Decomposition of Herbivore-Damaged Leaves of Understory Species Growing in Oak and Pine Stands

Round 1

Reviewer 1 Report

The paper contains a lot of data and analyses, used to investigate five hypotheses. The experiment and data seems to be solid, and increased knowledge about decomposition is important from many aspects. My main concern is that the structure and organization is a bit messy in some aspects. Since there are several hypotheses and a lot of data and analyses, a very clear structure is required throughout the paper, in order for the reader to be able to follow and understand the results. One important aspect in this is that the formulation of the hypotheses does not fully correspond to how the results are described.

I have given examples about this in “General comments”. In “Specific comments”, some more detailed comments and suggestions are given.

General comments:
Abstract: It is not clear in the abstract what the aim of the study is, and what you are going to analyze, which makes the results in the abstract more difficult to understand. And even when I read the hypotheses in the introduction, and compare with the results in the abstract, I don´t think I get clear answers to what the hypothesis testing gave. You don´t need to mention the hypotheses in the abstract, but the results should correspond to the formulation in the hypothesis. One example: You write that “We found that faster decaying leaf litter had a lower content of defensive compounds and a higher content of TNC and nitrogen, regardless of the plant species and leaf damage.” whereas the corresponding hypotheses (H4 and H5) is about how decomposition is affected by composition, not how the composition is in litter of different decay rate.

In my understanding, H1 and H2 are the most central ones. I suggest that you start to describe the results corresponding to H1 in the abstract (“Contrary to our expectations, leaf litter of resistant species decomposed faster than that of susceptible species.”), and then the results corresponding to H2. I don´t even find the results corresponding to H2 in the abstract, but in the end you write that  ”In our opinion, this is the main reason for easier decay of damaged leaves.”, although you never said in the abstract that the damaged leaves decay faster. Please clarify aim and results in the abstract.

Introduction:  You have five hypotheses, but it is difficult to get an overview, since they are “hidden” in a lot of text. It will be much easier to get an overview if you make them more visible, e.g. by trying to condense the text about the hypothesis, and add numbers, (1), (2), etc, for the hypothesis. You could even make a list of it:

(1) XXX
(2) XXX

When I look closer to the hypothesis, I think H3 appears a bit surprisingly, it doesn´t really fit in, based on the title and intro of the paper, which focuses on decomposition of litter related to insect damage. If you want to keep it, you need to introduce that question better in the intro.

Moreover, I think that H1 and H4 are very much connected, and also H2 and H5. The link between them should be made very clear. A suggestion is to have four hypothesis, e.g. 1a, 1b (1 and 4) and 2a, 2b (2 and 5). It would make it easier to write about it in a structured way, in the introduction as well as in the results and discussion.

Results: The content of chapter 3.2 and 3.3 do not correspond to hypothesis 4 and 5, or to the headings of the chapters. It should be about how metabolites (and substances beneficial for the growth and development of herbivorous insects) affect decomposition, but now it is more about how contents of metabolites etc vary with e.g. plant species. Please modify the text accordingly. Or change the hypotheses and the headings.

Moreover, I suggest that you relate to the hypotheses (true or not) already in the Results section, since it actually is the results. It will be much easier to read the paper if you do like that. Now it is difficult to find the results of the hypothesis testing (in the Discussion). For example, I didn´t find it at all for H4. Then, potential explanations for the results fit well in the discussion. And you don´t need to introduce the hypothesis as you now do in the discussion, e.g. “…we formulated our third hypothesis”. It is already done in the intro.

Discussion: See comment about hypotheses above. Also, it would help a lot with a few headings in the Discussion section. It is rather long, and difficult to read as it is now. Also, I think it would be easier to discuss H1 and H4 together, and H2 and H5 together (see General comments about Introduction, above).

Conclusions: The first sentence can be simplified and more straightforward than to say “…the negative effect on…and the positive effect on…. could not be confirmed.” Please go through the conclusions again, it is not very clear as it is now. Two “However” sentences in a row create some confusion.

Specific comments:
Page 1, line 19-20.  The last part of the sentence “Understory plants in Europe vary in leaf resistance to insect damage as follows, from the least to most damaged:…” needs to be reformulated. It should be about “resistance” not “damaged”.

Page 1, line 31: I suggest that you replace “In our opinion, this is…” with “We interpret this as….”. It sounds more scientific that “In our opinion”.

Page 2, line 38. I have problems with the first sentence. Do you mean that the understory vegetation is important in forest stands where the understory vegetation is mainly shrubs?

