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It is suitable for publication in PeerJ.
[# PeerJ Staff Note - this decision was reviewed and approved by Jörg Oehlmann, a PeerJ Section Editor covering this Section #]
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Title & Abstract
The revisions by the authors have significantly improved the paper. In the abstract, the important results on the effects of environmental variables as well as their interactions on net ecosystem productivity (NEP) are now included in the abstract. The grammatical errors in the previous version have been largely corrected. The abstract describes the methodology, main results, and highlights the region's overall role as a carbon sink. The study provides valuable insights to the key environmental drivers to ecological productivity in the region.
In the revisions, the authors included the alternate name of the study region, Tibet, into the keywords that should likely increase the international readership of the paper, as it is better known outside of the country.
Introduction
The key constituents of NEP, including gross primary productivity (GPP) and net primary productivity (NPP) are now sufficiently introduced with their estimations and definitions. The authors also provided the meaning of NEP > 0 and NEP < 0, which is extremely helpful for readers across different fields.
The relevance and ecological value of the southeastern region of Xizang in terms of endemic tree species and vulnerability under climate change are now mentioned by the authors, providing the reason for the focus on the specific region. This significantly enhanced the impact of the study by highlighting the critical importance of addressing the challenges posed to this sensitive ecological zone. The information generated from the study will likely aid future ecological management.
The novelty of the study approach using an improved CASA model and combining weather and topology data is now included in the last paragraph of the introduction. My previous concerns were addressed adequately by the authors, and I have no further comment on the introduction.
Figures & Tables
My previous concerns on clarity of the tables and figures have been adequately addressed by the authors.
Material and Methods
The authors addressed my comment on the ambiguity of the study location by including a map of the area in Figure 3. The important missing information on the normalized difference vegetation index (NDVI) is also provided, along with its calculation and its definition as an indicator of vegetation cover and quality. The other text that I highlighted that may cause confusion is no longer present. I particularly appreciate the detailed formulas provided for each metric used in the study, so that I do not need to frequently refer to other references.
Results
My previous major concern about the significance/novelty of the study has now been addressed by the authors and I have a much clearer understanding of the aims of the study. Moreover, the key results (i.e., interactions between environmental variables) and the meaning of NEP constituents are much better explained.
The figure 6 caption explains the figure on the interactions clearly and I have no further concerns.
Discussion
The discussion section reads clearly, mentioning the details on accuracy using the MOD17A3 data and consistency of the current results with the previous works on the same region. The limitations of the study are also discussed well, such as spatial artifacts that may affect precision and accuracy derived from the data. The spatiotemporal factors contributing to the observed NEP during the study period (e.g., solar irradiation and water availability, altitude and nutrient conditions) have been correctly identified and are consistent with those of other studies.
Conclusion
The conclusions section is written well and summarizes the study well, especially now that it is well-established earlier in the manuscript that the aim is to better understand the vegetation dynamics in this sensitive region in the context of climate change. I found the study to be generally sound following these revisions and have recommended it be accepted in current form.
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Given the journal's international audience, consider including more studies in different countries/regions in your references to place your findings in a global context and show where they can be applied or adapted.
I recommend rewriting a big portion from L365-377 and drawing on the considerable information on climate change implications to mountain ecosystems. The short discussion needs to be broadened to show the results' larger implications.
**PeerJ Staff Note:** Please ensure that all review, editorial, and staff comments are addressed in a response letter and that any edits or clarifications mentioned in the letter are also inserted into the revised manuscript where appropriate.
The figure image quality does not appear to be high resolution in the PDF I can view – please check all figures are sufficient quality for publication without blurring of text & other fine detail.
In at least one of the map figures (preferably all), show where the study region is relative to the whole country/Asia so that international readers can easily locate the study.
In all figure captions, spell out acronyms and state the convention for NEP i.e. >0 = carbon sink, <0 = carbon source.
Include more detail on L36-37 about China’s emission reduction targets with cited literature.
Wording of research question/aim (L79-84) is phrased awkwardly and should be rewritten for clarity, including removing full stop from L80.
Presumably one of the more important outcomes should be focused on managing/enhancing carbon storage in the region in addition to ecological conservation, which should be emphasised in this paragraph.
I do not have expertise in spatial modelling however the methods appear to be appropriate to the aims of the study.
Please add the versions and citations for the statistical software used in the study (e.g. Matlab, ArcMap, ArcGIS, GeoDetector).
