Short-Term Effects of Thinning on Tree Growth and Soil Nutrients in the Middle-Aged Chinese Fir (Cunninghamia lanceolata (Lamb.) Hook.) Plantations
Round 1
Reviewer 1 Report
This paper describes thinning responses to Chinese fir established at four locations each with different stocking conditions. Diameter, height and volume are used to describe stand structural responses to thinning while canopy gap analysis, soil total C/N and pH are used to describe shifts in resource availability. Authors conclude that thinning increases growth of retained trees, altered soil pH and opens the canopy. Growth responses are claimed to be larger in the three younger stands than in the one older stand and so they also recommend thinning earlier in the rotation. Some useful data on Chinese fir thinning response is presented, however insufficient number of stands and density measures are presented to permit conclusions about forest responses to age. Furthermore, there are insufficient variables measured to draw conclusions regarding responses in above vs. belowground resources. The writing is generally good, but there are turns-of-phrase and grammatical flaws to indicate that the paper should be thoroughly edited by a native English-speaker.
The experimental design and the collected and reported measurements are insufficient to draw conclusions about timing of thinning and relative responses by growth resources. A major concern is that the only measure of stand density presented is the number of trees per hectare. As the authors note, competition among trees increases as trees grow in size so it is necessary to include metrics that account for both size and number. Basal area would at least provide some measure of the stocking, but Reineke's Stand Density Index is preferred, especially if it is expressed as a fraction of maximum SDI. Additionally, the authors don’t report the actual number of trees per hectare, only some target as evidenced by all the plots having the same round number for TPH in every plot per site. The thinning treatment is expressed as a percentage of those round values, so that we don’t even have a sense of what is the variation is the density. Considering the authors inventoried all trees in measurement plots, it is certainly possible to provide adequate measures of stand density with the data in hand.
Conclusions are drawn about the timing of thinning within the rotation. Early rotation (11-yr-old) stands are well represented in this study with three being included. However, only one other age (20-yr-old) stand is included in the study, so it is not possible to draw conclusions based on stand age. The 11-yr-old stands have different TPH, so there is really only one stand representing each age-stocking combination. Although, various stocking levels among the 11-yr-old stands provides a minimal test on the effects of stocking on stand responses if analyzed in a regression type of design rather than a replicated treatment (Anova) design. We may also find that the densities,, as measured with percent of maximum SDI, form a gradient in stocking. For instance 11-yr, 2000 TPH representing open-grown stand structure while 11-yr, 4000 TPH and 20-yr 2000 TPH represent higher levels of density. In that case, it may be possible to draw conclusions about when to thin based on stand density, but the analysis would differ substantially from that presented.
Conclusions are also drawn about the relative responses of growth resources to thinning. The gap analysis is sufficient to quantify changes in light environment. However, pH, and total C and N are not sufficient to represent availability of belowground resources. Critical resources to measure include soil moisture and nutrients (primarily N) that are available for uptake. Moisture could (still) be measured with periodic or one-time spot sampling to determine if thinning affected available soil moisture. Unfortunately, no measure of moisture was presented. While pH and soil carbon are master variables that largely control belowground processes, they are indirectly related to nutrient availability. The best variables to measure nutrient availability are extractable concentrations. For N, that would primarily include salt extracted nitrate and ammonium concentratins. Total N is very slow to respond because it represents large unavailable pools of N that must be mineralized to become available. Chase et al 2016 (doi.org/10.1016/j.foreco.2015.12.014) demonstrate how to make relative comparisons among various growth resources in response to thinning treatments.
I notice that the gap analysis program also represents LAI which is a more useful, standardized metric of forest production potential. Unfortunately, only ‘openness’ is reported in the paper. Please consider switching to LAI and of course, comparing values obtained to other LAI values measured in Chinese fir and other forest thinning studies from the literature.
Detailed comments are listed below listed by line, table or figure.
L 23. Stability is what sense? Drought tolerance, wind-throw, be specific.
L 31. Many instances where spaces between words are missing, e.g. importantconifers”. Attention to detail is important so that we can focus on the science.
L 37. “quality of the economic value” be specific
L 43-4. References 16-19 are of questionable support for the statement.
L 46. References on density management diagrams illustrate the need to determine density based on both tree size and tree number. Given knowledge of this concept it is surprising the authors only express density as TPH. Are DMD available for Chinese fir from which to obtain maxSDI?
