Nitrogen and Phosphorus Resorption in Planted Forests Worldwide

Round  1

Reviewer 1 Report

The authors attempt to look at resorption efficiencies of N and P in planted forests of the world. This is a very interesting topic and may yield many interesting results.

However, the statistics are a) insufficiently described and b) biased. Let me explain:

a) The stats section for comparing NRE and PRE in different ways only mentions ANOVA. This has conditions and it has not been explained whether and in which way these have been tested. Nor has any post-doc test and p-value adjustment been described for the tests between multiple "treatments", e.g. the climate zones.

b) The studies used have a different amount of individual values. This can lead to a bias as studies with more values will be over-represented. There may be two solutions for this. Either the authors use a mean for each study (or tree species in the study), or they can use an LME, taking care of the problem with assigning "study" as a random effect. This is even more important as this study is used to draw global conclusions. Statistics have to be carefully done and described in detail!

Therefore, I have not read the whole manuscript as the methodological basis is not yet sound. Just two more thing at this stage:

1) I think Figs 2 and 3 should be presented in a table as this will give the actual values that further research can use and the same information will be transferred.

2) The supplementary dataset should include a variable to replicate the information currently in Fig. 3, i.e. boreal, subtropical etc.

I am very much looking forward to again look at this very interesting manuscript after the statistical description and analysis has improved.

Author Response

Point 1: The stats section for comparing NRE and PRE in different ways only mentions ANOVA. This has conditions and it has not been explained whether and in which way these have been tested. Nor has any post-doc test and p-value adjustment been described for the tests between multiple "treatments", e.g. the climate zones.

Response 1: We have re-written this section as “Before statistical analysis, the normality and homoscedasticity of the NRE and PRE were verified by the Kolmogorov-Smirnov test and Levene's test, respectively.” in Line 130, Page 3 and “If the differences were significant, post hoc multiple comparisons were subsequently conducted using Duncan’s test (P = 0.05).” in Line 136, Page 3.

Point 2: The studies used have a different amount of individual values. This can lead to a bias as studies with more values will be over-represented. There may be two solutions for this. Either the authors use a mean for each study (or tree species in the study), or they can use an LME, taking care of the problem with assigning "study" as a random effect. This is even more important as this study is used to draw global conclusions. Statistics have to be carefully done and described in detail!

Response 2: We have used the LME methods and assigned “study” as a random effect to solve the bias and described in detail in Line 132, Page 3. 

Therefore, I have not read the whole manuscript as the methodological basis is not yet sound.

We analyzed the data according to the comments and the results showed that our major conclusion that “variations in NRE were mainly regulated by climates, while variations in PRE were dominantly controlled by green leaf nutrient concentrations” stays constant.

Point 3: I think Figs 2 and 3 should be presented in a table as this will give the actual values that further research can use and the same information will be transferred.

Response 3: We discussed and preferred to present the data in Figures.  Additionally, we have uploaded all original data as supplementary materials which could be used by further research.

Point 4: The supplementary dataset should include a variable to replicate the information currently in Fig. 3, i.e. boreal, subtropical etc.

Response 4: Done as suggested.

Reviewer 2 Report

No comments or suggestions. Clear and valuable article worldwide!!

Author Response

Point 1: No comments or suggestions.  Clear and valuable article worldwide!!

Response 1: We appreciate the comments.

Reviewer 3 Report

GENERAL COMMENTS

This exploratory study, performed at global level, fits perfectly within the scope of “FORESTS”. Improving our understanding of nutrient resorption patterns in plantations is a pressing task due to the uncertainties that limit the secure predictions of productivity in forest ecosystems and its relation with biogeochemical cycles.

In my opinion, this research accomplishes the objectives set out, and I consider it a good precursor for future research in this line. The research has been well conducted, for which the authors have gathered a sufficiently large database, to which they have applied appropriate and correct statistical tools, obtaining consistent results. The article is well structured, clearly explained, correctly illustrated, supported by a wide range of literature, and well written in English.

Because the conclusions obtained should be made known to the scientific community.

I have some concerns, which could be included in the final version.

SOME CONCERNS AND QUESTIONS

1-Building a database is an arduous task. I suppose that the authors have taken some cautions to prevent bias or have used specific filters. However, they make no comments in this regard. For instance, were any cases of possible "fake senescences" (droughts, pests, ...) detected?

2-The fact that the "age" variable was excluded in all stepwise regressions is rather unusual, because this factor could have an a priori influence on senescence and, consequently, on N and P resorption efficiency. Any comment?

3-The values of r² are low. We could probably expect these values because they come from planted forest worldwide, and its significance is more important. However, some comment on this point would be useful because it is striking.

Author Response

Point 1: This exploratory study, performed at global level, fits perfectly within the scope of “FORESTS”.  Improving our understanding of nutrient resorption patterns in plantations is a pressing task due to the uncertainties that limit the secure predictions of productivity in forest ecosystems and its relation with biogeochemical cycles.