The conclusion is lacking any statement about the implications of the study for the environment and its management, or how these findings may apply in the broader research context. It should be expanded upon.
Remove decimal places in abstract for the area reporting and add a comma or space eg 70 119 km2. (Throughout paper)
Check formatting of all references is correct, e.g. L31 ‘IPCC. 2007’ should be ‘IPCC, 2007’, L32 ‘LUO’ should not be in all caps. Check reference list formatting also – there are a lot of minor errors which need to be corrected, e.g. use of capitalisation.
Given the international audience of the journal, it would be worth considering adding more diversity in your references, where appropriate, to include more studies in different countries/regions, this will ensure your findings are set in a global context and highlight where they can be applied or adapted outside of the study region.
Check language throughout is scientific, e.g. in L96 ‘The region enjoys abundant moisture’ is not in the scientific style of writing.
Remove duplicated citations e.g. L319-321 & L327 (check throughout the paper).
There is a large section from L365-377 that would benefit from relating back to the broader literature rather than discussing general patterns alone – I suggest this section is rewritten and that the extensive literature on climate change impacts to mountain ecosystems is thoroughly drawn upon. Overall, the discussion is quite short and needs to be expanded upon to highlight the broader implications of the results.
The study focuses on the spatiotemporal patterns and drivers of vegetation NEP in southeastern Tibet (Nyingchi) during 2000–2020. The topic is regionally significant, the time span is long, and the conclusions have practical implications for ecosystem management and conservation.
1 Since Landsat-7 has SLC-off striping and data gaps after 2003, please describe the full quality-control workflow: cloud/shadow masking, gap filling or compositing strategy (e.g., seasonal/annual composites), BRDF and topographic corrections, and any thresholds or QA flags used. If other sensors (e.g., Landsat-8/9 or MODIS) were used for gap filling or harmonization, please state how cross-sensor consistency was ensured and discuss potential impacts on NEP estimates.
2 Please detail how datasets with differing spatial (and temporal) resolutions were aligned. Specify the target grid, resampling/aggregation methods (e.g., nearest neighbor, bilinear, area-weighted), treatment of categorical layers (e.g., land cover), and any temporal aggregation steps. Briefly comment on how these choices may affect uncertainty.
3 Please specify how continuous predictors were discretized (e.g., natural breaks, equal-frequency, equal-interval, expert-defined), the number of classes used, and provide a sensitivity analysis showing how alternative binning schemes affect the q-statistic and the inferred interaction types. Consider moving detailed robustness results to the supplementary material.
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Please standardize the notation of all physical quantities across the manuscript. Instances such as “gC × m -2 × a -1” and “gC.m-2.a-1” should be unified to the SI-compliant form “g C m⁻² a⁻¹”. Ensure consistent use of NEP/NPP/RH symbols in the text, tables, and figure captions.
Please add units to Figures 3–6 .
Title & Abstract
This study assesses the spatio-temporal patterns in carbon sources and the mechanisms explaining the patterns in southeastern Xizang (i.e., Tibet Autonomous Region of China). While the intro makes good points in arguing that a better understanding of productivity and carbon cycle in the area is important for ecological conservation, especially in the context of climate change, the biggest weakness of the study is that the contribution to knowledge offered has not been communicated well. Moreover, the results on the interactive effects among environmental variables (driving factors) on NEP is missing.
There are preceding studies by Zhang et al., (2021) and Dong et al., (2024) that assessed the NEP of the Qinghai-Tibet Plateau within the same timeframe (2001-2017 and 2000-2024 respectively), yet no acknowledgement was given to these studies in the references. These previous studies assessed more broadly the entire autonomous region, and as such, the area fully eclipses that of the current study and similar results would be expected. In the introduction, please discuss the former studies in detail and outline explicitly what new information the authors aim to uncover in the current study.
See reference:
Dong S, Du S, Wang XC, Dong X. .2024. Terrestrial vegetation carbon sink analysis and driving mechanism identification in the Qinghai-Tibet Plateau. Journal of Environmental Management. 360, 121158.
While the title is appropriate for the study, the use of a slash means that it is “sources OR sinks”, but both are considered in the study. I suggest writing “carbon sources, sinks, and the driving factors influencing vegetation".
"Spatial and temporal” can be shortened to “Spatio-temporal” as done elsewhere in the manuscript.
