The Photosynthesis of Populus euphratica Oliv. Is Not Limited by Drought Stress in the Hyper-Arid Zone of Northwest China

Round 1

Reviewer 1 Report

This study reported the physiological measurements of Populus euphratica, a keystone species in oasis ecosystems. The author found that this species can exhibited stomatal limitation to photosynthesis to reduced midday transpiration, which acts as an adaptive water use strategy. However, there are some issues regrading this draft. For example, the sampled trees only consider a fixed DBH class, thus this experimental design could not account for age effects on tree water use. Also, it seems that the sap flow data was not used in this study, why mention them in the Method sections. In general, the Results are not very concise, most of them did not catch the key findings of the Figs or Tables. At last, language editing is needed. Considering that this study reported tree physiology of an important species in oasis ecosystems, I would suggest a major revision before it can be accepted for publication.

L12-14, these sentences are confusing as its meaning is unclear.

L22, define ‘decoupling coefficient’.

L30, replace ‘facing’ with ‘for’

L32-34, Low water availability reduces plant carbon balance and limits plant growth through its negative effects on photosynthesis.

L34-35, delete this sentence as it is well-known.

L36, the limitations of photosynthesis consist of two different processes:

L42, stomatal closure is a trait?

L44-45, I don’t get the meaning of this sentence.

L52, may modify morphological and ….

L56, what is ‘minimal water’?

L60, replace ‘constructive’ with ‘foundation’

L65, these riparian forests, which aimed to increase runoff to 0.95 billion 66 m3/year.

L74, to this end,

L80-81, the effects of seasonal fluctuations of temperature on photosynthesis of P. euphratica in the lower reaches of Heihe River

L03-104, We measure Vs using the heat ratio method (SFM1, ICT Inc., Armidale, Australia) by inserting two sensors into the xylem tissue of the trunks at dbh height.

L117, how long or what is depth of the sensor? Did they cover the entire sapwood? This is important for quantifying the whole tree water use.

L161, why Γ can be neglected, please explain it with Ref.

L193, delete ‘For the most part’

L199, varying between 17.54% and 75.21%.

L201-204, To make it more clear about the background conditions, I would suggest mark the specific date in Fig.2, as well as in Fig. 3, when the environmental and physiological parameters were measured.

L211-213, The meaning of these sentences is unclear, please rewrite it.

L214, changes than those

L250-263, If Figs. 3-4 were measured at the same date, combing them into one Figure rather than the current separated figures will make it more easily for reader to see how they covary.

L276, did you test the relationship between them?

L278-281, First, midday depression of photosynthesis is not a novel finding. On the other hand, midday depression of photosynthesis was not obvious in Fig. 3. Why it did not occur in June and August, 2013, as well as in June and September, 2014?

L313-314, Refs are needed.

Author Response

Dear Editors and Reviewers:

Thank you for your letter and for the reviewers’ comments concerning our manuscript entitled ‘The photosynthesis of Populus euphratica Oliv. is not limited by drought stress in the hyper-arid zone of Northwest China’ (ID: forests-2037168). Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have studied comments carefully and have made corrections which we hope meet with approval. The main corrections in the paper and the responses to the reviewers’ comments are as follows:

Reviewer #1:

1. The sampled trees only consider a fixed DBH class, thus this experimental design could not account for age effects on tree water use.

Response: We conducted our experiments in the P. euphratica Forest Reserve on the Ejin River, where is a fixed study site with a total of 80 trees. We selected three sample trees that were representative of the surrounding stand; the selected trees were not significantly different in height, DBH, and A_s to the stand means (t-test) (Table 1), and the ages of the three trees were very nearly the same. (Line 114-117, 135-136)

2. It seems that the sap flow data was not used in this study, why mention them in the Method sections.

Response: The sap flow data was used to calculate transpiration (T) (Line 132-133), then the canopy stomatal conductance (g_c) was calculated (formula 8), and finally the decoupling coefficient (W) was calculated (formula 7) to analyze whether P. euphratica was facing drought stress combining with water potential (y_L) and groundwater table (GWT).

