Drought or Severe Drought? Hemiparasitic Yellow Mistletoe (Loranthus europaeus) Amplifies Drought Stress in Sessile Oak Trees (Quercus petraea) by Altering Water Status and Physiological Responses

Round 1

Reviewer 1 Report

The toppic of manuscript is quite interresting. The article is aimed to describe the impact of mistletoe on the drought stress and other parameters in oak trees. However I found incomplete the methodology part. The description of trees selection is missing. The research design is not described. Comparison of infested and non-infested trees brings only the general information. I suggest to rearrange the results part. 

However, the journal Water is a open access journal on water science and technology, including the ecology and management of water resources. The article fit not into the scope of the journal.

 

 

Specific comments:

L.90 How the evaluated trees were chosen? Describe the placement of trees in the forest.

L 91 Describe the forest stand more precisely. Give the information about acreage and management of the chosen the oak forest. 

L 94 How the diameter of trees was calculated? How the height of trees was measured?

L. 97 How was the increment measured? Were the trees cored by Pressler increment borer? Give the detailed informations to radial increment measurement.

L. 102 The range of mistletoes density on the trees belongs to the results section. However, the mistletoe density is poorly described. The trees have to be sorted according the mistletoe damageand the water potential and other characteristics should be evaluated according to mistletone density on trees. 

L. 119 The leaf samples collection for all tests should be described clearly in one paragraph or summarized in table. There should be written details aimed to number of samples for each type of test, the description of samples collection and the description aimed on leaf position in the crown.

L. 197 The results brings simple evaluation in boxplots. The authors compare the infested and non-infested trees. However, the important question is how high rate of mistletoe presence could affect the growth of tree or the other tested parameters (i.g. water potential, assimilation rate...). The relationship between the rate of mistletoe presence and values of tested parameters should be tested for example by regression analysis how it was evaluated in the case of diameter at brest height (L. 283). The trees could be also sorted according to number of observed mistletoe in the crown into the few groups. This distribution on infested and non-infested trees simplifies the results of the study. I highly recommend the rearrange the results part.

L. 288-292 It belongs to the introduction part.

L. 490 The discussion part is long. The most important results should be summarized in the separate Conclusion part.

Author Response

Comments and Suggestions for Authors

The topic of manuscript is quite interesting. The article is aimed to describe the impact of mistletoe on the drought stress and other parameters in oak trees. However I found incomplete the methodology part. The description of trees selection is missing. The research design is not described. Comparison of infested and non-infested trees brings only the general information. I suggest to rearrange the results part. 

However, the journal Water is a open access journal on water science and technology, including the ecology and management of water resources. The article fit not into the scope of the journal.

We appreciate generally positive opinion and constructive criticism regarding the MS. We did our best to improve the manuscript accordingly. Concerning the journal scope, we are aware that the journal Water is primarily focused on water science and technology. However, the Special issue the manuscript was submitted to, deals with ecological and socio-economic impact of natural hazards on forest ecosystems under climate change. The content of the article had been discussed and approved by a guest editor before the submission and we believe it can be interesting for readers of the journal. We also suppose that the editors basically confirmed suitability for the special issue by sending the manuscript to reviewers.

 

Specific comments:

L.90 How the evaluated trees were chosen? Describe the placement of trees in the forest.

Thank you for the valuable comment. As suggested, we have included more detailed information regarding the criterion for the selection of the trees chosen for this study (L120).

 

L 91 Describe the forest stand more precisely. Give the information about acreage and management of the chosen the oak forest. 

Same as in the previous comment, we added a more detailed description of the study stand. The thinning intensity of 24 m³/ha (9% intensity) was performed in 2010.

 

L 94 How the diameter of trees was calculated? How the height of trees was measured?

According to the suggestion, both these measurements were described in more detail in MS (L123-L125).

 

L 97 How was the increment measured? Were the trees cored by Pressler increment borer? Give the detailed informations to radial increment measurement.

We did our best to clarify and describe this issue in MS better (L125-L128).

 

L 102 The range of mistletoes density on the trees belongs to the results section. However, the mistletoe density is poorly described. The trees have to be sorted according the mistletoe damage and the water potential and other characteristics should be evaluated according to mistletone density on trees. 

We agree the number of yellow mistletoes might be included in the Results. However, the observation of the abundance of mistletoe is out of the main scope of the study even though it was assessed for 50 infested trees within the stand. For most of the assessed physiological parameters (excluding only growth), just trees with a similar degree of infestation were used (8-11 mistletoes). The information was therefore used for a more detailed description of the study site and thus is not connected directly with most of the presented results. Regarding the second part of the comment, please see answer bellow dealing with proposed results rearrangement.

 

L 119 The leaf samples collection for all tests should be described clearly in one paragraph or summarized in table. There should be written details aimed to number of samples for each type of test, the description of samples collection and the description aimed on leaf position in the crown.

Thanks for the good point, we fully agree that the M&M was difficult to follow in some parts. Therefore, we tried to rephrase them in a more appropriate manner. Moreover, for easier comprehension, we added the summary table with a more detailed description of the sample size for each measured trait (Table 1).

