Changes in Forest Net Primary Productivity in the Yangtze River Basin and Its Relationship with Climate Change and Human Activities

Round 1

Reviewer 1 Report

General comments

The research demonstrates the forest net primary productivity relationships with the temperature and precipitation, and it is quite interesting to regional scale readers interested in Yangtze River basin productivity changes and potential relations to predicted climate changes. This research merits publication and needs to address reviewers’ concerns aimed at improving the manuscript. The title does not make a clear understanding, and the title should be improved by replacing “response to” with “relationship with”. Current title translates that there were changes in NPP in the river basin (may be due to unknown factors) and then how those changes responded to climate change (then what caused these changes).  

Generally, this research discusses regional net primary productivity of forests and puts in global net primary productivity-climate change perspective, but did not even mention the NPP-climate change interaction research work in dense boreal forests in North (mainly Canadian boreal forest)

 

Specific comments

Line 17: “NPP is the basis for ecosystem material” is vague sentencing and confused starting. It should be correctly worded with transparent sentencing.

Line 31: replace greater with stronger (especially when you are not giving the R2 values)

Line 33: authors have not provided any vegetation dynamics before, now talking on vegetation dynamics at the conclusion. What do they mean vegetation dynamics in addition to productivity?

Line 38-58: NPP is defined several times from various angles, and most of them conclude the same. Also, the importance and roles of NPP in regional/global C cycles and climate change are described in detail. This para is a basic one, but a bit too long

 Line 72-100: a lengthy discussion on spatial and temporal NPP in relation to climate change. Don’t think authors were required to discuss these in the intro part. These intro paras are somewhat too long, and all around

Line 101: even the last para was also started with a similar sentence and context

Line 115: LUE is the acronym for?

Line 122: add to the bracket as  [for example, ABC et al.   ……] or give several references here

Line 125-136: The study area is comprehensively described (although not required herein that detail) in intro part; however, this is an uninspiring finish of the introduction as to why this study was needed and what it would add to the current state of knowledge are barely touched. Additionally, there is a flaw as to how authors could validate the LPJ model that supposedly was developed under the soil, atmospheric and CO2 conditions different from the Yangtze river basin soil, vegetation and climate (e.g., typhoons) complex. Moreover, where the model assumptions, limitations and bounds were taken care of (I am expecting the details in the methods section anyway; however, a brief on these could be given here (especially when extra details on NPP, forests, productivity and climate change are given when not even needed).

Authors should explicitly provide specific objectives and hypothesis

Line 144: provide the reference

Line 144-147. This is a general intro, what is the need in the methods section?

Line 195: what is the difference between observed and estimated. To North American understanding of NPP research, data is estimated from observations (after calculations). Authors need to clearly define (and argue) for readers what they mean observed and estimated

Fig 2: in part a R2 and in part b, r value is used why?

Line 226: where are the vegetation structure and composition are described with nine PFTs. Authors must describe all those towards the end of the study area

Figure 9. The precipitation-NPP correlation graph is missing, probably a weak correlation?

In the conclusions part, authors want to say there was an interaction between temperature and precipitation that resulted in significantly enhancing the NPP. If there was an interaction, the data should be statistically analyzed using simple ANOVA tests and they would reveal the significance and would make this research even more interesting and valuable.

Author Response

1) The research demonstrates the forest net primary productivity relationships with the temperature and precipitation, and it is quite interesting to regional scale readers interested in Yangtze River basin productivity changes and potential relations to predicted climate changes. This research merits publication and needs to address reviewers’ concerns aimed at improving the manuscript. The title does not make a clear understanding, and the title should be improved by replacing “response to” with “relationship with”. Current title translates that there were changes in NPP in the river basin (may be due to unknown factors) and then how those changes responded to climate change (then what caused these changes).  

Reply: It has been revised. Thank you.

2) Generally, this research discusses regional net primary productivity of forests and puts in global net primary productivity-climate change perspective, but did not even mention the NPP-climate change interaction research work in dense boreal forests in North (mainly Canadian boreal forest).

Reply: Good suggestion. It has been revised. Thank you.

3) Line 17: “NPP is the basis for ecosystem material” is vague sentencing and confused starting. It should be correctly worded with transparent sentencing.

Reply: It has been revised. Thank you.

4) Line 31: replace greater with stronger (especially when you are not giving the R2 values).

