the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
09 Feb 2021
09 Feb 2021
Ecohydrological travel times derived from in situ stable water isotope measurements in trees during a semi–controlled pot experiment
- 1Hydrology, University of Freiburg, Freiburg im Breisgau, Germany
- anow at: Empa − Swiss Federal Laboratories for Materials Science and Technology, Technology and Society Laboratory, St. Gallen, Switzerland
- 1Hydrology, University of Freiburg, Freiburg im Breisgau, Germany
- anow at: Empa − Swiss Federal Laboratories for Materials Science and Technology, Technology and Society Laboratory, St. Gallen, Switzerland
Abstract. Recent advances in in situ measurement techniques for stable water isotopes offer new opportunities to improve the understanding of tree water uptake processes and ecohydrological travel times. In our semi–controlled experiment with 20–year–old trees of three different species (Pinus pinea, Alnus incana and Quercus suber) placed in large pots, we applied in situ probes for stable water isotope measurements to monitor the isotopic signatures of soil water and tree xylem before and after two deuterium labelled irrigations. Additional destructive sampling of soil and plant material complemented the in situ measurements and allowed for a comparison between destructive (cryogenic vacuum extraction and direct water vapour equilibration) and in situ isotope measurements. For the first labelling pulse, the tracer based travel time at a stem height of 15 cm was 0.7 days for all three tree species but at 150 cm height tracer based travel times ranged between 2.4 (for Alnus incana) and 3.3 days (for Quercus suber). The tracer based travel time from the root zone to 15 cm stem height was similar to the sap flow based travel times (i. e., for all trees 0.7 days). However, sap flow based travel times were 1.3 days (for Alnus incana) longer than tracer based travel times at 150 cm stem height. In terms of different between tree species, we found similar tracer movement in Pinus pinea and Alnus incanca while in Quercus suber tracer travel times were longer which is likely due to lower water uptake rates of Quercus suber. The comparison of destructive and in situ isotope measurement techniques suggests notable differences in the sampled water pools. In situ measurements of soil and xylem water were much more consistent between the three tree pots (on average standard deviations were by 8.4 ‰ smaller for δ2H and by 1.6 ‰ for δ18O for the in situ measurements) but also among the measurements from the same tree pot in comparison to the destructive methods (on average standard deviations were by 7.8 ‰ and 1.6 ‰ smaller for δ2H and δ18O, respectively). Our study demonstrates the potential of semi-controlled large scale pot experiments and high-frequent in situ isotope measurements for monitoring tree water uptake and ecohydrological travel times. It also shows that differences in sampling techniques or sensor types need to be considered, when comparing results of different studies and within one study using different methods.
David Mennekes et al.
Status: final response (author comments only)
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RC1: 'Comment on hess-2020-674', Anonymous Referee #1, 04 Mar 2021
General comments
Mennekes et al. present an experimental study in which they apply in-situ water isotopic monitoring techniques to estimate water travel times through the soil-plant-atmosphere continuum in three species with contrasting wood anatomy. They compare water travel times derived from in-situ isotopic monitoring with those obtained with sap flow measurements. At the same time, they also compare the isotopic results from the in-situ monitoring system with those of more traditional (destructive) approaches. The in-situ monitoring of isotopic composition appeared to estimate travel times consistently as travel times were very similar as those estimated with sap flow sensors. On the other hand, the isotopic composition as measured with the three different techniques was highly divergent.
This manuscript presents an experiment of high technical quality and that makes use of the most advanced methodologies to address scientific questions from a new perspective. Their results have thus the potential to advance our knowledge in ecohydrological travel times and provide new elements for the continuous development of in-situ isotopic monitoring techniques. The study falls well within the scope of HESS and has the novelty standards required for the journal. Despite this scientific high quality (both methodological and conceptual), I feel that the manuscript is not currently written in a way that it can take advantage of its strengths to make it the excellent contribution it could be. I recommend a thorough revision of some sections of the text but in my opinion, it is mainly the discussion that needs to be substantially improved. The experiment and all methodologies applied are well described and allow their replication. Please find below more specific comments.
1) In the Introduction, you present a rather broad context of the field, i.e. research on plant water relations using the isotopic composition of water. However, some of the open questions in this field are not actually addressed by your experiment. For instance, you mention the two water worlds hypothesis but do not provide any related new data. Also, you talk about improving plant water uptake models but never discuss the implications of your results on that. On top of that, I believe that your data allow you to make some inferences about potential isotopic effects during and after root water uptake (L41) but I did not find such discussion later in the text. Therefore, I would focus the Introduction on these questions in which your findings can matter most.
2) The Results subsection beginning at L340 should be synthesized and perhaps reformulated. In its current form, there is too much detail in the description of the results. This complicates the identification of the most relevant results and so, it is difficult to get the big picture. I understand that because you only have three trees, most of the results must be descriptive, and quantitative comparisons are limited. But for instance, figure 6 is very comprehensive but does not allow to identify the more general differences between and within methods, species and campaigns. A possibility would be to reframe the results along these three axes (species, method and time) while highlighting their interaction. I do not know if this is possible, other alternatives might be better.