 

L 197 The results brings simple evaluation in boxplots. The authors compare the infested and non-infested trees. However, the important question is how high rate of mistletoe presence could affect the growth of tree or the other tested parameters (i.g. water potential, assimilation rate...). The relationship between the rate of mistletoe presence and values of tested parameters should be tested for example by regression analysis how it was evaluated in the case of diameter at brest height (L. 283). The trees could be also sorted according to number of observed mistletoe in the crown into the few groups. This distribution on infested and non-infested trees simplifies the results of the study. I highly recommend the rearrange the results part.

We agree that this would give us even better understanding of how physiological processes are restricted by the activity of yellow mistletoe. However, experimental design does not allow us to do so. We focused only on non-infested trees and trees infested by 8-11 mistletoes (most heavily infested or less infested trees were omitted in case of the detailed measurements of physiological traits). This is basically determined by two limiting factors: (i) the daily dynamics of the most assessed traits is relatively high unlike the mentioned diameter at breast height. Trying to capture the whole spectrum of the infestation without great noise in data would require much more trees to be measured. It would be feasible to measure fewer trees per more groups (degree of infestation) but we believe that data would be less reliable. (ii) Being restricted by professional climbers too, we decided to evaluate as many similar trees as possible in order to limit the impact of the natural noise resulting from daily dynamics. This allows us to compare just two groups reliably.

 

L 288-292 It belongs to the introduction part.

As suggested, this part was moved to Introduction.

 

L 490 The discussion part is long. The most important results should be summarized in the separate Conclusion part.

Thank you for your valuable suggestion. It´s true, that the separate Conclusion part could help for a better readability of the MS, therefore we separated the Conclusion from the Discussion part. Similarly, we shortened the Discussion.

Reviewer 2 Report

The study analyses variability of different functional traits related to C and water physiology in oaks infested and non-infested by a mistletoe species in Central Europe. The paper includes some interesting information but the material and methods and the discussion need to be better worked out before the manuscript can be considered suitable for publication. The authors need to better explain the methods and improve the discussion (see for instance comment on lines 190-…). I add more specific comments below.

- First, the authors do not mention the oak species studied, which needs to be fixed. I assume it is Quercus petraea (Matt.), Liebl., therefore, the discussion should be focused in this species. Along the introduction and the discussion they should reformulate the use of the term ‘oaks’ and be more specific because they are just referring to ‘temperate oaks’ (or just one temperate oak species). When they discuss their drought tolerance they should refer in relative terms within European temperate forests. Please, be sure to constrain these statements accordingly within the paper. They do not necessary apply to other biomes where Quercus spp. occur. Think about the ecology of evergreen oaks either in Southern Eurasia or America, it can have little to do with that of temperate oaks and what it is discussed.

- The hypotheses do not match the functional traits analysed and the ideas discussed in the abstract. They need to be reframed accordingly with the analyses and ideas discussed in the manuscript.

- The authors need to better characterise their methodology, otherwise it is not possible now to judge whether the physiological traits are correctly assessed or not (I specify several comments below). For instance, Asat and gs oak values in Figure 2 (and line 334) seem very low for a temperate oak in full sun leaves (unless they are under very severe drought conditions, as discussed in lines 355-…). The authors need to characterise their measurements in order to explain any potential bias in their results and prove that they are correct. In addition, see my last two comments below: where do authors analyse tree growth? I did not see any mention in the methods, then they discuss about stem and height growth.

- The discussion and the general language expression should be improved. For instance, I did not see any reference to potential anisohydric behaviour of oaks in relation to the results related with mistletoe infection. This needs to be properly discussed and in general the physiological measurements better framed within the many literature on the species under stressed and non-stressed conditions. In general, the authors need to support their statements with proper references and/or analyses. See e.g. line 99.

More specifically:

- Line 26: please rephrase, e.g. ‘were higher than those of healthy oaks’, ‘reached’ is not clear to show that those traits increased in declinining trees (this is also repeated in the discussion later).

- Line 43: it is not correct to state that oaks in general are among the most drought tolerant species. The authors just give one specific reference comparing with Fagus sylvatica. Oaks can be more tolerant in relative terms that some co-occuring taxa, but not others, and there are many examples of more drought-tolerant species.

- Line 61-62: specify that those responses correspond to host trees.

- Line 64: I do not understand this sentence, spreading of what?

- Line 70: what do ‘habitus’ and ‘more massive branches’ mean? Please, rephrase this sentence and explain better (and demonstrate with appropriate references) what this ‘defensive mechanism’ accounts for.

- Line 73: what is ‘accelerated ageing’? Along the paragraph there are several assumptions that do not necessarily hold and look more like hypotheses or speculations.

- Line 78: what is ‘amplified drought’? Please, characterise and explain your statements.

- Line 92-104: those are results, not M-M. And all that information could easily be summarised in one table.

- Line 99: any reference or analysis to support these differences in growth? Otherwise they remain speculative.

