the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 23 Feb 2026
How is the functioning of saplings limited by soil nutrients in tropical rainforests?
Abstract. This study was conducted in an unmanaged tropical rainforest in French Guiana, in which either nitrogen (N), phosphorus (P) or a combination of N and P fertilisers were experimentally added to the soil. Here, we focused on the effects of fertilisation on photosynthetic capacity and growth of saplings in the understorey of the rainforest. We measured sapling photosynthesis and stem dimensions (height and diameter) after eight years of fertilisation, and compared these with pre-fertilisation data. In addition, we measured overstorey leaf area index, soil nutrients, foliar leaf nutrients, leaf morphology, total sapling leaf area and the leaf area/wood volume ratio. The results showed that an increase in available soil nutrients contributed to an increase in leaf area of tree canopies, reducing the light availability for saplings in the understorey, eliciting shade adaptations, such as an increase in specific leaf area and an increase in height increment. There was no fertilisation effect on the growth and photosynthesis of saplings that overrides the increase in light limitation. Nonetheless, the effect of fertiliser was significantly different among species, making it likely that fertilisation on the long-term will lead to a change in the composition of tropical rainforests.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-682', S. Joseph Wright, 23 Mar 2026
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RC2: 'Comment on egusphere-2026-682', Anonymous Referee #2, 02 Apr 2026
egusphere-2026-682
Comments to the authors:
This manuscript reports the impacts of nitrogen + phosphorus (N+P) fertilisation on the saplings of tropical rainforest after 8 years of fertilisation. The authors measured soil nutrients, sapling stem, height growth and volume, sapling leaf N and P concentrations, sapling photosynthetic capacity and overstorey leaf area index. Their findings showed that the combined N+P fertilisation increased soil P availability, increased sapling leaf N and P in some species, but had no observable effect on photosynthetic capacity, stem growth and volume. Fertilisation also increased overstorey leaf area, thereby reducing light availability to the saplings, which in response increased in height and specific leaf area. This study is indeed relevant in understanding the responses of saplings to elevated nutrient inputs in tropical forests. Generally, the manuscript reads well, but a few recommendations for improvement are provided below.
I am wondering why the authors excluded the N fertilisation only and the P fertilisation only from the study. This would have helped disentangle the effects of the individual nutrient additions on the sapling parameters measured.
L187-190. What constituted the six plots used in this study, if there were 12 plots for N, P, and N+P (L. 172), I’m guessing the number of replications was four. Did you study six plots in total or six plots per topographic location? Clarify.
I find the description of the statistical analysis section insufficient and rather general. The section lacks information on how the different parameters were handled. These are important to ensure transparency and reproducibility.
- 162 “..(sometimes ANOVA, when overstorey LAI had no statistically significant)”. It is unclear what the authors mean here.
L186. Is there a reason why height and not diameter was used as a criterion of what a sapling is?
- 135. ‘..around the world’ yet all the references were mainly neotropical. Either rephrase or include references from other tropical regions.
L.451. What does “deeply " mean?
L468. “leaded” Do you mean led?
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- 1
The authors evaluated the impact of combined nitrogen and phosphorus addition (+NP) for soil N and P concentrations, foliar N and P concentrations, photosynthetic potentials, overstory leaf area index (LAI), and stem diameter, stem volume, and height growth for saplings of four tree species. The experiment took place in an old-growth tropical rain forest in French Guiana. The +NP treatment was maintained for eight years and increased overstory LAI and sapling height growth. These are important results that will be of particular interest for readers concerned with nutrient limitation and tropical forests.
I have one important suggestion, one problem with the presentation of statistical analyses that really must be fixed, one problem with the emphasis placed on topographic position, and several moderately important and minor suggestions.
My most important suggestion concerns the growth analyses. Stem diameter, stem volume, and height do not grow independently. All three are allometrically related. Consider the immediate implication. If +NP increased height growth but not stem diameter growth, then +NP also changed the allometric relationship between stem diameter and height. I believe the authors should recast their growth analysis in an allometric framework. Stem volume should be dropped entirely because the authors calculate stem volume from stem diameter and height.
Also, the authors really must describe their statistical analyses and present the results of those analyses in standard fashion. Section 2.5 describes the statistical analyses and devotes just one sentence to this crucial description. That one sentence states: “The main statistical model was an ANCOVA model, with treatment, species, topography and overstorey LAI as key explanatory variables (sometimes ANOVA, when overstorey LAI had no statistically significant effect).” That is not acceptable. For example, the reader is never told whether the authors evaluated interactions among their “key explanatory variables” (see Figure 5 for an interaction that might be significant). The full results of ANCOVA models are never presented. Appendices E, F and G present ANCOVA model results but in a non-standard and uninformative format. The presentation of all statistical analyses and results must be improved.
I believe the authors place too much emphasis on topographic position. There is just one control plot and just one +NP plot in each of three topographic positions. Treatments are not replicated within topographic positions! This is important. Lacking replication within topographic positions, the experiment cannot convincingly demonstrate effects of topographic position. Rather, I believe the authors should treat their experimental design as a blocked design with respect to topographic position. The question becomes whether experimental results are consistent across topography and NOT whether experimental results are stronger in one topographic position than another.
Three moderately important suggestions follow:
Line 249-250: The authors wrote “The increment was then calculated by subtracting the 2016 measurements from the 2024 measurements.” The increment refers to a sapling diameter. Diameter growth (or increment) depends strongly on initial diameter. If the authors decide to stick with a separate analysis of stem diameter, then the authors should consider an analysis that uses initial diameter as a covariate.
The format of Figure 3 (and similar figures) is ineffective. The black edges cannot be seen on many plotting symbols, and the journal will not publish a figure in this large format. The format of Figure 2 is effective and might be adopted for Figure 3.
Lines 399-400: The authors wrote “We therefore conclude that hypotheses 1 and 2 could not be rejected and that N and P addition increased the availability of these elements and subsequently also their foliar concentrations.” Statistical analyses can lead one to reject the null hypothesis. In this case, the null hypothesis is that soil and foliar nutrient levels did not change. The null hypothesis is rejected. The authors must fix this! The same problem recurs at line 448.
Minor stuff follows.
Given the study motivation presented in the first paragraph of the Introduction, the authors should cite Peter Hietz’ work on N deposition impacts for tropical forests (see Hietz et al 2011 Science 334: 664-666).
Line 35: The authors wrote tropical forests are “absorbing about 40 billion tons of CO2 annually”. The value given exceeds 90% of all anthropogenic CO2 emissions. The standard inventory suggests the value should be about 30%, not 90% (Pan et al. 2024 Nature).
Line 234: The authors wrote “… the stomatal ratio was set at 0.5.” What does this mean? I am familiar with the LI-6400 being used but have never heard of setting a “stomatal ratio”.
Lines 343-345 describe an interesting correlation. Please consider adding a scatterplot that shows the data to the supplementary material.
Lines 425-430: The authors should reference the meta-analysis of Ostertag and DiManno (2016. Detecting terrestrial nutrient limitation: a global meta-analysis of foliar nutrient concentrations after fertilization. Frontiers in Earth Science 4.)
Signed: Joe Wright