Page 2, line 43. Remove “etc”, since you already wrote “such species include”.

Page 2, line 52. I don´t understand why phytomelioration is mentioned. Please add a reference if you keep it.

Page 3, line 139-140. You only specify what type of the tree species for Scot pine, not for oak, beech, etc. Is there a specific reason for that? Otherwise you should just write pine.

Page 5, line 205: To clarify, I think you should specify in the title that it is species of plant litter, and stand species. E.g. “Influence of plant litter type, leaf damage, and stand species on litter decomposition”
Page 5, line 215-217: How can the decay rate be as high in S. Nigra as in the other species with low susceptibility, whereas the reduction of litter mass is not correspondingly high in S Nigra? There are also other cases later where decomposition rate and reduction of litter mass is not correlated, I don´t understand how that can happen. Please discuss that in the Discussion section.

Page 5, line 226: I suggest: “There was a significant Sp*LD interaction, indicating that the impact of damage on the mass of the remaining litter was affected by the shrub species (Sp)”. Now the first part of the sentence “The impact of damage on the mass of the remaining litter was affected by the  shrub species (Sp)…” says the same as the second part …and there was a significant Sp × Ld interaction.”

Page 5, line 226-229. I don´t fully understand the sentence: “A comparison of the remaining litter mass (average in both stands) showed that, compared to control leaves, the mass loss of damaged leaves was the largest in P. serotina (by 27.1%), followed by S. nigra (6.1%) and C. avellana (1.7%)”. Was the mass loss 27.1 % larger in damaged leaves than in not damaged leaves in P serotine?  Please clarify the sentence.

Page 5, line 229-231:  The two sentences “The opposite was true of other species. They were faster to decompose control than damaged leaves, in F. alnus by 7.1% and P. padus by 4.3%, and 230 in C. sanguinea there was no difference.” need to be worked through. It needs to be (1) clarified in the same way as the sentences above (see comments for  line 226-229), (2) “They were faster to decompose control than damaged leaves…” sounds strange since “they” refer to litter of  “other species”. (3) The opposite was only true for two of the other species, in the first sentence it looks like it is true for all other species.

Page 6, line 238: You write “The stand did not significantly….”. I suggest that you write “stand type” or “stand species”. This should be done throughout the paper. It sounds strange to say that “the stand did not affect….”

Page 6, Table 1: I suggest that you split the Table into two (one with k values and one with ANOVA). The first part of the Table (above ANOVA) is not fully explained as it is now (it is not written what is within the brackets after the “k” values). If you split the Table, I suggest that you put the info from Page 5, line 206-208 in the first table, so that you can start the Results section with the interesting things: “Susceptibility and plant species had a significant impact….”

Page 7-8, Table 2: I suggest the same type of split as in Table 1. And the formatting makes it difficult to read, needs to be fixed (<0.001 in two rows as it is now).

Page 7, line and page 9, line 287: The headings only reflect part of the content: effects of metabolites, not effects of substances beneficial for the growth and development of herbivorous insects (e.g. nitrogen). Should be included (or just lump it together and call it chemical composition).

Page 10, line 316: Remove “been”

Page 11, line 335-337: Improve sentence “…, because as a result of the earlier feeding of insects, they would…” Also, start the sentence with “We hypothesized…” instead of  “We thought….”. And change the sentence after to “This second hypothesis was not confirmed”, or similar.

Page 11, line 343-344: Improve sentence: “. In P. serotina, almost exclusively shade type leaves are eating by herbivorous insects compared to sun type”. Should be “eaten”, and “compared to “ should be replaced, e.g. with “instead of”.

Page 14, line 430-432: You write “In the last hypothesis we assumed that since leaves damaged by insects due to defensive reaction would have more defensive metabolites (phenolic compounds) and less carbohydrate or nitrogen, they would be slower to decompose.” This formulation seems to be a mix of H2: “the rate of decomposition of damaged leaves by insect would be slower compared to undamaged leaves.” and H5: “leaves that contain higher amounts of defensive metabolites and fewer beneficial substances for the growth and development of herbivorous insects would decompose more slowly.” Please make sure that the hypotheses are the same throughout the paper. I think the formulation here on page 14 strengthens my argument to somehow lump H2 an H5 together (as well as H1 and H4).