- Line 124: where all phi measurements carried at the same hour within the day? Midday measurements? For instance in line 299-302 the authors discuss with predawn potentials from other studies. Be careful and describe properly your methods.

- Lines 125-130: I could not understand how many samples they used and how they measured water potential. Please, explain better and specifically state the number of samples (leaves? Twigs?) used for each of the three groups (infested and non-infested oaks and mistletoes). All this is basic to prove that your results are robust.

- Line 131-…: were those measurements carried out on unshaded leaves? If this was the case, did authors cut the leaves or carried out the measurements directly on the branches? In the latter case I assume they would need some scaffolding or climber with the IRGA. Please explain better. As the authors know, the light conditions of the leaves need to be homogeneous among groups in order to make them comparable. The same applies for the next paragraph after line 143.

- Line 163: how could authors sample ‘100 leaves per mistletoe shrub were collected’ ? That must be a mistake. And mistletoes are not shrubs.

- Line 175: soil from infected and non infected trees?

- Line 187-188: 100 leaves per mistletoe? Do authors mean a total of 100 leaves for mistletoes? How many plants accounted for those 100 leaves? How many total samples? Please, revise and specify. For instance, the difference in SLA between the two oak groups shown in Fig 5a (surprisingly that is not significant with that box plot… this goes back to describing properly also the statistical methods) would be remarkable, but needs to be supported by an appropriate sampling design.

- Line 190-196: it is really difficult to understand how the analyses were carried out and to which samples. This section needs to be better explained in order to judge the quality of the manuscript.

- Line 251: differences in what?

- Line 294-299: Needs to include some reference, those results were already discussed in the reference Zweifel et al. (2012) [22], among others.

- Line 302: do the authors report predawn potentials? Otherwise these lines are not correct. And please, add more references, there are many similar studies in the physiology of Q. petraea (assuming this is the species studied).

- Line 367: this is not totally true, iWUE refers to the leaf level, meaning that it can remain unchanged while tree WUE and transpiration be modified e.g. if there is a decrease in leaf area. It would be good to discuss this issue and provide some appropriate references on the topic.

- Line 379-…: Ci, iWUE, gs, A and all the gas exchange traits measured with the IRGA are interrelated and some calculated based on the others. This means that they can be easily discussed together, rather than adding independent paragraphs for results that are not independent. And please be careful along the discussion in that paragraph… the A and gs obtained are really low and would express much stress also in non-infected trees, unless the leaves samples were shaded (which could explain such low values).

- Line 429-430: where are these results? How can authors report growth increments just with one sampling in 2019?

- Line 492: what is ‘logging stem increment’? and where are those conclusions demonstrated in the study?

Author Response

Comments and Suggestions for Authors

The study analyses variability of different functional traits related to C and water physiology in oaks infested and non-infested by a mistletoe species in Central Europe. The paper includes some interesting information but the material and methods and the discussion need to be better worked out before the manuscript can be considered suitable for publication. The authors need to better explain the methods and improve the discussion (see for instance comment on lines 190-…). I add more specific comments below.

Firstly, we would like to thank for a very thorough review. We believe that we were able to incorporate most of the comments and that it helped to improve the MS greatly.

 

- First, the authors do not mention the oak species studied, which needs to be fixed. I assume it is Quercus petraea (Matt.), Liebl., therefore, the discussion should be focused in this species. Along the introduction and the discussion they should reformulate the use of the term ‘oaks’ and be more specific because they are just referring to ‘temperate oaks’ (or just one temperate oak species). When they discuss their drought tolerance they should refer in relative terms within European temperate forests. Please, be sure to constrain these statements accordingly within the paper. They do not necessary apply to other biomes where Quercus spp. occur. Think about the ecology of evergreen oaks either in Southern Eurasia or America, it can have little to do with that of temperate oaks and what it is discussed.

Thank you for the good point. We were probably too careful with the specification of the species because we were unable to visually distinguish Q. petraea and Q. dalechampii reliably as they both (possibly even hybrids of those two) could be present in the evaluated stand. Moreover, there is still no exact consensus if Q. dalechampii is separate species or belongs to Q. petraea species. Therefore, we specified the measured oak species as the Q. petraea s.l.. Similarly, we highlighted in the introduction that we are dealing with the European temperate oaks, not with evergreen ones.

 

- The hypotheses do not match the functional traits analysed and the ideas discussed in the abstract. They need to be reframed accordingly with the analyses and ideas discussed in the manuscript.

We reframed the abstract to fit better with the discussed issues. However, we believe that the analyses correspond well with the proposed hypotheses. We would be grateful for getting more specific recommendations from the reviewer if the issue has not been solved properly.

 

- The authors need to better characterise their methodology, otherwise it is not possible now to judge whether the physiological traits are correctly assessed or not (I specify several comments below). For instance, Asat and gs oak values in Figure 2 (and line 334) seem very low for a temperate oak in full sun leaves (unless they are under very severe drought conditions, as discussed in lines 355-…). The authors need to characterise their measurements in order to explain any potential bias in their results and prove that they are correct. In addition, see my last two comments below: where do authors analyse tree growth? I did not see any mention in the methods, then they discuss about stem and height growth.