Reply: It has been corrected. Thank you.

5) Line 33: authors have not provided any vegetation dynamics before, now talking on vegetation dynamics at the conclusion. What do they mean vegetation dynamics in addition to productivity?

Reply: In addition to changes in vegetation productivity, vegetation dynamics mean vegetation cover changes, vegetation type changes, changes in vegetation indices, and changes in vegetation carbon storage. 

6) Line 38-58: NPP is defined several times from various angles, and most of them conclude the same. Also, the importance and roles of NPP in regional/global C cycles and climate change are described in detail. This para is a basic one, but a bit too long.

Reply: It has been revised. Thank you.

7) Line 72-100: a lengthy discussion on spatial and temporal NPP in relation to climate change. Don’t think authors were required to discuss these in the intro part. These intro paras are somewhat too long, and all around.

Reply: It has been revised. Thank you.

8) Line 101: even the last para was also started with a similar sentence and context.

Reply: It has been revised. Thank you.

9) Line 115: LUE is the acronym for?

Reply: LUE is the acronym for light use efficiency. It has been corrected. Thank you.

10) Line 122: add to the bracket as [for example, ABC et al.   ……] or give several references here

Reply: Some references have been added in the article. Thank you.

11) Line 125-136: The study area is comprehensively described (although not required herein that detail) in intro part; however, this is an uninspiring finish of the introduction as to why this study was needed and what it would add to the current state of knowledge are barely touched. Additionally, there is a flaw as to how authors could validate the LPJ model that supposedly was developed under the soil, atmospheric and CO2 conditions different from the Yangtze river basin soil, vegetation and climate (e.g., typhoons) complex. Moreover, where the model assumptions, limitations and bounds were taken care of (I am expecting the details in the methods section anyway; however, a brief on these could be given here (especially when extra details on NPP, forests, productivity and climate change are given when not even needed). Authors should explicitly provide specific objectives and hypothesis.

Reply: Good suggestion. It has been revised. Thank you.

12) Line 144: provide the reference.

Reply: It has been revised. Thank you.

13) Line 144-147. This is a general intro, what is the need in the methods section?

Reply: It has been revised. After the transformation and cutting of the project, different vegetation types were obtained. Through calculation and comparison, this result has been obtained.

14) Line 195: what is the difference between observed and estimated. To North American understanding of NPP research, data is estimated from observations (after calculations). Authors need to clearly define (and argue) for readers what they mean observed and estimated.

Reply: This article re-verified the LPJ model and rearranged the data. This part of the data can't support my verification, so the content has been deleted. Thank you.

15) Fig 2: in part a R2 and in part b, r value is used why?

Reply: The figure displays an error. It has been revised. Thank you.

16) Line 226: where are the vegetation structure and composition are described with nine PFTs. Authors must describe all those towards the end of the study area.

Reply: It has been revised. Thank you.

17) Figure 9. The precipitation-NPP correlation graph is missing, probably a weak correlation?

Reply: It has been corrected. Thank you.

18) In the conclusions part, authors want to say there was an interaction between temperature and precipitation that resulted in significantly enhancing the NPP. If there was an interaction, the data should be statistically analyzed using simple ANOVA tests and they would reveal the significance and would make this research even more interesting and valuable.

Reply: It has been revised. Thank you.

Author Response File: Author Response.pdf

Reviewer 2 Report

I have read the manuscript with great interest. This manuscript shows the variation of NPP and its response to climatic factors in the Yangtze river basin in china.

The author has explained the need of the study and comparison with other existing literature. I have following questions:

Its not clearly explained why the author used LPJ model. There are other vegetation simulation models like CASA, SimCycle etc. why the author didn't choose or compare all the existing model to know which model can simulate best NPP.

The author didn't mention about uncertainty in the model and how to improve it.

The author has used climatic data from National Meteorological Science Data Sharing Service Platform. But I would suggest the author should compare this data with existing meteorological station data to know the accuracy of data.

The author should show the vegetation types maps of two years like 1980 and 2013 so it would be easy for us to know what vegetation types are increasing and decreasing then it would be easy to understand the phenological changes in the vegetation and impacts of climatic factors.

Validation is not sufficient

Figure 9b should be revised with precipitation data

explanation about climatic driven or human activity driven change in NPP is not enough so the author should elaborate. 

