Predicted Future Changes in the Mean Seasonal Carbon Cycle Due to Climate Change
Round 1
Reviewer 1 Report
Comments and Suggestions for Authors1. This manuscript uses the 3D-CMCC-FEM model to simulate the NEE, GPP, and Reco of five forest ecosystems where flux measurements were conducted. The measurement data were used to validate the model.
2. Major concerns:
• The term “MSC” is used throughout the manuscript without a clear definition or quantitative index. For example, in Lines 235-236: “The NEE, GPP, and Reco, as modeled by 3D-CMCC-FEM, exhibit strong correlations with the observed daily and monthly eddy covariance data and for the overall MSC across all five sites.” What does MSC mean in this context? The authors should avoid using the term unless clearly defined.
• In Section 3.2, the change in NEE requires a clearer description. When the authors state, “Overall, across all the sites and scenarios considered, there is a consistent reduction in NEE over time with changes in the source/sink and sink/source switch over the year,” it is unclear whether NEE values are becoming more negative or positive. The terminology of NEE can be confusing since a negative NEE value indicates net carbon uptake. A reduction in NEE typically means decreased carbon sink strength, but literally, “reduction in NEE” could imply more negative values. Please clarify this point.
• In Section 3.3, the entire section on NEE dynamics is fundamentally incorrect. A carbon sink corresponds to NEE < 0, not NEE > 0, and a carbon source corresponds to NEE > 0, not NEE < 0. Given this significant error, the calculations of days of carbon sink and source are incorrect, which in turn affects the interpretation in the discussion. The section needs to be thoroughly revised to accurately reflect these definitions.
• The phrase “Reco variations range from 37-106% for RCP 2.6 and 60-121% for RCP 6.0 across all forests examined” should be revised to “Reco changes.” The term “variation” is typically used to describe consistency within a population, not changes over time.
3. The current citation style makes reading difficult. For example, “The most recent code versions since [17] adopt the biogeochemical photosynthesis model of [50] to compute gross primary productivity (GPP).” Revise to: “The most recent code versions [17] adopt the biogeochemical photosynthesis model of Farquhar et al. [50] to compute gross primary productivity (GPP).”
4. The correlation coefficients between observed and simulated NEE, GPP, and Reco are impressively high (Tables A1-A3). However, an initial visual inspection of Figure 2 suggests that the green and black time series do not match as closely as the high correlation coefficients would indicate. Please include detailed statistical analyses in the appendix to address this discrepancy.
5. The Relative Mean Bias (RMB) and Modeling Efficiency (ME) are not percentages. Correct the legends of Tables A1-A3 and the text (Lines 246-272).
6. L288-292: Please provide evidence for these statements.
7. Table A1-A3 contain only daily and monthly values, no yearly values. Correct the legends!
8. Minor corrections:
• L122-123: Change ‘less’ to ‘minus’ for clarity.
• L148-151: Italicize species names.
• L158: Use subscript for CO2. Check this throughout the manuscript.
• L177: Use superscript for -1
• L177: μmol 666.4 mol-1 should be 666.4 μmol mol-1.
• L240: 5 gC m day should be 5 gC m-2 d-1
• L240: the overestimation occurs around 180 DoY, not 200 DoY
• L240: change 200th to 200.
• L340: Change “frequency of days” to “number of days.”
Author Response
We thank the Editor and the two Referees for their insightful comments and suggestions that have greatly improved our manuscript.
In response to the points raised by both Referees, the manuscript has been thoroughly revised and checked for grammar and typos to fully address all their comments. Detailed responses to each comment are provided in the attached file.
Specific replies to the Reviewers’ comments are provided in red and start with “R:”.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsGeneral comments:
This manuscript examines an interesting problem and uses an adequate method to reach a conclusion. I write adequate because the underlying observational data has some very large limitations, the primary one being an underestimate of Reco. The other problem is that the carbon dynamics of forests are limited to what can be detected using eddy covariance. This leads to various processes (i.e., combustion, erosion, and dissolved transport) being ignored. This results in a very distorted, misleading description of forest carbon dynamics in the introduction.
