The premise of the study:

The authors confront a dynamic vegetation model with experimental data from a nutrient fertilization experiment in Costa Rica and assess the validity and consequences of alternative root allocation parameterization. The overarching question the authors address is how root allocation responds to changes in nutrient availability, which is a very valid question to pursue. Evaluating models and their underlying assumptions with direct field observations and experiments is necessary to advance our understanding and gain confidence in model predictions. I appreciate the effort the authors have undertaken with the demographic model ED2, and their joint effort with experimental findings and expertise to advance root dynamics and their interactions with nutrient availability. However, I currently cannot recommend publishing the manuscript due to some concerns.  

Main concern:

1. The ecological underpinnings of the chosen parameter sets in regard to fine root allocation are not sufficiently clear. This includes the ecological theory to justify choosing the parameter sets, the discussion of the consequences of these parameter sets, and placing the chosen parameter set and findings within the literature and previous modeling efforts.
2. The study covers short-term and long-term process effects of the different parameterizations, whereby we can expect different outcomes. The experiment finds that fine root production increases with fertilization. That does not necessarily mean that higher fertile sites will be characterized by higher fine root production. The others touch on these aspects of timing in the discussion, and the significance of this difference is crucial for the study, however insufficient emphasis is placed on this in working out the premise of the study and discussing the findings. Associated with this, the experiment takes place in a young forest stand, the implications of this deserve more discussion.
3. Direct nutrient acquisition via fine roots is only one of several possible mechanisms of how plants can acquire nutrients. The authors mention that the trees are associated with arbuscular mycorrhizae but the implications of this in regard to the outcome of the experiment, and how this might have affected the model performance are not addressed.

Additional comments methodology:

The chosen parameter sets reflect negative, constant, and positive relationships between fine root production and soil P. The negative one would reflect the resource-dependent parameterization, however not via internal plant demand and supply (as commonly done) but depending on external P supply, in a linear fashion. This is quite different from any of the previous model approaches, and the parameterizations are all based on a linear relationship between soluble soil P and root allocation. A discussion of the ecological underpinning of this model and the parameterization approach would be helpful. The authors touch upon the ecological theory they are addressing only in the discussion part.

Parameters a and b are coordinated to yield a similar 0.3 root-to-shoot ratio in control plots, but to different settings in fertilized plots. The reason for not testing different parameterization settings in the control plot was not clear to me, it would be helpful if the authors could elaborate on this.

The allocation process as a whole in ED2 is not sufficiently explained. The calculation of the daily leaf and root allocation, and the allometric equations to determine maximum leaf and root allocation, need to be described and included in the discussion. Similarly, the PFTs that are modeled and their parameterizations are not described.

Additional comments results:

One positive parameterization was the only one that agreed well with experimental observations over the 3-year period. Positive parameterizations led to the highest AGB over 30 years, however, not due to alleviation of nutrient limitation but water stress. It would be helpful if the reader would understand how that occurred process-wise. What was the actual allocation to fine roots? And why did that not alleviate nutrient stress? Similarly, with P fertilization, positive parameterizations led to too many fine roots over the 30-year time scale. As is described in the abstract as the main finding. Does that not indicate that the chosen parameterization works well for the short term but, importantly, not well in the long term? Is such an assumption about root allocation advisable then? It would be helpful if the authors go beyond “experiments need to measure leaf, root and wood production” in their main take-away for the paper, and rather elaborate on recommendations for model development.

Given that the experiment took place in a tropical dry forest, the anticipated interactions between soil fertility and water stress are intriguing and it would be helpful if the authors elaborate on the alleviation of water stress in the 30-year simulation that has been touched upon.

Maybe the agreement at the 3-year time scale is not sufficient to evaluate. Can the control plots not also be used to evaluate the parameterization, representing rather long-term dynamics? Should the parametrizations not be tested on the control and experimental plots, equally? The authors state that ED2 has been validated before. Did the constant parameterization work well before? Did ED2 employ a resource-dependent allocation scheme before? It would be helpful to outline which allocation approaches agreed well with observations and which did not in previous studies with ED2.

The resource limitation theory postulates plant allocation is adjusted to acquire the most limiting resource. Many ecosystem models to date, adopt a resource-dependent allocation scheme, so they would predict that more roots would be produced if soil nutrients were limited, irrespective of the return. The authors find that the experimental results contradict this hypothesis. I believe this apparent contradiction might be a time issue, so that roots grow to acquire the soluble P, while root allocation would decrease once P limitation is alleviated. It might also have to do with mycorrhizal interactions, previous outsourcing of phosphorus acquisition to mycorrhizae now becomes less beneficial with increased nutrient supply and plants switch to “do-it-yourself”. Even if we cannot be certain about any of these hypotheses, a discussion thereof would be helpful.

Line comments:

1. 336 please specify: allocation parameterization sensitive to external nutrient availability
2. 383- 341 what is the implication of that finding?
3. 344 the model is microbial-explicit. That is an important aspect of the model. It would be helpful if this is included in the manuscript and discussed.
4. 365 For future efforts, the authors can consider evaluating the model and different parameter sets at different locations in the tropical biome, and including an evaluation along a soil fertility gradient, as well as experimental changes. The combination of both would be helpful to discern short-term and long-term effects.
5. 369 The experiment near Manaus found increased primary productivity in response to fertilization, indicating that production was limited by phosphorus. To my understanding, this finding is still consistent with the resource limitation hypothesis, since plants allocated carbon to roots to acquire the nutrients that they were in demand of.
6. 375 As the authors note, roots are there for acquiring multiple resources at once. A discussion of these interactions would be helpful. The analogy to rain roots is interesting, the authors could elaborate here. The root production after fertilization could potentially be a similar short-term effect to acquire the limiting resource.
7. 379 The paragraph on field observation is helpful, however, root stock and root production seem to be mixed up. Since the model evaluation deals with root production, the authors could elaborate on those aspects of the measurements.
8. 387 It would be helpful if the authors elaborate on the interactions with mycorrhizae, here, and the implications for this study.
9. 389 The “supply-limited” hypothesis, which is the basis of this study, should be introduced earlier and placed in context with the alternative hypotheses.
10. 390 This section could benefit if it discussed what has been learned from this and the previous approaches.
11. 398 Similarly, in this section, it would be helpful if the authors go beyond the summary of the results here and discuss the implications of these findings. See the main comments above.
12. 424 Here it is crucial to discuss that resource-dependent model approaches are not exactly comparable to the negative parameterization applied in this study. See the main comments above.