Author Response

1) Its not clearly explained why the author used LPJ model. There are other vegetation simulation models like CASA, SimCycle etc. why the author didn't choose or compare all the existing model to know which model can simulate best NPP.

Reply: In the “Introduction” section, the reason for choosing the LPJ model has been added and compared with other models. Thank you.

2) The author didn't mention about uncertainty in the model and how to improve it.

Reply: This part has been added in the article. Thank you.

3) The author has used climatic data from National Meteorological Science Data Sharing Service Platform. But I would suggest the author should compare this data with existing meteorological station data to know the accuracy of data.

Reply: My meteorological data such as air temperature and precipitation were collected from 175 meteorological stations in the Yangtze River basin from the China Meteorological Science Data Sharing Service Network. I compared these data with relevant data, which has certain accuracy. Thank you.

4) The author should show the vegetation types maps of two years like 1980 and 2013 so it would be easy for us to know what vegetation types are increasing and decreasing then it would be easy to understand the phenological changes in the vegetation and impacts of climatic factors.

Reply: Vegetation type maps have been added in “Results” part. Thank you.

5) Validation is not sufficient.

Reply: It has been revised. Thank you.

6) Figure 9b should be revised with precipitation data.

Reply: It has been revised. Thank you.

7) Explanation about climatic driven or human activity driven change in NPP is not enough so the author should elaborate.

Reply: Explanation about climatic driven or human activity driven change in NPP has been added in “Discussion” section. Thank you.

Author Response File: Author Response.pdf

Reviewer 3 Report

The paper with the title “Changes in forest net primary productivity in the Yangtze River basin and its response to climate change” provides a study that estimates NPP in the Yangtze River basin using the LPJ model and analyses the relationship between climate change and forest NPP. Although there is a worthy topic, there are several issues that need to be addressed

 

First, what is the unique contribution respect to what is already published in literature? At first glance, the manuscript does not provide the hypothesis and the aims of the study.

Second, the manuscript needs a lot of editing, grammar and writing review. I found grammatical mistakes, some sentences are in the incorrect tense (a mix of the past and present tense), and some paragraph is repetitive, which makes the reading very hard to follow.

 

I have a problem seeing the units of NPP as they are (gC/m2/a). I am assuming that “a” refers to annual, am I right? I suggest changing the units to gC/m2/y; where “y” means year because most of the literature published refers to NPP as gC/m2/y

 

Early the authors mentioned that the basin is a big afforestation area. Is there any forest plantation? Alternatively, major afforestation projects? Do you know the age of trees? Mature forests tend to have minor productivity compare to young stands, and maybe that could explain the spatial differences in NPP estimations.

The validation strategy is a little bit confused. NPP data observed was compared to NPP estimated. According to the authors, NPP observed comes from two sources Luo PhD dissertation and 60 literature sources, but also NPP data came from Oak Ridge National Laboratory, USA. I am assuming is MODIS products? Alternatively, what kind of data/observations? An explanation here is needed because as the authors mentioned, validation is a key step in NPP research. For instance, how do you relate remote sensing data (MODIS products are available since 2001) to your estimations (1961- 2013) (e.g. spatial and temporal resolution). Also, authors need to provide the reference(maybe in a table) of those 60 literature sources

 

Please revise the methods and results section. In some part, there are repetitive ideas, and some sections are mixed. Results need to go to the results section and methods needs to go to the methods section. You need to explain carefully (more than a sentence).

 

Please revise equation 1 and provide the proper reference. As far as the knowledge of this reviewer NPP = GPP-Respiration. (Sala et al. 2000. Methods in ecosystem science. Springer)

 

The methods section also needs to describe how the NPP estimations are going to relate to climate change. One sentence is not enough. For instance, How do you assume that a simple correlation explains a relationship between climate change and NPP? Is the spatial and temporal distribution attributable to other factors?

Please revise the conclusion section. The conclusions need to be concise. Also, they need to reflect the major findings of your study supported by your results.

Particular

Lines 125-130. “The Yangtze River…to climate change” I suggest moving this paragraph to the methods section in the description of the study area.

Line 171-185. Consider moving these paragraphs to the introduction section.

Line 186. Why are you assuming that the forests reach maturity? Is there any bias for doing so?

Line 189. Could explain a little bit more about these variables? For instance, how did you measure the Leaf area index? How many sample sites (if you measure on the field)?

Line 204-210. These paragraphs and figures need to go to the results section.