3) The Discussion (and Conclusions) is the section of the manuscript that in my opinion, requires more work. First of all, it is unnecessarily long (7 pages). These are my humble suggestions to improve it (but see further comments in the “Specific comments”):
- Avoid repeating the aims/goals of the study. This information should be in the Introduction.
- Start the Discussion with an opening paragraph in which in a couple of sentences, you report the two or three most relevant results of the study but without going into much detail. Importantly, compare these main results with the most relevant literature. Currently this would be in the paragraph starting at L400, but I am not sure that you are putting everything relevant there. Obviously, my proposal for this first paragraph is questionable. In any case, I find it useful for the readers to make it easy to find the most relevant results and conclusions in the beginning of the Discussion.
- Try to avoid as much as possible the repetition of the results (L475-484 but check the rest).
- Do not report others’ results if they are not clearly linked to yours (Millar et al. 2018).
- Structure the paragraphs (not only the subsections) with clear and identifiable concepts, do not leave isolated two-line sentences.
- The Conclusions are well-written but somewhat repetitious with the rest of the Discussion. I would use this briefer conceptual scheme for the rest of the Discussion, while embedding the relevant literature. In any case, I think that the Discussion should be shortened, retaining only the most relevant points to discuss.
Specific comments
Abstract (L9): Perhaps I miss a sentence on the main goal of the experiment.
L21 Notable differences in the sampled water pools, is this related to the consistency of the measurements? Is it based on the differences of the average isotopic composition obtained by each method?
L30-32 I am not sure of the meaning of this sentence, please split it in two or rephrase it. Also, the last part after “by different tree species” is not well-connected with the rest.
L32-34 I suggest to reshuffle this sentence the other way around. For example, “The ecohydrological processes related to water uptake and usage at the tree- or plant-level are not yet fully understood”.
L35 Not clear what do you mean by “patterns” here. I guess that you meant “the depth of plant water uptake and its spatiotemporal patterns”. I would end the sentence here and start a new one: “The isotopic composition of water allows to link plant water to its putative…”.
L37 Unfortunately, each water pool has not always its own unique isotopic composition.
L40-41 I agree that it used to be widely accepted that root water uptake does not entail isotopic fractionation, and that there are some exceptions. But in many cases unknown fractionation processes occur before, during or after water uptake. Many recent papers reported that. Nowadays, I would not say that the absence of isotopic effects is widely accepted, but the opposite.
L42 Some of those cited studies are not so recent (e.g. Wigmosta et al. 1994).
L42-48 The message of this paragraph is rather vague.
L49 “Isotopic information” is not a precise term. I would say “isotopic composition”.
L50 Why consecutive? Usually, each water sample is analysed just once.
L52 What is the particular effect of organic contamination here? Did you mean the interference of organic compounds in laser-based isotopic analysis?
L56 It is not controversial? What do you mean?
L66-68 Please rephrase these sentences.
L93-94 “Water” travel times.
L27-128 Perhaps watering several times would have been more effective in removing the antecedent water in the soil. I don’t see the problem as long as you know the isotopic composition of irrigation water.
L130 I don’t understand what do you mean by “split the total amount”.
L212 It could be easier to provide extraction temperatures and times for soil and plant water samples here (without having to search for another paper).
L280 Why do you think this is due to sensor failure? Is this sap flow velocity not consistent with the literature?
Results: synthesise, simplify
Figure 3: It would be helpful to somehow indicate the signal of each labelling (as done for the precipitation and the irrigation).
L341-244 It is not clear if the differences you mention are between xylem water of the same species, or within an individual xylem water and its corresponding soil water.
L378 “For instance” instead of furthermore?. Also, “soil xylem”, what is this?
L374-385 This paragraph is difficult to read. Also, the L386-388 short paragraph summarises the result but the main idea that I got is that there are “partial” differences between methods.
L388-392 Good!
L396 I would say “for deriving ecohydrological…”.
L403 soil depths, in plural?
L408 Could you define more precisely what you mean by “path diffusion”?
L416 Martín-Goméz et al (2016) discussed Branch evaporative enrichment. These others you mention might be perhaps Barbeta et al. 2020?
L450 “European beech”. In fact, better if you give the scientific names and optionally, the English common name which might not be known by everyone.
L457 Quercus suber is a Mediterranean species, and it probably has denser wood than Alnus incana. Also, the lumen area per trunk section might be also smaller, which implies a smaller hydraulic conductance. I think that this could explain the different travel times.
L478 Which previous findings?
L480-484 Please synthesise the explanation in a manner than it is not required to jump from figure to figure to follow the argument.
L486 This sentence should not be a single paragraph.
L502-506 You should go straight to the point and avoid repeating your results.
L509 “less for d2H”. Please be more precise here.
L511 Is this result statistically significant?
L531-543 I think that the details of the results of a previous study over more than ten lines should not be provided in the discussion. In my opinion, you should stick to discussing your results, in the light of the previous literature, true, but the link between Millar et al. (2018) and your study is not even clearly done.