As specified below, we tried to improve the methodology according to the suggestions of reviewers. We used the standard protocol for measuring gas exchange (see e.g. Kučerová et al. 2018, iForest; Pšidová et al. 2015, Photosynthetica; Ďurkovič et al. 2014, Plant Biology), so we believe that the results are correct and reliable. Moreover, values of stomatal conductance of yellow mistletoe are high and this could also indicate that protocol and machine worked just fine. The remarkably low values of Asat and gs, which we also noticed, we assign to long-lasting drought period during which the experiment was performed.

There is indeed one source of noise which is nearly impossible to avoid in an observative study of this kind: daily dynamics of physiological parameters. We are aware of that and thhis is why the stand with both, infested and non-infested trees was chosen and as many trees as possible were sampled by two professional climbers. For more information about sampling, position of measured branches in the crown, and about the stem/height growth, please see answers below.

 

- The discussion and the general language expression should be improved. For instance, I did not see any reference to potential anisohydric behaviour of oaks in relation to the results related with mistletoe infection. This needs to be properly discussed and in general the physiological measurements better framed within the many literature on the species under stressed and non-stressed conditions. In general, the authors need to support their statements with proper references and/or analyses. See e.g. line 99.

We did our best to improve the discussion accordingly (L349-L354).

 

More specifically:

- Line 26: please rephrase, e.g. ‘were higher than those of healthy oaks’, ‘reached’ is not clear to show that those traits increased in declinining trees (this is also repeated in the discussion later).

The formulations were rephrased.

 

- Line 43: it is not correct to state that oaks in general are among the most drought tolerant species. The authors just give one specific reference comparing with Fagus sylvatica. Oaks can be more tolerant in relative terms that some co-occuring taxa, but not others, and there are many examples of more drought-tolerant species.

We fully agree that this statement is “too strong”, therefore it was rephrased.

 

- Line 61-62: specify that those responses correspond to host trees.

We agree that those are responses typical for the drought conditions. However, they are more pronounced with the co-occurring of yellow mistletoe. Of course, this statement was clarified in MS.

 

- Line 64: I do not understand this sentence, spreading of what?

The formulation was corrected.

 

- Line 70: what do ‘habitus’ and ‘more massive branches’ mean? Please, rephrase this sentence and explain better (and demonstrate with appropriate references) what this ‘defensive mechanism’ accounts for.

We apologize for the incomprehensible formulation. We tried to clarify the formulations and the appropriate reference was added (L73-L76).

 

- Line 73: what is ‘accelerated ageing’? Along the paragraph there are several assumptions that do not necessarily hold and look more like hypotheses or speculations.

Same as in the previous case, we corrected the formulation.

 

- Line 78: what is ‘amplified drought’? Please, characterise and explain your statements.

By ‘amplified drought’, we meant that the water deficit was stronger because of the presence of yellow mistletoe, which sucks the water from the host plant. Nevertheless, this statement was clarified in MS.

 

- Line 92-104: those are results, not M-M. And all that information could easily be summarised in one table.

We fully agree with the comment, this part was moved to Results and is presented as a summary table as suggested (Table 3).

 

- Line 99: any reference or analysis to support these differences in growth? Otherwise they remain speculative.

Thanks, we fully agree. As suggested, we rephrased this statement and support them by the ANOVA and the Tukey's post-hoc test. All these results are presented in the Results part in a table (Table 3).

 

- Line 124: where all phi measurements carried at the same hour within the day? Midday measurements? For instance in line 299-302 the authors discuss with predawn potentials from other studies. Be careful and describe properly your methods.

Due to the nature of our experiment, as we wanted to compare the adult trees infested and non-infested by yellow mistletoe, we were not able to measure all individuals at the same time. Anyway, we did our best to measure both groups under as comparable conditions as possible (please see also below). The mature trees were necessary to be used for such an experiment because the yellow mistletoe does not infest young trees and seedlings. As stated in the MS: “Measurements were taken on subsequent days from 8:30 to 12:00 and from 15:00 to 18:00”. We wanted to avoid midday depression of photosynthesis. Whereas part of measured physiological parameters is tightly coupled, we measured them for each sampled branch at the same time (immediately after branch was cut down). We discuss range of values measured by other authors (both, predawn and midday water potential). It would be extremely complicated to build up scaffolding in a high-grown managed forest and even then it would be barely feasible climbing up fast and safely enough to get data from few trees before sunrise. Nevertheless, we agree that some details were not mentioned, and we added more information about the experimental design.

 

- Lines 125-130: I could not understand how many samples they used and how they measured water potential. Please, explain better and specifically state the number of samples (leaves? Twigs?) used for each of the three groups (infested and non-infested oaks and mistletoes). All this is basic to prove that your results are robust.

