Spatio-Temporal Evolution, Future Trend and Phenology Regularity of Net Primary Productivity of Forests in Northeast China
Round 1
Reviewer 1 Report
The manuscript “Spatio-temporal Evolution, Future Trend and Phenology Regularity of Net Primary Productivity of Forests in Northeast China” appears relevant for the journal Remote Sensing. I do think that the introduction is very sparse in terms of background information, which left me wondering what the motivation behind this study was. It would help to get more detail on the ecological civilization, what recent studies concerning forest NPP have been finding and how the models the authors mention are used to inform economic decisions. From reading the conclusions section it appears that the main differences of this study compared to other was that it accounted for land use changes using some indices, but this doesn’t become clear until the methods and the end of the discussion section. On the contrary it appears that the novelty of this study was the higher temporal resolution, yet most of the results focus on annual NPP values. The introduction mentions several stories but does not go into detail for any of them, so the reader is left wondering what they found (i.e., starting at line 59). Throughout the manuscript certain sections appear to require more information, or at least a citation to guide the reader to more readings. The paper references climate change and rising temperatures a lot, yet there was no analysis or mention of meteorological data for the time period the authors looked at, to show that temperature significantly increased in this region. The discussion section is lacking an in-depth discussion especially comparing results from other studies to the current one, especially with only 10 citations in this section. In summary, the study seem sound, but the manuscript would benefit from an improvement in organization, more explicit descriptions and an improved discussion section.
specific comments
Abstract
The abstract is a bit vague, descriptions like “environmental quality” and “future evolution trends” or “ecological environment construction” leave a lot to the imagination of the reader. What is meant by “occupied a larger area” on line 18 or sentence starting on line 22 is not clear to me.
Introduction
More in depths descriptions about what it means to have an “ecological civilization” might help some more readers, thus expanding upon the second part of the sentence on line 31 would be useful. Additionally, nowhere in the introduction do the authors mention what methods they intended to use (aka MODIS remote sensing data).
Line 67: Please describe “CASA model”
Line 71: the sentence needs a citation
Line 78: Please be more specific about existing environmental issues
Lines 73-84: This section appears to be the rational for the study, but I do find myself wondering what the authors are specifically looking at in the paper (other than NPP). It would be helpful to describe a specific objective in more detail and what timeframe the authors are studying. I am left wondering how the data will inform ecological civilization
Line 79: please define GLO-PEM, in addition a short description of the model would be useful
Line 82: Please define environment quality
Methods
Section 3.1.1: This is the first-time hearing about a MODND1D MODIS product. A quick search only leads me to the gscloud.cn website and other publications, mainly from China, mentioning the same product. I don’t doubt its validity, but I am wondering why the authors chose to use this, instead of the NDVI product by MODIS (MOD13, MYD13, etc.). A short description about the product would help to understand its benefits, since the website the authors cite will likely be inaccessible by readers who do not speak Chinese.
Line 176 require citations to the original papers and what is We(x,t)?
Line 183, section Sen-Mann-Kendall, again this section needs some citations
Line 192 section 3.2.3, the Hurst index needs a better explanation. After reading this section I do not understand how this index works, or how it is calculated, aka, what variables go into the calculation, NPP?
Line 237 section 4.2: The MOD17 NPP product seems to come out of nowhere, please describe this in section 3.1.1 as well.
Results
Line 250: Is that an increase every year, or 2019 compared to 2001?
Line 255: Please describe ecological environment in this context, and what continuous improvement means
Line 260: This seems very hypothetical and would require a citation or some data on the rising temperatures in this region.
It is unclear to me what the differences between figures 8-10, especially 8 and 10 are. I am also confused whether figures 9-10 show future trends or the period 2001-2019, the captions seem misleading.
Line 395: What does 1st NPP refer to?
Figure 11, there might be a better way to show the differences by year. With this picture it is very hard to distinguish different years.
Figure 1213: I am not entirely sure what I am looking at here. Am I to assume that the black line shows the NPP at day 97-137 and 265-289 for each year? Are they accumulated NPP up to day 97, etc? Why are the units g C m-2 per year then?
Discussion
Section line 463-471 needs several citations for given statements
Line 467-471: This section is hard to understand, I would also argue that intercropping of forests and its effects on NPP, water and energy cycles should be established before this management strategy could be implemented
Line 497-500: This sentence goes in circles and doesn’t really describe how low biodiversity impacts productivity. Is low productivity always an “ecological problem”?
Section 6.3 seems a bit confusing and jumps around. Additionally, it does not talk about the uncertainty in phenology observations, but only mentions one limitation at the end of the last paragraph, but not how it may have influenced the uncertainty in their results
Section 6.4 now talks about limitations of model inputs and outputs, resolution, etc. which makes me wonder how high the uncertainty of their results could be. But because the authors did not do a sensitivity or uncertainty analysis, we are left believing the results as is
Author Response
Please see the attachmentAuthor Response File: 
Author Response.docx
Reviewer 2 Report
The manuscript has merit to be considered for publication. The authors studied net primary productivity in forests in China. The object of study and the location are extremely relevant. Below I send some important comments:
- citations throughout the manuscript are not in numerical format, required by RS;
- Although I have enjoyed the Introduction, I missed seeing how this research fills the gap in the assessment of NPP in China;
- I would like to see a paragraph with the hypotheses and objectives at the end of the Introduction;
- The authors could use the Pettitt test to identify which year the trends identified by the MK-test began;
- The authors could also use the ARIMA model to make a future modeling of the NPP values.
Author Response
Please see the attachmentAuthor Response File: Author Response.docx
