Dear Dr. Cawse-Nicholson,

I have read the three reviews as well as your responses and how you will address those in the revised version of bg-2018-73. All reviewers acknowledge the importance of this study and its novelty filling an important gap in understanding how plants and ecosystems will respond to continually rising CO2 coming up with different results as experiments, such as FACE, limited in space and time. I will ask you to address the valuable comments made by the reviewers and decided to accept bg-2018-73 for publication after a minor revision.

Dr. A. P. Ballantyne notes the importance of this study in testing different remote-sensing techniques to test vegetation responses to elevated CO2 concentrations at the long-term. He focuses in his review much on the rather unexpected negative relationship between NDVI and soil CO2 flux which is an important observation. Possible factors, e.g., nutrient limitation, extreme climate events, oxygen deprivation of roots, soil acidification and plant vigor (see also comments of both anonymous referees) should be discussed indicating the need for further studies in this complex system.

The author’s should also address the influence of elevation reducing the CO2 effect.

Please also indicate some additional statistics and the order of magnitude of change in NDVI per change in Soil CO2 flux in the abstract.

I encourage the authors also to indicate the limitations of the FACE experiments around the world in contrast to the long-term emissions of volcanic CO2 emissions especially focusing on the warming trend.

Even if 99% of the emitted gas volume is CO2, the author should mention what other gases are emitted with the respective numbers. Even if CO2 is a trace gas in the atmosphere it has a tremendous impact on the Earth system!

Please do also consider the minor concerns indicated my Dr. Ballantyne.

Anonymous Referee #1 notes the novelty of this study filling an important gap in understanding how plants and ecosystems will respond to continually rising CO2 by overcoming limitations of previous studies in spatial and temporal scales. As he indicated, the authors should indicate some explanation to why looking at several different vegetation indices and comparing each individually to enhanced CO2 may be beneficial for understanding how plant physiology and growth is impacted and what methodologies may be selected in investigating other ecosystems.

As Dr. Ballantyne, anonymous reviewer #1 also focuses a lot on possible negative impacts (oxygen deprivation of roots, soil acidification, plant vigor) on elevated CO2 (contrasting results obtained by the FACE experiment) indicating the need for further studies especially on soil pH and O2 and citing important literature which should be considered by the authors.

The strength of the paper is the long-term monitoring vs. short-term observations as in FACE, and this should be evidenced in the discussion (as pointed out by the other reviews).

Please consider also the minor concerns and technical corrections indicated by reviewer #1.

Anonymous referee #2 acknowledges the long-term character of this monitoring, but also criticizes the empirical model used for the analysis of many covariates (which are closely related to each other as pointed out by referee #1) to discern the effects of eCO2 on structure and dynamic.

This reviewer has two major concerns. The first focuses on the variability of eCO2 over time and the authors give an interesting response indicating the history of measurements of CO2 emissions at Mammouth Mountain and I would find in interesting to include this in the Material and Method section. The second one was addresses soil conditions as important factor on structure and dynamics of vegetation which was already indicated by the other two reviewers and should be sufficiently addressed in the revised version of bg-2018-73.

Please to also consider the minor concerns indicated by referee #2.

I am looking forward to the revised version of bg-2018-73. Thank you for choosing Biogeosciences to publish this fascinating study.

Best regards,

Jochen Schöngart
(Associated Editor of Biogeosciences)

Dear Dr. Cawse-Nicholson,

on 14th August I send you my decision based on the three reviewes and how you will respond the critics and suggestion in a revised version of the manuscript.

I would like to receive a manuscript and a author´s response how you addressed the points I indicated in my decision.

The documents I have available on the platform indicate again how you responded the reviewers´concerns, but no comment regarding my indications.

Best regards,
Jochen Schöngart

Publish subject to minor revisions (review by editor) (14 Aug 2018) by Jochen Schöngart
Comments to the Author:

Dear Dr. Cawse-Nicholson,

I have read the three reviews as well as your responses and how you will address those in the revised version of bg-2018-73. All reviewers acknowledge the importance of this study and its novelty filling an important gap in understanding how plants and ecosystems will respond to continually rising CO2 coming up with different results as experiments, such as FACE, limited in space and time. I will ask you to address the valuable comments made by the reviewers and decided to accept bg-2018-73 for publication after a minor revision.