The use of grammar throughout is generally good, although there are a few areas requiring improvement. I have highlighted these throughout.
Abstract
The abstract is generally clear regarding the methods and overall results from the analyses. An area that can be improved is clarifying the factors identified in L 22-23 in the context of the results in L 18-20. Using a line or two, please elaborate on in what way do altitude, precipitation, and temperature explain those patterns in NEP across space?
L 11: Change to "is a crucial measurement for"
L 19: “Higher values in southern areas and lower values in northern areas”
L 20: higher values in eastern areas compared to western areas"
L 23: Correct to plural “interactions”
Keywords
Some keywords are already present in the title and are therefore redundant. “NEP” should be written in full. The authors might consider including the alternative name for this autonomous region of China in the keywords to broaden the readership.
Introduction
The introduction is well-structured, providing background information on the study region, initiatives on reducing emissions and assessment of terrestrial carbon cycle processes in China, and various methods used to measure productivity.
However, there are a number of areas that need further elaboration or clarification. The text mentions that there has been a progress from using gross primary productivity (GPP), net primary productivity (NPP), to net ecosystem productivity (NEP). However, the details about each of these estimation methods is missing for a better understanding of the NEP. Furthermore, an explanation of “heterotrophic respiration” (L 48) is missing. Including the definitions, background and/or explanation of their calculations, meaning, and practical applications can help readers understand the context of these metrics. For example, it can provide more meaning to positive and negative NEP values (L 50-51).
L 28: Elaborate on the types of “technology” responsible for increasing atmospheric CO2.
L 35: A bit too general – clarify further what is meant by “Earth's environmental conditions.”
L 36-37: Explain these specific goals and provide reference values for context, such as reducing CO2 emissions by x %, or reducing emissions from x to y.
L 48: NPP doesn't need to be written in full again.
L 53: It is unclear what is meant by “impact of NEP on terrestrial ecosystems”. NEP is a measure of productivity of a given ecosystem and so it is a property of that ecosystem. Environmental factors (such as climate change) may have an impact on NEP; therefore, it does not make sense to state that NEP has "impact on terrestrial ecosystems". Please rephrase.
L 71-72: Even if true, the intro should introduce the studies that have done it in detail (see Geng et al., 2024).
The paragraph from L 53-72 is quite dense, presenting a large number of previous studies involving NEP with a focus on the specific study geographical region. However, only a few of these example literature aligns with the study goals, making this section of the introduction a bit unfocused and scattered. I suggest to reduce or condense this general information. Include some background introduction on carbon source and sink environments and their relevance for conservation. Moreover, discuss the impact of climate change on shifts between sources and sink states.
L 73-78: Provide some direct information related to species diversity or endemic species to enhance the conservation aspect. Moreover, this paragraph describes the whole region, so please explain why the current study is focusing on the southeastern area only?
Focus on (1) the background information on understanding sources and sinks through NEP, and (2) the current understanding of carbon source and sinks especially in the context of the Qinghai-Tibet plateau. Doing so would put the current study into context and help identify/better explain its novelty. For instance, describe the advantages of the current study compared to those studies that assessed a broader area
I provide some references, in addition to Dong et al., (2024), that may be of interest to the introduction.
See references:
Geng YX, Yi GH, Zhang TB, Bie XJ, Li JJ, Wang GY, Lin MN. 2024. Impacts of climate change on grassland carbon sink/source patterns in the Qinghai-Tibet Plateau. Journal of Natural Resources. 39, 1208-1221.
Xu M, Guo B, Zhang R. 2024. Variations of terrestrial net ecosystem productivity in China driven by climate change and human activity from 2010 to 2020. Forests 15(9), 1484.
Figures & Tables
In all of the figures, the county names are not legible. Please change to a clearer font, such as Arial, enlarge, and use white color for the text.
Figure 3-7 captions: Remove the excessive “the” before NEP.
Figure 9 caption: Writing “total” is a bit confusing as it could mean total (sum) of all study years. Do the authors mean “annual precipitation (mm)”? Include the time span for the average total precipitation.
Figure 11 caption: Write DEM in full.
Figure 14 caption: This is too brief. Explain what q is, and is the meaning of a to e. Provide the meaning of the colored scale on the right hand panel (i.e., meaning of the colors).
Table 1 caption: Explain all abbreviations.
