Enhancing Solar-Induced Fluorescence Interpretation: Quantifying Fractional Sunlit Vegetation Cover Using Linear Spectral Unmixing

Round 1

Reviewer 1 Report

The paper is well-written and in good shape. Please find some minor comments below:

Page 1: Is APAR flux limited to green? Or are we concentrating on the green APAR flux?

Page 2, Line 45: There is a grammatical error in the sentence starting with “SIF is in a first order driven by the amount.”. Please rectify.

Page 2, line 55, What do you mean by distorting incoming light and emitted fluorescence? Does it mean that the linear relationship between them is lost? It is confusing, please consider rephrasing it.

Page 2, Line 62: What do you mean my quality of the PAR?

Page 5, Line 223, Can you briefly recollect the protocols used from [35]? What was the criteria used for classifying the pixels?

Page 4,5: For, fig 4and 5 Could you provide some more information in the figure label?

The English is good.

Author Response

Please, see the cover letter and other reviewer answers on the attachment

Reviewer 1:

Dear reviewer, thank you for your suggestions, please, find below the answers to your comments.

Comments and Suggestions for Authors

The paper is well-written and in good shape. Please find some minor comments below:

Page 1: Is APAR flux limited to green? Or are we concentrating on the green APAR flux?

The main focus is on obtaining the green APAR flux by normalizing the APAR by the sunlit FVC.

Page 2, Line 45: There is a grammatical error in the sentence starting with “SIF is in a first order driven by the amount.”. Please rectify.

We have changed to: (Line 46-47) SIF is primarily controlled by the amount of light reaching the photosynthetic surface (absorbed by chlorophyll a (Chl a) molecules).

Page 2, line 55, What do you mean by distorting incoming light and emitted fluorescence? Does it mean that the linear relationship between them is lost? It is confusing, please consider rephrasing it.

We have changed to: (Line 54) However, at the canopy level, the vegetation structure distorts both, the light reaching the photosynthetic surface (affecting the photosynthetic reactions) and the fluorescence signal (emitted by the vegetation) measured at top of canopy (TOC).

Page 2, Line 62: What do you mean my quality of the PAR?

By quality, we refer to the spectral characteristics of the light.

Page 5, Line 223, Can you briefly recollect the protocols used from [35]? What was the criteria used for classifying the pixels?

The measurement protocols were different in [35]. The word "protocol" was incorrect. We have changed it to "methodology" to avoid misleading the reader. The criteria used to classify the pixels is detailed from line 231 to 236.

Page 4,5: For, fig 4and 5 Could you provide some more information in the figure label?

We have improved the captions to give more information about the concepts presented.

Author Response File: Author Response.pdf

Reviewer 2 Report

Dear Author,

Thank you for this interesting study.

I am aware the fact that you are measuring reflectance properties and the canopy behavior under the optimal conditions, however, it is still not clear how you did manage to relate this study with the natural systems to use the results for environmental studies (e.g. different stress stress conditions, backscattering properties of NDVI, the soil properties of the pots with respect to the natural conditions, etc.). 
Please define every abbreviations at first use regardless of how commonly used it is (i.e. TOC). 

The article needs some additional related references about its necessity within the introduction. Furthermore, the article requires information about relating an experimental study with its application on the environment within the discussion. It can be accepted after a minor revision.

Author Response

Please, see attached the cover letter with other review answers.

Reviewer 2;

Dear reviewer, thank you for your suggestions, please, find below the answers to your comments.

Dear Author,

Thank you for this interesting study.

I am aware the fact that you are measuring reflectance properties and the canopy behavior under the optimal conditions, however, it is still not clear how you did manage to relate this study with the natural systems to use the results for environmental studies (e.g. different stress conditions, backscattering properties of NDVI, the soil properties of the pots with respect to the natural conditions, etc.). 

Please define every abbreviations at first use regardless of how commonly used it is (i.e. TOC). 

We have defined TOC in the abstract (line 33) and introduction (line 57) sections, and FQE in the introduction (line 51).

The article needs some additional related references about its necessity within the introduction.