L 51. Example of improper grammar. Use “not well known”
L 51-6. Uncertain connection to thinning and management. This new topic should be introduced and better explored is a separate paragraph.
L 59-60. What are ‘improved soil nutrients’? Quantity, balance, be specific.
L 62. What is ‘soil nutrients development’?
L 62-4. Research is not well justified by the introduction. The concepts in this section-concluding statement about short-term processes and sustainable forestry is vague without concrete thesis or testable hypotheses.
Table 1. Include site names. TPH can’t be exactly the same in every plot. This table should include actual tree counts from initial inventory where heights and diameters were measured. Some measure of density that includes both tree size and tree count should be displayed. Preferably stand density index (SDI), or even basal area (BA).
L 91-3. How were thinned trees selected? Were they smallest trees (from below), largest trees (from above) or randomly? Justify the selection of 35 or 43% thinning intensity. Using percentages of the number of trees to thinning target is simplistic. What was the original SDI or BA? How does that relate to the maximum SDI or BA, and what is the target SDI or BA for the thinning treatment? Since trees were measured in Oct 2020 and thinning was applied in Dec 2020, it’s not clear what trees were included in results for thinned plots.
L124. Explain how O-horizon was handled; included or excluded from soil samples.
L 128. Describe standards, duplicate samples used (i.e. quality control).
L 130. Statistical analysis is not adequately described. Other than normality tests only Wilcoxon rank sum is described which is a pair wise non parameteric test. This might be valid as a post-hoc test for means comparison, but it is not appropriate to understand the experiment-wide effects of thinning and sites. A two-way factorial using the plot averages is most appropriate for this type of data. Results should display F-statistics and P-values for main effects and the thinning by site interaction. There is no mention of analysis for canopy openness, LAI, Total C/N or pH. Description for analysis of those should also be included.
L 142. Such a response is consistent with thinning from below.
L 140. Text describes age and density but tables and figures don’t list. It is annoying to readers to have to go back and find. Include this type of information in all table and figure captions. Why does figure show DBH and table list DQ. No purpose for this is explained. Decide on one or the other and use consistently throughout (preferably DQ).
L 142. Describe how diameter responded to thinning before describing increment. Expecting that if plots were thinned from below then 2020 measurements in thinned plots would be slightly larger than unthinned plots, but that is not the case. Table 1 shows no consistent trend between Con and Thin. Fig 1 shows in bottom bars at Xiao that Thin decreased relative to Con, but increased in all other sites. Maybe the thinning was not from below. Or, is it possible that 2020 values in thinned plots included all trees measured in Oct, even those that were removed during thinning in Dec? The thinned trees must be removed from 2020 measurements, otherwise increments would be inaccurate because the removed trees would no longer exist in 2021. From her on I will assume that thinned trees were removed from Thin plot measurements.
L 143. Based on calculating growth percentage as increment/2020 x 100, the growth percentage in Con at Jin was 7.29%, making it the highest rate and growth at Xiao was 6.95%, meaning Xiao is never the highest. Please check the calculation and accuracy of statements regarding values.
L 144. Delete “After one-year thinning”. It is confusing because controls were also measured after one year. Removal will eliminate any confusion.
L 153. Rephrase the description for upper case letters; ‘different treatments in the same plots’ can’t be correct since plot is the experimental unit.
Table 2. I calculate somewhat different values for increments in Con plots at Xiao and Yang. Please check all calculations.
L 157. Table 3 shows that increments were actually higher in Con plots except at Yang; however, letters and standard error overlap indicate that means are statistically equivalent between Con and Thin treatments at all locations, thus they should not be spoken of as being different in either direction.
L 175. This requires presentation of statistical test showing this trend is significant. For instance, what is the P-value for a linear regression of pH as a function of TPH or include letters distinguishing sites in Table 4.
L 181. Notice soil samples were for mineral soil and exclude O-horizon (should be described in methods, see L 124). It is rare to see changes in soil carbon in short-term responses to management. Indicates the need to describe quality control for total CN analysis (see L 128).
L 182. Figure numbers from here on appear to be incorrect.
L 184-7. Delete. See L 188.
L 188. This indicates that C content change was insignificant, so it is not possible to speak of any relative increases or decreases and invalidates previous statements.
L 188-9. Delete. See L 188.
L 196. Total N did not change at Xiao.