We fully agree that the M&M was difficult to follow in some parts. Therefore, we tried to rephrase them in a more appropriate manner. Moreover, for easier comprehension, we added the summary table with a more detailed description of the sample size for each measured trait (Table 1). However, we decided to keep both groups of oaks in the one column as the sampling was exactly the same.

 

- Line 131-…: were those measurements carried out on unshaded leaves? If this was the case, did authors cut the leaves or carried out the measurements directly on the branches? In the latter case I assume they would need some scaffolding or climber with the IRGA. Please explain better. As the authors know, the light conditions of the leaves need to be homogeneous among groups in order to make them comparable. The same applies for the next paragraph after line 143.

As mentioned in the above response, the measurements were conducted on the sunny leaves from the upper part of the crown. The information concerning sampling was probably overlooked: “For further physiological measurements, branches from 14 infested trees and 14 non-infested trees were cut in turns during the day by professional climbers from the upper, unshaded part of the crown and measured immediately.” To maintain the homogeneity of samples at the highest possible level, we used the fully sun-exposed branches/leaves from the upper part of the crown. As trees were more than 20 m high, we were unable to collect the samples with the telescopic scissors, and the assistance of professional climbers was required. Two climbers cut down the branches and to avoid fast desiccation of tissues, branches as long as possible were sampled (approx. 2-4 m long), immediately put into the water and measured. Each climber was assigned to an exact group of trees, and the infested and non-infested oaks were sampled and measured in turn as quickly as possible.

Measurement performed by a climber is not the option neither as the basal parts of branches have no leaf and manipulation with the machine would be basically impossible. Anyway, we kept the same light conditions in the chamber during the measurements by Li6400XT using cuvette with a light source.

To conclude, it appears extremely hard to eliminate the daily dynamics of physiological traits and light conditions and maintain the appropriate sample size. We believe that the relatively high number of evaluated trees (14+14) measured “in pairs” is a counterbalance of this disadvantage. We did try to describe the methodology more clearly.

 

- Line 163: how could authors sample ‘100 leaves per mistletoe shrub were collected’ ? That must be a mistake. And mistletoes are not shrubs.

That´s true, that the yellow mistletoe is not a shrub stricto sensu, therefore we replaced the "shrub" by "individual" or “plant”. As for the number of sampled leaves, the bigger individuals (attached to a sampled branch) have a diameter of 1.5m and thousands of small leaves, therefore we see no obstacle in sampling such a high number of leaves per one individual and use it e.g. for scanning or mineral nutrition analyses.

 

- Line 175: soil from infected and non infected trees?

Within the research plot, the infested and non-infested trees grow close to each other (not all individuals are infested and they grow in the very same stand together), therefore we used just one soil sample containing mixed samples from 10 different spots on the research plot. This procedure was clarified in M&M.

 

- Line 187-188: 100 leaves per mistletoe? Do authors mean a total of 100 leaves for mistletoes? How many plants accounted for those 100 leaves? How many total samples? Please, revise and specify. For instance, the difference in SLA between the two oak groups shown in Fig 5a (surprisingly that is not significant with that box plot… this goes back to describing properly also the statistical methods) would be remarkable, but needs to be supported by an appropriate sampling design.

As for the number of leaves, please see the above-mentioned comment. The information about the sample sizes was added in the 2.1 chapter in the table (Table 1). Similarly, we tried to clarify the data processing and conducted statistical analyses. The differences in SLA/LMA were double-checked but we cant confirm significant difference at α=0.05.

 

- Line 190-196: it is really difficult to understand how the analyses were carried out and to which samples. This section needs to be better explained in order to judge the quality of the manuscript.

We agree that this part was hard to follow, therefore we did our best to make it clear.

 

- Line 251: differences in what?

Corrected

 

- Line 294-299: Needs to include some reference, those results were already discussed in the reference Zweifel et al. (2012) [22], among others.

The references were included.

 

- Line 302: do the authors report predawn potentials? Otherwise these lines are not correct. And please, add more references, there are many similar studies in the physiology of Q. petraea (assuming this is the species studied).

Please, see answer above. References have been added.

 

- Line 367: this is not totally true, iWUE refers to the leaf level, meaning that it can remain unchanged while tree WUE and transpiration be modified e.g. if there is a decrease in leaf area. It would be good to discuss this issue and provide some appropriate references on the topic.

Thanks for the comment, it is true that overal WUE can not be connected with WUEi measured at leaf level. The paragraph was shortened and reference was added.

 

- Line 379-…: Ci, iWUE, gs, A and all the gas exchange traits measured with the IRGA are interrelated and some calculated based on the others. This means that they can be easily discussed together, rather than adding independent paragraphs for results that are not independent. And please be careful along the discussion in that paragraph… the A and gs obtained are really low and would express much stress also in non-infected trees, unless the leaves samples were shaded (which could explain such low values).

We intended to structure the paragraphs in a logical order. Honestly, our first attempt was to discuss the parameters together, but it turned out to be quite confusing in our opinion. We believe that separating paragraphs makes the discussion better readable and it is easier to find information about exact parameters, especially for the readers without deep ecophysiological background. If it is a sine-qua-non requirement of the reviewer, we will try to discuss all parameters together in the second revision.