Table 2 caption: Specify the interactions between what variables and what do the X and q indicate?
Material and Methods
The methods are adequately detailed for an understanding of the calculation of metrics used to assess NEP and are typical for a study with the goal of measuring NEP. The statistical approaches such as the Theil-Sen method is appropriate for the data, as the presence of outliers is likely. I appreciate that the value ranges and meaning of the metrics are explained very well. However, there are a few details that are still required to enhance reproducibility, and the better explanation of the RH metric is required.
L 87: Include a map of the study area within China to help readers understand the location.
L 89: Add “above sea level”
L 111-112: This sentence is cut-off and incomplete.
L 120: Explain what “NDVI” is and what it indicates - write in full.
L 181: Explain why this particular estimation for RH is chosen (equation 10). Does this estimation take account of the effects of leaf litter on respiration?
L 197: Is this the environmental factors (e.g., precipitation, altitude)? Please make this clear.
L 205: What program is this statistical tool? Name the company making the tool and the version number, etc. If it is freeware, provide the link to the tool. It would also be helpful to briefly describe how the program works to determine whether an interaction is synergistic, antagonistic or independent.
L 210: List the model parameters being referred to here in parentheses.
L 214: How were the correlations tested and does it involve statistical testing of the relationships? Specify the model used and the software used to run it.
Results
The results section provides a general overview of the NEP data across the study area, which is consistent with the expected patterns across the east and west environmental cline present across the area and the fluctuations in precipitation across the time period. The consistency of altitude differences with expectations in NEP responses also lend credibility to the data. The information provided by the study is potentially useful and presents a meaningful advancement to the field, given that the contribution to knowledge to the region or novelty of the study is better explained (see comments above). Moreover, the results on interaction effects between environmental variables on NEP is not explained clearly. The results reported in L 297-300 need to be associated with a table or figure showing those results (Figure 6 have been cited in error, and the correct figure is Figure 14). However, it is not clear what the figure is showing in terms of the type of interaction and the text is not very clear on this.
The authors have put the analyses and results into the same section (L 215). In studies, analyses are usually part of the methods section. For example, the lines from L 267-272 are clearly methodology. Please check that the section content adhere to journal guidelines.
There are several points that need clarification:
L 220: This R-sq value is relatively high and it should be mentioned that most of the variance is explained by the model. However, it would be helpful to contrast this value with those of previous studies.
L 229: The authors mentioned during the period from 2000-2010, NEP decreased over time due to low precipitation and humidity at the time, but this is not easily to understand without the supporting information - is there a reference or figure showing this?
L 244-251: In addition, these values can also be expressed as percentages of the total area to aid understanding.
L 250-251: Rephrase to “Areas with NEP values 0-300 gCm-2a-1 was most widespread within the study area.”
L 294-296: My interpretation from the wording here is that high precipitation, high temperature, and high altitude are more suitable for vegetation growth, but high altitude is more stressful.
L 303-304: Which factors are relatively less important?
Discussion
Rather than beginning the Discussion with uncertainty in estimates or limitations of the study, begin with the results in the context of the stated goal of the study. For example, starting the discussion at L 323 would be more suitable.
The discussion section describes the implications of the results well, providing an explanation for the indirect effects warming and waterlogging on NEP through RH. The biome-specific environmental conditions (e.g., high altitudes) are also explained in the context of NEP. However, there is a lack of statement on whether the goal of the paper has been achieved (due to the limited discussion on previous related work on this region, as I mentioned above), and also this section lacks discussion on future directions/work following the results of this study (this may include further experimental work).
A potential limitation that can be discussed is potential inaccuracy due to the lack of accounting for the effects of different leaf areas (e.g., per unit ground area, see Liu et al., 2018). Moreover, the effects of excess nitrogen in the soil on carbon cycling (e.g., in areas close to agricultural land) has not yet been explored or discussed, even though several of the references used by the authors explored this topic.
Moreover, consider discussing the results in the context of projected environmental changes in the study area, if that is available.
L 319-320: The citations inside the parentheses are not necessary.
Conclusion
The conclusions section summarizes the results of the study effectively. However, since a major motivation of the current study is climate change, I would expect much more emphasis on it. The significance of the study area on biological conservation should also be given some focus.
To finish the conclusions section, there should be a line or two on the steps going forward, such as avenues for future research or initiatives on securing ecological health in the face of climate change.
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