L 211-3. This section suggests that evidence presented of growth responses can be attributed to “aboveground resource availability”. This section must be much more specific to accurately represent what was measured. The growth increase was associated with increased canopy openness, and it was not associated with changes in pH, total N or total C. While N is a belowground resource, total N is not a measure of N availability. To support that there were not below ground changes, It would be necessary to show that extractable N did not respond to thinning. pH and C are indirectly related to nutrient availability and are not direct measures of belowground resources. Belowground resources also include soil moisture availability which was not measured in this study (although it could still be along with another set of stand measurements). Thus, it is not possible to draw the conclusion stated in these lines unless specific measurements are described and avoid using general terms such as above- and belowground resources. Chase et al 2016 and references cited within demonstrate that post-thinning soil resources are proportionally equal or more important than light interception.
L 222. In what sense are forests recovering? What is the desired target?
L 224. It’s necessary to include studies that measure nutrient availability. Nutrient stocks are large and unchangeable, nutrients availability are critical to this discussion.
L 228. Thoroughly present canopy results in Results section. Include statistics on site differences and any site x treatment interactions.
L 228-31. On what basis was growth in Thin stands slower than in Con at Yang? Table 2 shows diameter increment was three-fold higher in thinned than control. Table 3 shows > 2x increase in volume increment. The time perspective is also odd with respect to “in the long-term. Only one-year responses are reported here.
L 231-4. This is exactly the reason why thinning targets should be based on three size and tree number, i.e. SDI or BA. Pretzsch certainly understands this. See their section 2.2.2.
L 241. Please provide citations of references that describe changes in pH due to drought. I could only find such info for fertilized agricultural fields. This line of discussion must be support by appropriate authorities. Is it possible that pH for 21 samples were measured at a different time than 20 samples and measurement errors created an artifact in the data?
L 247. Sankey & Tatum measured moisture and temperature. This is a new topic unrelated to pH. They certainly demonstrate that thinning affects belowground resources which contradicts overly generalized conclusions in this paper (see L 211-3). But their results should not be said to be similar to any pH response.
L251. Explain what is meant by stability. It’s likely not mechanical stability. Be specific.
L 254-8. Thoroughly present understory diversity in Results section. Include full statistical treatment including P-values for regression lines in Fig 6.
Author Response
Dear Reviewer,
Thank you very much for the constructive suggestions and comment on our manuscript.
The point-by-point response is the Word version. Please see the attachment.
Best wishes,
Na Lin
Author Response File: 
Author Response.docx
Reviewer 2 Report
Dear Authors,
Your research work is interesting because it shows the short-term effects of thinning on tree growth and soil nutrients 2 in the middle-aged Chinese fir plantations. This paper is prepared in the usual way for scientific work. However, I have a few comments and remarks:
1. Title. “fir(Cunninghamia" - add space.
2. Abstract. Please rewrite this chapter - hypothesis first, then results.
3. Introduction. Please add information on sustainable forestry and the role of thinning.
4. Line 98. “(N)were” – please add space.
5. Line 112. Please move the equation (4).
6. Line 125. “form72” - please add space.
7. Line 128. Please add the country of manufacture of the analyzer used.
8. Line 129. In what solution did you determine the pH? Complete the information.
9. Table 1 and Table 3. A dot is missing.
10. Figure 1, 2 and 5 are illegible. Increase the name of the farm (fig. 1-2)
11. Figures 1-7. Unify the signature. According to the instructions it should be "Figure".
12. Tables 2-4. Please move the explanation below the table.
13. Lines 216-219. In the text, reference numbers should be placed in square brackets [ ], and placed before the punctuation; for example [1], [1–3] or [1,3]. Prepare a document according to the guidelines for Forests https://www.mdpi.com/journal/forests/instructions. à So, Xu et al. (2020) [37]. Please check the entire document carefully!
14. Line 247. Should be “and” instead “&”.
15. Please rewrite the chapter Discussion. In my opinion, the pH paragraph should come after C and N. In addition, the impact of thinning on the growth and development of fir plantations and sustainable forestry has not been sufficiently discussed. Please complete it.
16. Conclusions. „stage of middle-aged Chinese fir plantations” - How old is it?
The language appears to be correct, but I don't feel qualified to judge about the English language and style.
Good luck!
Sincerely yours
Reviewer
Author Response
Dear Reviewer,
Many thanks for your constructive comments and suggestions. We try our best to revise the manuscript. The point-by-point response to your comments as a Word file. Please see the attachment.
Best regards,
Na Lin
Author Response File: Author Response.docx
Round 2
Reviewer 2 Report
Accept in present form