Dr. A. P. Ballantyne notes the importance of this study in testing different remote-sensing techniques to test vegetation responses to elevated CO2 concentrations at the long-term. He focuses in his review much on the rather unexpected negative relationship between NDVI and soil CO2 flux which is an important observation. Possible factors, e.g., nutrient limitation, extreme climate events, oxygen deprivation of roots, soil acidification and plant vigor (see also comments of both anonymous referees) should be discussed indicating the need for further studies in this complex system.

The author’s should also address the influence of elevation reducing the CO2 effect.

Please also indicate some additional statistics and the order of magnitude of change in NDVI per change in Soil CO2 flux in the abstract.

I encourage the authors also to indicate the limitations of the FACE experiments around the world in contrast to the long-term emissions of volcanic CO2 emissions especially focusing on the warming trend.

Even if 99% of the emitted gas volume is CO2, the author should mention what other gases are emitted with the respective numbers. Even if CO2 is a trace gas in the atmosphere it has a tremendous impact on the Earth system!

Please do also consider the minor concerns indicated my Dr. Ballantyne.

Anonymous Referee #1 notes the novelty of this study filling an important gap in understanding how plants and ecosystems will respond to continually rising CO2 by overcoming limitations of previous studies in spatial and temporal scales. As he indicated, the authors should indicate some explanation to why looking at several different vegetation indices and comparing each individually to enhanced CO2 may be beneficial for understanding how plant physiology and growth is impacted and what methodologies may be selected in investigating other ecosystems.

As Dr. Ballantyne, anonymous reviewer #1 also focuses a lot on possible negative impacts (oxygen deprivation of roots, soil acidification, plant vigor) on elevated CO2 (contrasting results obtained by the FACE experiment) indicating the need for further studies especially on soil pH and O2 and citing important literature which should be considered by the authors.

The strength of the paper is the long-term monitoring vs. short-term observations as in FACE, and this should be evidenced in the discussion (as pointed out by the other reviews).

Please consider also the minor concerns and technical corrections indicated by reviewer #1.

Anonymous referee #2 acknowledges the long-term character of this monitoring, but also criticizes the empirical model used for the analysis of many covariates (which are closely related to each other as pointed out by referee #1) to discern the effects of eCO2 on structure and dynamic.

This reviewer has two major concerns. The first focuses on the variability of eCO2 over time and the authors give an interesting response indicating the history of measurements of CO2 emissions at Mammouth Mountain and I would find in interesting to include this in the Material and Method section. The second one was addresses soil conditions as important factor on structure and dynamics of vegetation which was already indicated by the other two reviewers and should be sufficiently addressed in the revised version of bg-2018-73.

Please also consider the minor concerns indicated by referee #2.

I am looking forward to the revised version of bg-2018-73. Thank you for choosing Biogeosciences to publish this fascinating study.

Best regards,

Jochen Schöngart
(Associated Editor of Biogeosciences)

The authors addressed sufficiently the comments and suggestions of the reviewers and the editor and I will accept bg-2018-73 for publication in Biogeosciences after some small technical corrections which are listed below:

Best regards,
Jochen Schöngart

Technical corrections of bg-2018-73:

In terms of in-text citations, the order can be based on relevance, as well as chronological or alphabetical listing, depending on the author's preference, however, it should me consistent. In some parts in-text citations are listed from the most recent study towards the most ancient, in other place it is vice versa.

There is no consistent use of capitals for words in the titles of sections.

Material and methods:
P. 6/L. 5-6: Is “Classic instrument” correct or necessary in this sentence?
P. 6/L. 17: “E.g.” at the beginning of the sentence should be avoided
P.12/ L. 4: add a comma after et al.
P. 12/L/.18: What does ”<<0.05” means? Below 0.01?

Discussion:
P. 15/L. 24: Please delete the comma after et al.
P. 17/L. 23: Should be “Tissue et al., 1999).”

In Figure 1, I am wondering about the atmospheric CO2 flux (blue line). Why is it constant over the 25-yr period?