3. L12-14, these sentences are confusing as its meaning is unclear.

Response: Photosynthesis is one of the most likely physiological processes to be affected due to drought stress in the arid area. The factors that lead to the decrease of photosynthesis can be divided into stomatal and non-stomatal limitations. As Populus euphratica Oliv. is the foundation species of the desert riparian forests in Ejin oasis, we aimed to investigate whether the photosynthesis of P. euphratica was limited there and, if so, whether this limitation was caused by drought stress. Relevant theoretical introductions have been added (Line 12-16, 19-20).

4. L22, define ‘decoupling coefficient’.

Response: The definition of decoupling coefficient has been added (Line 28-30).

5. L30, replace ‘facing’ with ‘for’.

Response: The word ‘facing’ has been replaced (Line 38).

6. L32-34, Low water availability reduces plant carbon balance and limits plant growth through its negative effects on photosynthesis.

Response: This sentence has been modified (Line 40-43).

7. L34-35, delete this sentence as it is well-known.

Response: This sentence has been deleted (Line 41-42).

8. L36, the limitations of photosynthesis consist of two different processes:

Response: This sentence has been modified (Line 45-46).

9. L42, stomatal closure is a trait?

Response: The word ‘trait’ is inappropriate, we have replaced it with ‘adaption’ (Line 52).

10. L44-45, I don’t get the meaning of this sentence.

Response: We have modified this sentence to ‘values of g_s will decrease to reduce water loss through transpiration, and plants will become relatively conservative water users with stomatal limitation of photosynthesis as the dominant mechanism’. (Line 53-55).

11. L52, may modify morphological and ….

Response: This sentence has been modified (Line 62).

12. L56, what is ‘minimal water’?

Response: We have replaced the word ‘minimal’ with ‘little’ (Line 67).

13. L60, replace ‘constructive’ with ‘foundation’.

Response: The word had been replaced (Line 72).

14. L65, these riparian forests, which aimed to increase runoff to 0.95 billion 66 m³/year.

Response: This sentence has been modified (Line 79).

15. L74, to this end.

Response: This sentence has been modified (Line 86).

16. L80-81, the effects of seasonal fluctuations of temperature on photosynthesis of P. euphratica in the lower reaches of Heihe River.

Response: This sentence has been modified (Line 92).

17. L03-104, We measure V_s using the heat ratio method (SFM1, ICT Inc., Armidale, Australia) by inserting two sensors into the xylem tissue of the trunks at dbh height.

Response: This sentence has been modified (Line 117-120).

18. L117, how long or what is depth of the sensor? Did they cover the entire sapwood? This is important for quantifying the whole tree water use.

Response: We have added this part. Because the thickness of the sapwood was less than the length of the probe, the outer (22.5 mm depth) V_s was selected for analysis. Data from the inner (7.5 mm depth) probe were always low and smooth, and even negative (data not shown), suggesting that it was inserted in heartwood, and the data were discarded. (Line 120-123).

19. L161, why Γ can be neglected, please explain it with Ref.

Response: G can always be neglected as its value is much smaller than that of C_a and C_i [42-44]. (Line 185-186).

20. L193, delete ‘For the most part’.

Response: This sentence has been modified (Line 219).

21. L199, varying between 17.54% and 75.21%.

Response: This sentence has been modified (Line 225-226).

22. L201-204, To make it more clear about the background conditions, I would suggest mark the specific date in Fig.2, as well as in Fig. 3, when the environmental and physiological parameters were measured.

Response: The specific date that environmental and physiological parameters were measured in both years were not the same in a month, the values in Fig.2 and Fig. 3 were averages for each parameter at the same hour on different observational days in each month. Introductions were added in the legends of Fig.2 and Fig. 3 (Line 229-232, 283-286).

23. L211-213, The meaning of these sentences is unclear, please rewrite it.

Response: This sentence has been modified to ‘Physiological factors (P_n, g_s, and C_i) were measured by an LI-6400 portable photosynthesis system on the same days with environmental factors collected from the meteorological station’. (Line 234-236).

24. L214, changes than those.

Response: This sentence has been modified. (Line 240).