Line 250. Change “between” to “among.”

Line 337-340. Needs to go to the methods section.

437. References needed.

467-468. Please add a reference to the sentence.

Author Response

1) The paper with the title “Changes in forest net primary productivity in the Yangtze River basin and its response to climate change” provides a study that estimates NPP in the Yangtze River basin using the LPJ model and analyses the relationship between climate change and forest NPP. Although there is a worthy topic, there are several issues that need to be addressed. First, what is the unique contribution respect to what is already published in literature? At first glance, the manuscript does not provide the hypothesis and the aims of the study. Second, the manuscript needs a lot of editing, grammar and writing review. I found grammatical mistakes, some sentences are in the incorrect tense (a mix of the past and present tense), and some paragraph is repetitive, which makes the reading very hard to follow.

Reply: The hypothesis and the aim have been added in “Introduction” part. Thank you. 

2) I have a problem seeing the units of NPP as they are (gC/m2/a). I am assuming that “a” refers to annual, am I right? I suggest changing the units to gC/m2/y; where “y” means year because most of the literature published refers to NPP as gC/m2/y.

Reply: It has been revised. Thank you.

3) Early the authors mentioned that the basin is a big afforestation area. Is there any forest plantation? Alternatively, major afforestation projects? Do you know the age of trees? Mature forests tend to have minor productivity compare to young stands, and maybe that could explain the spatial differences in NPP estimations.

Reply: The Yangtze River Shelter Forest Projects launched in 1989 in order to resist floods and reduce soil erosion. The second term of this project started in 2001 and ended in 2010. Therefore, most of the forests in the basin belong to young forests, and tend to have major productivity.

4) The validation strategy is a little bit confused. NPP data observed was compared to NPP estimated. According to the authors, NPP observed comes from two sources Luo PhD dissertation and 60 literature sources, but also NPP data came from Oak Ridge National Laboratory, USA. I am assuming is MODIS products? Alternatively, what kind of data/observations? An explanation here is needed because as the authors mentioned, validation is a key step in NPP research. For instance, how do you relate remote sensing data (MODIS products are available since 2001) to your estimations (1961- 2013) (e.g. spatial and temporal resolution). Also, authors need to provide the reference (maybe in a table) of those 60 literature sources.

Reply: I have re-verified the LPJ model and rearranged the data, and this analysis have been added in “Result” part. Thank you.

5) Please revise the methods and results section. In some part, there are repetitive ideas, and some sections are mixed. Results need to go to the results section and methods needs to go to the methods section. You need to explain carefully (more than a sentence).

Reply: It has been corrected. Thank you. 

6) Please revise equation 1 and provide the proper reference. As far as the knowledge of this reviewer NPP = GPP-Respiration (Sala et al. 2000. Methods in ecosystem science. Springer).

Reply: This equation was referenced from Yin et al. (2018) and Liu et al. (2009).

Ref:

Yin, Y.; Ma, D.; Wu, S. Climate change risk to forests in China associated with warming. SCI REP-UK 2018, 8, 1-13.

Liu, R.; Li, N.; Su, H.; Sang, W. Simulation and Analysis on future carbon balance of three deciduous  forests  in Beijing mountain area warm temperate zone of China. Chinese Journal of Plant Ecology 2009, 33, 516-534.

7) The methods section also needs to describe how the NPP estimations are going to relate to climate change. One sentence is not enough. For instance, how do you assume that a simple correlation explains a relationship between climate change and NPP? Is the spatial and temporal distribution attributable to other factors?

Reply: Thank you. I added the t-test. The data was analyzed using simple t-tests and would reveal the significance and would make this research even more interesting and valuable.

Spatial and temporal distribution was also affected by other factors, but this article mainly analyzed the relationship among climate change and human activities with net primary productivity.

8) Please revise the conclusion section. The conclusions need to be concise. Also, they need to reflect the major findings of your study supported by your results.

Reply: It has been revised. Thank you.

9) Lines 125-130. “The Yangtze River…to climate change” I suggest moving this paragraph to the methods section in the description of the study area.

Reply: The text of the lines 125-130 has been moved to the “Study area” section. Thank you.

10) Line 171-185. Consider moving these paragraphs to the introduction section.

Reply: The text of the lines 171-185 has been moved to the “Introduction” section. Thank you.

11) Line 186. Why are you assuming that the forests reach maturity? Is there any bias for doing so?