As mentioned in the answer below, we also noticed the remarkably low values of Asat and gs. The leaves were collected from the upper part of the crown and were not shaded for sure. Therefore, we attribute such low values to the long-lasting drought.

 

- Line 429-430: where are these results? How can authors report growth increments just with one sampling in 2019?

We apologize. These results were previously reported in the M&M section. However, they were moved into the Results section and the appropriate statistical tests were done to support our statements. Similarly, the procedure, how the stem increment is measured, was described more properly in the M&M section (L125-L128).

 

- Line 492: what is ‘logging stem increment’? and where are those conclusions demonstrated in the study?

We apologize for the confusing terminology. The term was unified by using ‘stem increment’.

 

 

Reviewer 3 Report

Thank you for giving me the opportunity to read and review the manuscript “Drought or Severe Drought? Hemiparasitic Yellow Mistletoe (Loranthus europaeus) Amplifies Drought Stress in Oak Trees (Quercus sp.) by Altering Water Status and Physiological Responses” by Kubov et al.

 

This study focus on the interesting interaction between a host tree and a plant that parasitizes a tree with a canopy called mistletoe from an ecological point of view. However, it is think that the manuscript has some points to be corrected. The details are shown below. I hope you find at least some of my comments useful -Thanks.

 

General comment: As the authors proposed that the goal of this study was to analyze a broad array of ecological properties and processes in oak trees infested by yellow mistletoe under drought conditions, I think that the research topics and questions of this manuscript are more in line with 'Forests' rather than with 'Water' of the MDPI journal. Why did the authors choose ‘Water’?

 

P2 L90: Is the stand of this study plantation forest? How did you determine the age of the stand?

P2 L97: How many individuals and how many years did you analyze the average stem increment per year and the mean height of the first branches? Please show the standard error in addition to their mean value.

P2 L102: The mean density of mistletoes per infested tree was 8.30. => The mean number of mistletoes per infested tree was 8.3.

P3L119: Please specify which measurement from 2.2 to 2.6 described later was measured in situ or on the subsequent days.

P4L143: Why was not the chlorophyll a fluorescence measurement in mistletoe?

P5 L183: 2.6 Leaf traits => 2.6 Leaf morphological traits

P10 L277: If the authors want to mention about the thicker leaves, LMA would be better to show rather than SLA.

P10 L280: In general, the coefficient of determination of the regression line is indicated by the r^2 value instead of the r value. The same applies to other parts of the manuscript.

P10 L296: As the author also discussed later, lower leaf water potentials in infected trees should make embolism more likely to occur rather than avoid it. Also, isn't it possible that this result is not necessarily an adaptive response of the infected tree, but is simply physically dragged down by the relatively low water potential of mistletoe?

P11 L320: Unlike host trees, mistletoes control their transpiration rate at very low levels… => Unlike host trees, mistletoes control their transpiration rate at very high levels…

P12L362: Yellow mistletoe can get carbon only from xylem sap, isn't it phloem transport? On the contrary, is it possible to transport carbon and nutrients from mistletoe to the host tree via phloem transport?

P13 L414, 415: For readers who are not familiar with this field, it is necessary to explain what NPQ and ROS mean. At least it shouldn't be an abbreviation. Especially for readers of Water rather than Forests.

P13 L421: I don't know the author's conclusion here. If we refer to Wang et al., isn't the change in photosynthesis of infected oaks interpreted as being caused by irreversible damage to PSII?

P13 L428: There are no results about height increment in this manuscript. And if the authors want to discuss about the differences in growth increment between non-infested and infested oaks, it would be better to move these results from Material and Methods to Results. And also, these results should be analyzed Tukey tests for the mean comparison between non-infested and infested oaks.

P13 L430: The text here is not appropriate, because the relationship between the diameter at breast height and the number of mistletoes in Figure 6 cannot not indicated Infested trees grow more slowly than non-infested ones.

P13 L433: The coefficient of determination of the regression line between the height and the frequency of mistletoe here (r=0.12) is not consistent with in Result (r = 0.07, P10 L280). Please check be careful.

P13 433: The text here is not appropriate, because immediate results mean that at least tree height is less important for mistletoe invasion of oak.

P13 L448: The result of oak tree distribution was not shown in Results of this manuscript. If the authors want to mention about it, please add the related results.

P14 L477 – L488: When comparing the results of this study with previous studies, information on the host tree or mistletoe studied there should be shown rather than the author name of the previous study. It is possible to guess whether the difference between the results in this study and in previous studies is due to the difference between the host tree or the mistletoe.

P14 L492, 493: The conclusion here is not partly appropriate, because there are no results about crown increment of oaks in this manuscript, and nutrient accumulation results was not negative effects in this study.

Author Response

Comments and Suggestions for Authors

Thank you for giving me the opportunity to read and review the manuscript “Drought or Severe Drought? Hemiparasitic Yellow Mistletoe (Loranthus europaeus) Amplifies Drought Stress in Oak Trees (Quercus sp.) by Altering Water Status and Physiological Responses” by Kubov et al.