25. L250-263, If Figs. 3-4 were measured at the same date, combing them into one Figure rather than the current separated figures will make it more easily for reader to see how they covary.

Response: Figs. 3-4 were measured at the same date, but Fig.3 is to analyze diurnal variations of physiological factors, while Fig.4 is to analyze diurnal variations of the values of stomatal limitation (L_s), relative stomatal limitation (RL_s) and non-stomatal limitation (Ci/g_s) of photosynthesis, so we make them separated two figures.

26. 26. L276, did you test the relationship between them?

Response: We have added the correlation coefficients. (Line 313)

27. L278-281, First, midday depression of photosynthesis is not a novel finding. On the other hand, midday depression of photosynthesis was not obvious in Fig. 3. Why it did not occur in June and August, 2013, as well as in June and September, 2014?

Response: Midday depression of photosynthesis (MDP) is not a novel finding indeed, we mentioned it here is mainly to analyze the stomatal and non-stomatal limitations to photosynthesis during MDP periods below (Line 330-334). As MDP not occurred on all observational days, the introductions about MDP in Line 316-320 could be deleted after reviewed, and then we could combine ‘4.1’ and ‘4.2’ together.

28. L313-314, Refs are needed.

Response: We have added the reference [51] (Line 359).

Except for the modifications mentioned above, we have further revised our full manuscript and improved the language expressions by the editing service again (the language of the original version was checked by a native English-speaking professor, and we further improved it by the editing service this time).

We appreciate for Editors/Reviewers’ warm work earnestly, and hope that the corrections will meet with approval.

Once again, thank you very much for your comments and suggestions.

Author Response File: Author Response.docx

Reviewer 2 Report

The paper refers to data taken 8-9 years ago, yet it deserves to be considered for publication in Forests. 

Major points to be changed are:

1) keywords should be different from those already present in the title.

2) A short description of the species will be helpful for the readers who are not familiar with it

3) in the M&M section the Authors reported to have measured sap flow but I could not find any results about it nor it's discussed in the discussion section. Have I missed something?

Please provide the species description and a thourough explanation of the measure you took with regard to sap flow 

Author Response

Dear Editors and Reviewers:

Thank you for your letter and for the reviewers’ comments concerning our manuscript entitled ‘The photosynthesis of Populus euphratica Oliv. is not limited by drought stress in the hyper-arid zone of Northwest China’ (ID: forests-2037168). Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have studied comments carefully and have made corrections which we hope meet with approval. The main corrections in the paper and the responses to the reviewers’ comments are as follows:

Reviewer #2:

1. Keywords should be different from those already present in the title.

Response: We have deleted two keywords that have already presented in the title, and added a new one. (Line 35-36).

2. A short description of the species will be helpful for the readers who are not familiar with it.

Response: The description of the species has been added. (Line 70-73).

3. in the M&M section the Authors reported to have measured sap flow but I could not find any results about it nor it's discussed in the discussion section.

Response: The sap flow data was used to calculate transpiration (T) (Line 132-133), then the canopy stomatal conductance (g_c) was calculated (formula 8), and finally the decoupling coefficient (W) was calculated (formula 7) to analyze whether P. euphratica was facing drought stress combining with water potential (y_L) and groundwater table (GWT).

Except for the modifications mentioned above, we have further revised our full manuscript and improved the language expressions by the editing service again (the language of the original version was checked by a native English-speaking professor, and we further improved it by the editing service this time).

We appreciate for Editors/Reviewers’ warm work earnestly, and hope that the corrections will meet with approval.

Once again, thank you very much for your comments and suggestions.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Although the current version was improved, some typos and the language need to be carefully fixed or edited.

Author Response

Dear Editors and Reviewer:

        Thank you for your letter and for the reviewer's comments again. We have  improved the language expressions by the editing service from MDPI (https://www.mdpi.com/authors/english) before the last submission of the revised manuscript. The edited version of the manuscript and related materials last time have been attached. We further modified our manuscript this time.

        We appreciate for Editors/Reviewer's warm work earnestly, and hope that the corrections will meet with approval.

Author Response File: Author Response.pdf