Another problem is that the study framing ignores the wider context of forests regarding atmospheric exchanges. For example, the authors seem to suggest that older forests that are not carbon sinks have little value in climate mitigation. The suggestion seems to be keep forest young to keep the forest a sink. However, that would involve disturbing the forest, most likely via harvests. That would likely increase the carbon sink ability of forests, not necessarily their effect on atmospheric carbon. That is because one has to understand the fate of the carbon removed from the forest to understand the effect on the atmosphere.
I believe these issues can be resolved by the authors by being more precise in how the dynamics of carbon in forests is actually controlled and by pointing out the limitations of system they examined.
In general I found the authors very lax in their explanations. Numerous examples are given below. I think they need to be more precise to improve the clarity of their explanations, but also to avoid setting a bad precedent (we already have way too many of those in the literature).
Specific comments:
2 Should this be “Predicted future changes” to recognize the fact these changes are predicted and have not already occurred as suggested by the current wording?
11 Why is the wording massive amounts needed? There is no indication of the amount released via respiration, which gives a very misleading impression. Global respiration is very close to that of photosynthesis, granted it is slightly lower, but this is very misleading.
13 However, this assumes that all the exchanges of carbon involve just gases. That is misleading as there are also losses via erosion and in dissolved forms not to mention losses via combustion. It would be proper to indicate the majority is likely captured by NEE, but a majority is not all.
18 I am not sure the model shows anything. I think “has” is more appropriate wording. Also the clause “under current climatic conditions” should be separated by commas.
19 isn’t this predicted climate change?
20 Modeled is not the correct concept. It should be predicted as models predict.
21 evergreen forests? Don’t adjectives need nouns?
24 Why is types in bold? But more significantly, forests that reach a maximum store can also have mitigation value. This seems to assume that only forests that take up carbon have a mitigation value. However, the way to cause a forest to be a sink is to remove carbon. Unless none of the carbon is released to the atmosphere, then one cannot assume that causing a forest sink leads to any mitigation value. One could indicate the future forest may have a reduced capability to store more carbon.
30 Again why large amounts for photosynthesis but nothing regarding amounts for respiration. And further, combustion also releases carbon back to the atmosphere. Why is that neglected?
34 Again, this describes an Earth without fires. Not sure that version of Earth exists.
50 not sure “and then” is needed in this sentence. It seems like a sentence fragment.
62 As written it is not clear if droughts are part of warming effects.
64 Not sure what growth up to refers to. Everything between growth and mortality? But photosynthesis does not necessarily mean growth. This should describe the full range of processes.
69 What does “more than the one” mean? I suggest “the one” be removed to make the meaning clearer.
70 Isn’t a microbial activity respiration? Not sure this makes sense as written.
72 Assuming a world in which there are no fires. Is that reasonable?
129 What exactly does litter contain? These models are known for not adequately modeling the dynamics of woody litter including branches, tree boles, and coarse roots. Were these included? And if so then how?
141 Is turnover the same as mortality?
225 The red triangles on red areas is difficult to see. I suggest another color, maybe black, be used instead.
282 Shouldn’t this be in past tense?
282-292 This seems more like methods than results.
352 “In order” is not needed in any sentence. Please remove here and elsewhere.
353 Not clear what “stating” has to do with this assessment. Do the authors mean examining or observing?
392 Do the authors means “does best”?
419 What does “NEE ability” mean? Are the authors suggesting that a positive NEE means less ability? I don’t think NEE as an ability. It is just balance.
443 Do the authors mean “finding”?
494 Wouldn’t a limitation also be the underestimate in Reco that the current data gives? Having a model that predicts the same underestimate does not inspire confidence.
504 wouldn’t this depend also on having a model that considered these processes? That is species succession and migration? If the model does not account for these processes, then why would more time matter?
511 Not sure the “then” is needed.
516 Although it is acknowledged that Reco is generally underestimated, so I am not sure this is a particularly positive finding regarding what is actually going on.
Author Response
We thank the Editor and the two Referees for their insightful comments and suggestions that have greatly improved our manuscript.
In response to the points raised by both Referees, the manuscript has been thoroughly revised and checked for grammar and typos to fully address all their comments. Detailed responses to each comment are provided in the attached file.
Specific replies to the Reviewers’ comments are provided in red and start with “R:”.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
Comments and Suggestions for AuthorsThe authors have by and large addressed my concerns.