 

This study focus on the interesting interaction between a host tree and a plant that parasitizes a tree with a canopy called mistletoe from an ecological point of view. However, it is think that the manuscript has some points to be corrected. The details are shown below. I hope you find at least some of my comments useful -Thanks.

 

General comment: As the authors proposed that the goal of this study was to analyze a broad array of ecological properties and processes in oak trees infested by yellow mistletoe under drought conditions, I think that the research topics and questions of this manuscript are more in line with 'Forests' rather than with 'Water' of the MDPI journal. Why did the authors choose ‘Water’?

We appreciate generally positive opinion and we believe that the insightful review of our manuscript can help to improve its quality. Concerning the journal scope, we are aware that the journal Water is primarily focused on water science and technology. However, the Special issue the manuscript was submitted to, deals with ecological and socio-economic impact of natural hazards on forest ecosystems under climate change. The content of the article had been discussed and approved by the editor before the submission and we hope it can be interesting for readers of the journal.

 

P2 L90: Is the stand of this study plantation forest? How did you determine the age of the stand?

The stand is a managed forest. The information about the age and the origin of the stand was obtained from the report of the forest stand management, which provides the basic information about each forest stand in the Slovak republic.

 

P2 L97: How many individuals and how many years did you analyze the average stem increment per year and the mean height of the first branches? Please show the standard error in addition to their mean value.

We apologize for not being clear. We added the summary table with a more detailed description of the sample size for each measured trait in 2.1 section of M&M (Table 1) and the results of the growth traits measurements in 3.3 section (Table 3). The results are presented as a mean ± standard deviations and appropriate post-hoc tests.

 

P2 L102: The mean density of mistletoes per infested tree was 8.30. => The mean number of mistletoes per infested tree was 8.3.

The formulation was changed as suggested.

 

P3L119: Please specify which measurement from 2.2 to 2.6 described later was measured in situ or on the subsequent days.

All measurements were done in situ, except leaf morphology traits – those were measured after field campaign in a laboratory. By formulation “subsequent days”, we mean that it was not feasible to perform the detailed physiological measurements within one day and we needed two days to do so. The main limiting factor was the necessity of professional climbers to sample branches from the crowns.

 

P4L143: Why was not the chlorophyll a fluorescence measurement in mistletoe?

Thanks for the comment, we agree that it would be worth focusing more on the physiology of mistletoe in the future. However, we were strongly restricted regarding the time during the presented field experiment and it was barely feasible to measure all presented data immediately after the branches were cut. Therefore, we set priorities as described: we focused primarily on methods revealing a complex impact of mistletoe on the physiology of infested trees (by comparing with the group of non-infested trees) and secondarily, we focused on methods enlightening the direct relationship between mistletoe and infested tree (e.g. water status; transpiration and stomatal conductance of mistletoe measured by gas exchange system; mineral nutrition etc.). We suppose that effectivity of the light-dependent stage of mistletoe does not reveal much about the direct impact on tested oak trees. Taking to account that the measurements are relatively time-consuming (30 minutes long dark adaptation + approximately 2-minute-long measurement per each leaf) we decided not to measure it.

 

P5 L183: 2.6 Leaf traits => 2.6 Leaf morphological traits

The title was changed as suggested.

 

P10 L277: If the authors want to mention about the thicker leaves, LMA would be better to show rather than SLA.

Thank you for the good point. We agree that the LMA better reflects the thickness of leaves, therefore, we remade the results and used LMA instead of SLA.

 

P10 L280: In general, the coefficient of determination of the regression line is indicated by the r^2 value instead of the r value. The same applies to other parts of the manuscript.

Thank you, we change this through the MS.

 

P10 L296: As the author also discussed later, lower leaf water potentials in infected trees should make embolism more likely to occur rather than avoid it. Also, isn't it possible that this result is not necessarily an adaptive response of the infected tree, but is simply physically dragged down by the relatively low water potential of mistletoe?

Thank you for a valuable recommendation, we reformulated this part to be more precise. The decrease in leaf water potential increases the gradient between roots and leaves, thus increasing water uptake, which may positively impact the overall water balance of an infested tree. This could be achieved by production of osmoprotective compounds, as an adaptation strategy. On the other hand, the decrease in leaf water potential results in more negative water potential in xylem which could result in cavitation occurrence. Identification of sources responsible for lowering leaf water potential is rather difficult because of naturally coupled processes. It needs more detailed study, which is beyond the scope of this manuscript. Anyway, we agree with the reviewer’s interpretation and consider the yellow mistletoe to be the main factor influencing water potential. We have changed the text in lines 327-330 to avoid possible misinterpretation.

 

P11 L320: Unlike host trees, mistletoes control their transpiration rate at very low levels… => Unlike host trees, mistletoes control their transpiration rate at very high levels…

We apologize for the misunderstanding. We wanted to say, that the yellow mistletoe controls his own transpiration at very low levels (doesn’t close stomata), not the transpiration of the host tree. Of course, this statement was clarified also in MS.

