line 152, 159. Replace “where”

We corrected.

Table 1. use the normal font

We corrected.

Line 308-313. compare the results with other scholars

Thanks, we revised the description.

Round 2

Reviewer 4 Report

Dear authors,

Thank you for considering all comments and suggestions and improving your manuscript accordingly.

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Effects of Forest Gaps on Abies faxoniana Rehd. Leaf Litter Mass Loss and Carbon Release along an Elevation Gradient in a Subalpine Forest

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