We have rewritten the first and last paragraphs of the introduction to clarify the complexity of measuring FQE at TOC. We have also included additional references to support the need to go beyond fluorescence for the early detection of stress.

Furthermore, the article requires information about relating an experimental study with its application on the environment within the discussion. It can be accepted after a minor revision.

We have rewritten the last paragraph of the discussion: (Line 643).

“This experiment tested a methodology capable of retrieving the FQE from hyper-spectral TOC data, with a protocol to validate the spectral-based FQE values with image-based measurements. However, before extrapolating the proposed methodologies to a larger and more complex ecosystems, further studies are needed to properly characterize and correct for the effects of canopy vertical structure in more heterogeneous surfaces, and to improve fluorescence retrieval to down-scale TOC measurements to the leaf - photosynthetic surface - level. In addition, further characterization of green (i.e., specific pigment absorption) and non-green (i.e., woody material and soil absorption libraries) endmembers as well as the atmospheric correction should be incorporated into the proposed methodology. Given the proposed improvements, it is expected that the next studies will apply this methodology to larger scales, such as airborne or satellite measurements, allowing the remote estimation of FQE and consequently the early detection of crop and global vegetation stress.”

Author Response File: Author Response.pdf

Reviewer 3 Report

The authors applied simultaneous measurements of Solar-induced chlorophyll fluorescence and reflected radiance. the measurements were performed both at leaf and canopy levels for Salvia farinacea and Datura stramonium plants. To disentangle the proportion of sunlit and shaded absorbed Photosynthetically Active Radiation, an ad hoc experimental setup was designed to provide a wide range of Fraction Vegetation Cover canopy settings.

The authors have conducted thorough experiments, employed appropriate methodologies, and analyzed the data effectively.

Material and methods are well described 

The results are well presented.

Unfortunately, the discussion is very limited. I suggest building specific research questions and hypotheses so the discussion will be more structured. You have a lot of concepts in your introduction and lit review, but they are often lists of concepts not well linked.

The authors should add the implication of this study in the conclusion section.

Author Response

Please, see attached the cover letter with the answers to other reviewers.

Reviewer 3

Dear reviewer, thank you for your suggestions, please, find below the answers to your comments.

The authors applied simultaneous measurements of Solar-induced chlorophyll fluorescence and reflected radiance. the measurements were performed both at leaf and canopy levels for Salvia farinacea and Datura stramonium plants. To disentangle the proportion of sunlit and shaded absorbed Photosynthetically Active Radiation, an ad hoc experimental setup was designed to provide a wide range of Fraction Vegetation Cover canopy settings.

The authors have conducted thorough experiments, employed appropriate methodologies, and analyzed the data effectively.

Material and methods are well described 

The results are well presented.

Unfortunately, the discussion is very limited. I suggest building specific research questions and hypotheses so the discussion will be more structured. You have a lot of concepts in your introduction and lit review, but they are often lists of concepts not well linked.

We have modified the introduction and discussion, identified the main research questions, to linked them between sections and highlighted when we address each research question in the discussion. 

The authors should add the implication of this study in the conclusion section.

In the conclusion section, line 669 until the end, we have modified the text highlighting the implications of the study:

“In this study the spectral signature of the sunlit vegetation fraction was used to de-scribe the vegetated surface, which is crucial for Cal/Val activities. From the point of view of the remote sensing methodologies, the quantification of the components presents inside the measured area overcome the limitation of the structural effects on the fluorescence signal, demonstrating that the proposed spectral approach has the potential to unravel the intricate complexities of underlying vegetated surfaces with hyperspectral sensors. The development of more robust methods to obtain products beyond fluorescence, such as surface effective PAR and APAR, and most importantly FQE, will allow remote quantification of the vegetation energy balance. Closing the gap between the flux of photons absorbed, dissipated, and used for photochemistry, which is critical for remote quantification of photosynthesis. Future research lines are needed to investigate the extrapolation of this method to larger scales, with the aim of studying FQE temporal dynamics and variable stress conditions for global monitoring of vegetation using orbiting satellites.”

Author Response File: Author Response.pdf