  In my opinion, this research accomplishes the objectives set out, and I consider it a good precursor for future research in this line.  The research has been well conducted, for which the authors have gathered a sufficiently large database, to which they have applied appropriate and correct statistical tools, obtaining consistent results.  The article is well structured, clearly explained, correctly illustrated, supported by a wide range of literature, and well written in English.

Response 1: We appreciate the comments.

Point 2: Building a database is an arduous task.  I suppose that the authors have taken some cautions to prevent bias or have used specific filters.  However, they make no comments in this regard.  For instance, were any cases of possible "fake senescences" (droughts, pests, ...) detected?

Response 1: It is really true as you suggested that we have used specific filters to prevent bias.  Therefore, we have added “(4) Any data from cases of possible ‘fake senescences’ (e.g., drought, pests,…) were also eliminated.” in Line 105, Page 3.

Point 2: The fact that the "age" variable was excluded in all stepwise regressions is rather unusual, because this factor could have an a priori influence on senescence and, consequently, on N and P resorption efficiency. Any comment?

Response 2: We included the “age” variable in all stepwise regression analyses.  However, it did not enter into the final model.  With different species, climate, and soil conditions worldwide, variations in NRE were mainly regulated by climates [1-4], while variations in PRE were dominantly controlled by green leaf nutrient concentrations [4-6].  We made this point clear in Line 182, Page 8 and discussed it in 4.2 on page 10.

Point 3: The values of r² are low. We could probably expect these values because they come from planted forest worldwide, and its significance is more important. However, some comment on this point would be useful because it is striking.

Response 3: The values of r² are low because our data come from plantations worldwide, and its significance is more important.  Moreover,  previous studies on nutrient resorption worldwide also showed low r² values.  For instance, r² = 0.18 for NRE and r² = 0.10 for PRE in Vergutz et al. study [1]; r² = 0.08 for NRE and r² = 0.07 for PRE in Yuan et al. study [7]; r² = 0.07 for NRE and r² = 0.02 for PRE in Yan et al. study [2].

Reference

1.    Vergutz, L.; Manzoni, S.; Porporato, A.; Novais, R.F.; Jackson, R.B. Global resorption efficiencies and concentrations of carbon and nutrients in leaves of terrestrial plants. Ecol. Monogr. 2012, 82, 205-220. doi:10.1890/11-0416.1

2.    Yan, T.; Zhu, J.J.; Yang, K. Leaf nitrogen and phosphorus resorption of woody species in response to climatic conditions and soil nutrients: a meta-analysis. J. For. Res. 2018, 29, 905-913. doi:10.1007/s11676-017-0519-z

3.    Zhang, H.; Guo, W.H.; Yu, M.K.; Wang, G.G.; Wu, T.G. Latitudinal patterns of leaf N, P stoichiometry and nutrient resorption of Metasequoia glyptostroboides along the eastern coastline of China. Sci. Total Environ. 2018, 618, 1-6. doi:10.1016/j.scitotenv.2017.11.030

4.    Augusto, L.; Achat, D.L.; Jonard, M.; Vidal, D.; Ringeval, B. Soil parent material-A major driver of plant nutrient limitations in terrestrial ecosystems. Glob. Chang. Biol. 2017, 23, 3808-3824. doi:10.1111/gcb.13691

5.    Yan, T.; Lu, X.T.; Zhu, J.J.; Yang, K.; Yu, L.Z.; Gao, T. Changes in nitrogen and phosphorus cycling suggest a transition to phosphorus limitation with the stand development of larch plantations. Plant Soil 2018, 422, 385-396. doi:10.1007/s11104-017-3473-9

6.    Wang, Z.N.; Lu, J.Y.; Yang, H.M.; Zhang, X.; Luo, C.L.; Zhao, Y.X. Resorption of nitrogen, phosphorus and potassium from leaves of lucerne stands of different ages. Plant Soil 2014, 383, 301-312. doi:10.1007/s11104-014-2166-x

7.    Yuan, Z.Y.; Chen, H.Y.H. Global-scale patterns of nutrient resorption associated with latitude, temperature and precipitation. Glob. Ecol. Biogeogr. 2009, 18, 11-18. doi:10.1111/j.1466-8238.2008.00425.x

Reviewer 4 Report

Dear Authors!

After careful reading of the MS I am impressed by your work. Definitely this MS is an important step to understand global pattern of N and P resorption. I have basically only one comment:

You take into account only N and P from needles/leaves, yes? It should be said in a very clear way already in abstract that you concentrate only on aboveground parts of the plants. I do not have other comments which coudl improve the readability of the MS.

Author Response

Point 1: After careful reading of the MS I am impressed by your work. Definitely this MS is an important step to understand global pattern of N and P resorption.

Response 1: We appreciate the comments.

Point 2: You take into account only N and P from needles/leaves, yes? It should be said in a very clear way already in abstract that you concentrate only on aboveground parts of the plants. I do not have other comments which coudl improve the readability of the MS.

Response 2: Yes, we take into account only N and P from needles/leaves and have said in a very clear way in abstract.