Reply: Thanks. This sentence was taken from Miao et al. (2010) and this assumption was conducive to the accuracy of the results of the model operation. 

Miao, Q.; Huang, M.; Li, R. The net primary productivity of vegetation in the Yangtze River basin in response to future climate change. Journal of natural resources 2010, 25, 1296-1305.

12) Line 189. Could explain a little bit more about these variables? For instance, how did you measure the Leaf area index? How many sample sites (if you measure on the field)?

Reply: These variables have been explained: Biomass is the total number of living organisms in a given area, expressed in terms of living or dry weight per unit area. Leaf area index (LAI) can be simply defined as the amount of leaf surface area per unit ground area. Soil carbon storage is defined as soil organic carbon storage. Evapotranspiration (ET) represents the loss of water from the earth’s surface through the combined processes of evaporation (from soil and plant surfaces) and plant transpiration (i.e., internal evaporation). Runoff refers to the flow of water that flows along the surface or underground under the action of gravity during rainfall and snow melt or watering. Soil moisture content refers to the amount of water contained in the soil.

We measured the Leaf area index (LAI) using the LAI-2200C plant canopy analyzer.

We show comparisons between the mean annual NPP from the LPJ model and from observations at 172 sites across China

13) Line 204-210. These paragraphs and figures need to go to the results section.

Reply: These paragraphs and figures have been moved to the “results” section. Thank you.

14) Line 250. Change “between” to “among.”

Reply: It has been corrected. Thank you.

15) Line 337-340. Needs to go to the methods section.

Reply: These paragraphs have been moved to the “methods” section. Thank you.

16) 437. References needed.

Reply: The references have been added in the article. Thank you.

17) 467-468. Please add a reference to the sentence.

Reply: The reference have been added in the article. Thank you.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

I think the author has made sufficient revision and revised version can be accepted. 

Author Response

Thank you.

Reviewer 3 Report

Many thanks to the Authors to carefully respond to all my reviews. However, there are still some minor precisions that need to be made:

 

For instance:

Revision no 3. Early the authors…

The intention of this reviewer was to make a point about how the forest configuration is affecting NPP values across the basin. Could you explain how the spatial distribution of forest plantations (afforestation projects) influence the results?

 

 

The validation strategy needs to be described in the methods section. What happened to the 60 literature sources?

 

I still do not see how NPP estimations are related to climate change? I know you added the simple t-tests but what does it mean in terms of Climate change ? please revise methods section.

Author Response

1) Revision no 3. Early the authors…

The intention of this reviewer was to make a point about how the forest configuration is affecting NPP values across the basin. Could you explain how the spatial distribution of forest plantations (afforestation projects) influence the results?

Reply: Good suggestion. It has been revised. The afforestation projects have increased the forest area and canopy density in the Yangtze River basin. The Yangtze River Shelter Forest Project was a major afforestation project in the Yangtze River basin. This project launched in 1989 in order to resist floods and reduce soil erosion. The second term of this project started in 2001 and ended in 2010. Therefore, most of the forests in the basin belong to young forests, and tend to have major productivity (Lu et al., 2018). Thank you.

Ref:

Lu, F.; Hu, H.; Sun, W.; Zhu, J.; Liu, G.; Zhou, W.; Zhang, Q.; Shi, P.; Liu, X.; Wu, X.; et al. Effects of national ecological restoration projects on carbon sequestration in China from 2001 to 2010. Proc. Natl. Acad. Sci. 2018, 115, 4039-4044.

2) The validation strategy needs to be described in the methods section. What happened to the 60 literature sources?

Reply: The 60 literature sources come from the Ph. D. dissertation of Luo (1996). However, there were no detailed descriptions about these articles. Therefore, I have deleted it. And we got new data to verify the observed and simulated NPP. Thank you.

Ref:

Luo, T. Patterns of biological production and its mathematical models for main forest types of China. Ph. D Type, Committee of Synthesis Investigation of Natural Resources, Chinese Academy of Sciences, Beijing, 1996.

3) I still do not see how NPP estimations are related to climate change? I know you added the simple t-tests but what does it mean in terms of Climate change? Please revise methods section.

Reply: It has been revised. The changes of the precipitation and temperature for two periods has been analyzed and the analysis of the two extreme droughts has been added in “Results”. Thank you.

Author Response File: Author Response.pdf