 

P12L362: Yellow mistletoe can get carbon only from xylem sap, isn't it phloem transport? On the contrary, is it possible to transport carbon and nutrients from mistletoe to the host tree via phloem transport?

Yellow mistletoe as a representant of hemiparasitic plants can get carbon only from xylem sap as there is a missing “phloem bridge” between hemiparasite and host. Minerals retrieved from xylem sap are trapped in mistletoe. As mistletoe acquires N and other compounds (micro and macro nutrients) in solute form from xylem sap, mistletoe also automatically receives carbon naturally attached to them. Such a heterotrophic carbon gain could range up to 62% of parasite dry matter (Tennakoon and Pate 1996).

 

P13 L414, 415: For readers who are not familiar with this field, it is necessary to explain what NPQ and ROS mean. At least it shouldn't be an abbreviation. Especially for readers of Water rather than Forests.

Thanks for the comment, we clarified this terminology.

 

P13 L421: I don't know the author's conclusion here. If we refer to Wang et al., isn't the change in photosynthesis of infected oaks interpreted as being caused by irreversible damage to PSII?

We apologize for the misunderstanding. We tried to clarify this conclusion. Please, see L455-462.

 

P13 L428: There are no results about height increment in this manuscript. And if the authors want to discuss about the differences in growth increment between non-infested and infested oaks, it would be better to move these results from Material and Methods to Results. And also, these results should be analyzed Tukey tests for the mean comparison between non-infested and infested oaks.

We apologize for the mistake and wonder how it can have happened. The height increment was not measured at all and we meant height. We corrected the formulation in MS. Moreover, as abovementioned, the results of the growth traits measurements were moved to the Results section and presented in Table 3 with the post-hoc tests.

 

P13 L430: The text here is not appropriate, because the relationship between the diameter at breast height and the number of mistletoes in Figure 6 cannot not indicated Infested trees grow more slowly than non-infested ones.

We agree, the formulation has been corrected. Please, see L467-468.

 

P13 L433: The coefficient of determination of the regression line between the height and the frequency of mistletoe here (r=0.12) is not consistent with in Result (r = 0.07, P10 L280). Please check be careful.

All regressions were double-checked.

 

P13 433: The text here is not appropriate, because immediate results mean that at least tree height is less important for mistletoe invasion of oak.

We fully agree that the formulation was not appropriate. It has been changed.

 

P13 L448: The result of oak tree distribution was not shown in Results of this manuscript. If the authors want to mention about it, please add the related results.

This part was deleted.

 

P14 L477 – L488: When comparing the results of this study with previous studies, information on the host tree or mistletoe studied there should be shown rather than the author name of the previous study. It is possible to guess whether the difference between the results in this study and in previous studies is due to the difference between the host tree or the mistletoe.

We prefer general claim because there were many species evaluated across the world and listing them would be distracting from the main point of the idea. There is not enough literature dealing with Q. petraea and yellow mistletoe relationship and sometimes we have no other choice but discuss general knowledge about hemiparasite and its host. We used host instead of oak in the manuscript.

 

P14 L492, 493: The conclusion here is not partly appropriate, because there are no results about crown increment of oaks in this manuscript, and nutrient accumulation results was not negative effects in this study.

Thank you for the comment. The whole Conclusion section was rephrased in a more appropriate manner.

Round 2

Reviewer 1 Report

 

Dear authors,

    vast majority of suggestions was included into the manuscript which was significantly editted. The most edited section was the methodology which is more understandable now. One of my last suggestion was foccused on the reconsidering the journal because the "Water" deals with topic of water science and technology. However, this question is on Editors of journal. Manuscript could be accepted after minor corrections or in Below are two point which could be improved.

 

L 120 Give the clear statement if the all measured trees were comparable - predominant (with full crown on sunlight), dominant or codominant trees...

 

L 128 The stem increment per year should be specified according to cores taken by Pressler auger. The differences between two measurements (DBH measurement) in spring and autumn in one year could be distort by measurement on the other place of the stem or by the changes on the bark. However, if I understand well, you do not have this data, this suggestion should be considered in future research.

Author Response

L 120 Give the clear statement if the all measured trees were comparable - predominant (with full crown on sunlight), dominant or codominant trees. Information added to the manuscript.

The stand is relatively uniform and for the analysis only dominant trees fully exposed to sun were chosen for the analysis. Text was added to the manuscript.

L 128 The stem increment per year should be specified according to cores taken by Pressler auger. The differences between two measurements (DBH measurement) in spring and autumn in one year could be distort by measurement on the other place of the stem or by the changes on the bark. However, if I understand well, you do not have this data, this suggestion should be considered in future research.

The DBH changes were derived from band dendrometer records. We do not have Pressler auger data. Thank for your advice, we will perform tree coring in next assessment.

Reviewer 3 Report

I think the revised manuscript has been improved by the efforts of the authors.

Author Response

Thank you for your effort to improve our manuscript.

With regards

PF et al.