Author Response

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Reviewer 2 Report

Thank you for your work on this manuscript. I do have questions about the biological/ ecological significance of the study. The introduction and conclusion do not fully explain why this study is relevant to forests or their management. Furthermore, without additional clarification on the amount of samples remaining for analysis (as you state, 'several bags were destroyed by forest animals') it is not clear how robust these analyses are.

Also, there are sentences with awkward wording that inhibit reading (and understanding) of this manuscript, and table 2 is very difficult to interpret.

Comments for author File: Comments.pdf

Author Response

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Reviewer 3 Report

Below are some of the issues in this manuscript:

a) The choice of 0.3mm mesh size litterbags needs to be justified.

b) The study also does not mention the control litterbags to measure if there is any ingress of organic matter into the litterbags during the period of incubation.

c) Chemical methods mentioned in the chemical analyses section should be elaborated more. e.g. what wavelengths, what standards etc.

d) In the results section, the average mass remaining value is reported. I am not sure what this signifies. How was this estimated or calculated? Would the average value not change depending on when in the incubation the measurements were made?

e) I also saw that some of the litterbags were lost. Because the number of bags per replicates is one, how confident should we be regarding the results?

f) Why are all graphs missing error bars? Please add error bars throughout.

g) Table 2.  What is the value represented by column Mass?

h) There is a clear distinction between 'content' and 'concentration'. The authors have mentioned content in the manuscript and yet have presented concentration data. It would be better to present both the content and concentration change over time.

 

 

 

Author Response

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Round 2

Reviewer 3 Report

The authors have addressed some of my prior concerns. However, I still see areas in the manuscript that needs improvement.

Major comments:

a) ANOVA analysis on initial values needs to be presented to ascertain some of the hypothesis the authors assume they tested. For instance, damaged vs undamaged leaves having defensive compounds. A table with initial values for species*damage*resistance would be meaningful.

b)The authors suggest that Nitrogen content increased with litter decomposition. Why would you expect the content to increase? Deposition? or Movement of N from soil to the litterbags? Content needs to be calculated based on mass remaining and N concentration. 

c) The authors also do not mention C:N ratio in their manuscript. The revision should also focus on the initial C:N ratio of litters and how they correlate with decay rates or the treatment variables like damage and susceptibility. Most of the microbial decomposition of organic matter is driven by C:N stoichiometry.

d) Additional exploratory analyses may need to be conducted/or presented. For example, in Line 667-671, were there any significant linear relationships to suggest that defensive compound explained faster decay rates in damaged vs. undamaged?

e) This is just my thought that would there be not a difference in surface area that is accessible for soil microbes in damaged vs. undamaged leaves? I see you discussed SLA in the discussion section, addressing how leaf damage affected SLA in your leaves may help explain decay differences between damaged vs undamaged leaves.

f) Section 4.3 of the discussion needs further elaboration and clarification. For instance, how about background phenolics, microbial communities, etc. You could add some sentences and targeted citations to prove your point here about "home field advantage".

g) Some of the statements in the conclusion are rather not clear and beyond the scope of this experiment. For instance, the conclusion about rapid transformations between leaves vs. fallen compounds. Also, the last statement in the conclusion is also confounding.

Minor comments:

Line 20: change 'choose' to 'chose'

Line 118: what is 'small fauna'? Does your choice of mesh allow small fauna access to litter in the litterbags? Or do you mean microfauna?

Line 127: add some references for 'home field advantage'

Line 173: Add a statement about "the assumption that organic matter ingress through the litterbag was minimal."

Line 199: Content=Concentration*Mass

Line 218-219: Provide your linear model here.

Line 229: Be consistent 'analyzes' or 'analyses'

Line 252: Add results from simple ANOVA on initial values here.

Figure 1: n=? ; add in the figure caption

Line 310: undamaged before 'control'

Figure 2. Not clear the y axis is mass lost or mass remaining

Line 445-448: Stated earlier. Please recast.

Line 471-472. Not clear what you mean by much original research.

Line 476-477: Not clear what the sentence is trying to state. Recast.

Line 536-538: You could estimate N content in your litter over time with your litter mass and N concentration. This would allow you to see whether the increase in concentration suggests an increase in content over time.

Line 641-643: Would the damaged leaves not have more surface area per unit mass that is exposed for microbes to consume? See my earlier comments.

Line 683-685: Is it the initial or the mean that you are talking about here?

Line 692-693: Add some information about C:N ratios for damaged vs undamaged leaves. This might provide additional insights.

 

Author Response

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Author Response File: Author Response.pdf