Effects of Tree Diversity, Functional Composition, and Large Trees on the Aboveground Biomass of an Old-Growth Subtropical Forest in Southern China

Round 1

Reviewer 1 Report

 

Dear authors,

I really like this manuscript. The manuscript is well written. Nonetheless, I have some comments. The authors should highlight more what is really the novelty in their research.

It is a well written introduction. The working questions are clearly stated at the end of the introduction. However, I would appreciate to have clear hypotheses associated to each question, indicating which effects are expected and therefore which tests/analyses can be conducted, but without the figures reported in the introduction (in my opinion they need to be moved in the methods parts)

M&M

Please add the dimension of small plots

 

I think, the focus on the DBH and diversity of tree species is very interesting. Thus, I suggest to use for the introduction and discussion also the comparison and the results found by other researchers in Europe using the CAP approach (see for example:

De Cáceres et al., 2019, A general method for the classification of forest stands using species composition and vertical and horizontal structure. Ann. For. Sci. 76 (2), 40;

Angiolini, Claudia, et al. "Assessing the conservation status of EU forest habitats: The case of Quercus suber woodlands." Forest Ecology and Management 496 (2021): 119432.

 Yılmaz et al., 2018, Effects of the overstory on the diversity of the herb and shrub layers of Anatolian black pine forests. Eur. J. For. Res., 137 (2018), pp. 433-445.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

General comments:

Wang et al sought to elucidate the BEF relationships by analyzing forest survey data from an old-growth subtropical natural forest. The forest survey data were collected from a 80 m * 200 m plot which was divided into 40 small quadrats. A total of 10083 woody plants (including branches), representing 84 species, were recorded in the plot. They used structural equation models to test the effects of tree diversity (species, phylogenetic, functional and size inequality), functional composition, large trees and environmental factors (topography, soil nutrients and understory light) on AGB.

 

They found that: (1) Size inequality, the community-weighted mean of maximum DBH and large trees had significant, positive effects on AGB; (2) Large trees are the driver of effects of tree size inequality and functional composition on AGB; (3) Lower soil phosphorus content promoted an increase in AGB. They further concluded that retaining an appropriate number of large trees and dominant species, as well as maintenance of a complex stand structure, are key measures for improving the productivity of subtropical natural forests.

 

My major concern is the writing. First, I would rephrase parts of Abstract (lines 14-17) and Introduction to switch the focus from BEF relationship to large trees to match the key findings in results, showing the importance of large trees. Otherwise, it may confuse readers. For example, in lines 50-51, it highlighted the uncertainty about the effects of abiotic conditions on BEF relationships, but these results were not mentioned in the abstract at all. Second, please rephrase the introduction to focus on the knowledge gaps addressed by the results. For example, lines 76-78 showed a knowledge gap about how tree diversity promotes niche complementarity which I don’t think can be addressed by variables used in this study, i.e., previous studies on the link between biodiveirsty and niche complementarity used specific eco-physiological measures to quantify niche complementarity, e.g., using plant stem water isotopes to quantify niche complementarity on plant water use.

 

Specific comments:

1.     Line 57-58:

I was confused by greater intra-specific competition than inter-specific one described here because there were no variables used in this study to count for the intra-specific competition. Note that all stand attrbutes used here are based on inter-specific level, not intra-specifc one. Please rephrase this part. 

2.     Line 76-78:

Here highlighted a knowledge gap about how tree diversity promotes niche complementarity. But this knowledge gap cannot be addressed by the variables used in this study because none of them are measures of niche complementarity. I would rephrase this part to focus more on knowledge gaps addressed by this study.  

3.     Line 229-230:

Were the photos from cloudy and sunny days comparable to calculate Co and LAI?

4.     Line 232:

What are the values under the the two PCA axes? Correlation coefficients?

5.     Line 234:

Aboveground biomass is a key variable in this study. I would use species-specific equations instead of the general one. Or please explain why it is appropriate to use the general equation and cite previous studies using general equations for similar research questions.   

6.     Line 311-312:

Please rephrase this part to make a smooth transition from randome forest to SDM.

7.     Line 351-360:

Both the relative effects from bivariate regression in the Fig.3 and variable importance from random forest here are related to the comparisons in the effects of different variables on AGB. I would either combine them together in one paragraph or only focus on the random forest outputs.

8.     Line 361-382

I am not an expert on SEM. Most of the papers I read before only showed one SEM explaining most variance for the final response variable, e.g., AGB in this study. So please add some explanation in the methods why using this multiple-SEM approach.    

9.     Line 414

Note that DBH was used here, instead of height. Please rephrase this part.

10.  Line 453-454

 

I would add one more subtitle for this paragraph about the big tree effect, which is one of  the key messages from this study. 

 

 

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

Dear Authors!

Please find enclosed below few minor comments/suggestions, and request to clarify part of the methods.

L39-40 – In this statement „land“ should be inserted before „…surface area of the globe“

Fig 1 L129 – please consider using „biotic“ here instead of „biological“

L141 (and later in) – „hm2“ should be replaced by „ha“ (hectares, 1 ha = 10000 m2)

L202-211 – I did not understood here whether you have constructed the DEM (Digital Elevation Model) or you calculated terrain parameters in some other way? I expect that you have used DEM, in which case you should state the spatial resolution of the DEM grid.

L228-229 – I have two questions here. I cannot follow how you ended up with 600 hemispherical images. You have 40 quadrat that are spatially adjacent to each other, meaning that neighbouring quadrats has common edges, and consequently corners. This means that there are unique 95 locations for taking photo, and if you took three photos at each location this ends up with 285 photos, and not 600 photos. Latter should actually mean that you have altogether processed 600 images (because same image could be used multiple times in combination with other for each particular quadrat). Furthermore, if you have made three photos on each location, and for each quadrat you have used five location, how have you processed and used those images? Have you calculated mean LAI from three photos at each location, and used those from five location to get mean value per quadrat, or all 15 photos at once, or something else?

L 263 – I guess here some word(s) missing between „…whether the mean values of“ and „are evenly…“

L366 – capital „S“ in „Simpson's indeks“

Author Response

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Author Response File: Author Response.docx