L549-550 I suggest that this sentence should be joined to another paragraph.
L551-568 So, what we can conclude from these results?
L536-656 This sentence arises from the results of your study? I do not find the evidence supporting that DVE is more reliable than CVE.
L568 This is a good point of the study, but I believe that it does not belong to the Discussion but instead, to Material and Methods or the Introduction.
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AC1: 'Reply on RC1', David Mennekes, 16 Apr 2021
We thank Referee #1 for the comments, which were very useful to prepare an improved version of the manuscript. We answer below to each comment in a point-to-point reply.
(comments by RC are bold, answers are normal font)
General comments
1)
Thank you for this comment. We agree and connected the first part of the introduction better with the importance of the here presented in situ method. Furthermore, we mention and transfer arguments in the introduction again in the discussion.
2)
Thank you for this comment. We shortened the results, e.g. for the climate, soil conditions subsection we deleted more specific air temperature information.
For the isotope label arrival section, we summarized / shortened information about specific arrival patterns (starting L292).
We changed the order of results in the subsection “comparison of in situ vs destructive”, talking first about 2H and then 18O which is more consistent with the other text.
Regarding figure 6: We understand the problematic of the figure. We tried to rearrange axes and categories. However, we think that in the current version one can see best, especially for 18O, that differences between methods (in situ vs. destructive) are much higher than between species. Furthermore, one sees well the variability for 2H. Regarding differences between the cryogenic extraction and water vapor equilibrium method, we agree that differences are hard to detect in the graph. However, we argue that this was not our main focus since here also more data would be needed for a fundamental research.
3)
Thank you for this comment. We generally shortened the discussion and start with an introductory sentence now. Regarding your specific points:
Avoid repeating the aims/goals of the study. This information should be in the Introduction
We did delete the repetition of our aims/goals.
Start the Discussion with an opening paragraph in which in a couple of sentences, you report the two or three most relevant results of the study but without going into much detail. Importantly, compare these main results with the most relevant literature. Currently this would be in the paragraph starting at L400, but I am not sure that you are putting everything relevant there. Obviously, my proposal for this first paragraph is questionable. In any case, I find it useful for the readers to make it easy to find the most relevant results and conclusions in the beginning of the Discussion.
Thank you for your suggestions. We followed your suggestion and start the discussion with an introductory sentence.
Try to avoid as much as possible the repetition of the results (L475-484 but check the rest).
We checked the discussion for possible repetition and deleted those where possible.
Do not report others’ results if they are not clearly linked to yours (Millar et al. 2018).
Thank you for your remark. We deleted this section since it is clearly not relevant for our paper.
Structure the paragraphs (not only the subsections) with clear and identifiable concepts, do not leave isolated two-line sentences.
We improved the structure of this paragraph.
The Conclusions are well-written but somewhat repetitious with the rest of the Discussion. I would use this briefer conceptual scheme for the rest of the Discussion, while embedding the relevant literature. In any case, I think that the Discussion should be shortened, retaining only the most relevant points to discuss.
Thank you for this comment. We have combined conclusions and implications into one section and considerably shortened it
Specific comments
Abstract (L9): Perhaps I miss a sentence on the main goal of the experiment.
Rephrased the beginning of the abstract (L9 – 11) to make our main goal clearer.
L21.
Thank you for pointing this out. It is based on the measured stable water isotope signals. We added this to the sentences.
L30-32
We rephrased the sentences to: “Challenges are partly the lack of sufficiently resolved data but also the lack of ecohydrological process understanding, e. g. the origin of water used by different tree species (Brinkmann et al., 2018; Ellsworth et al., 2007; Sprenger et al., 2016a; Volkmann et al., 2016b).“
L35
Thank you for your comment. We rephrased the sentences according to your suggestion.
L37
We agree with your comment and rephrased the sentences: “For the separation of water pools based on the concept that potentially each water pool has its own unique stable water isotope signature due to underlying physical or chemical fractionation processes, highly precise and / or frequent stable water isotope measurements are needed (Dubbert et al., 2019; Ehleringer and Dawson, 1992; Evaristo et al., 2015).”
L40-41
We totally agree with your comment and changed the text.
L42
We deleted the study by Wigmosta et. al. in our citation.
L42-48
We added some words to the paragraph to make our message more precise.
L49
Thank you for your suggestion! We changed information to “composition”
L50
Yes, “consecutive” was the wrong word. We changed it to “subsequent”.
L52
Yes. We changed it according to your suggestion.
L56
We changed the beginning of the sentences to: “However, it is undebatable that….”
L66-68
we rephrased the sentences to: “Furthermore, for membrane–based systems no isotope fractionation could be observed when water vapour passes through the membrane. However, it should be considered that a considerable amount of air / vapour is withdrawn from the soil or xylem media by the necessary flow rates of the isotope analyser.”
L93-94
Thanks. We added the word “water”
L127-L128
Yes maybe… However, we also wanted to start the experiment a bit earlier but ran into some troubles just before starting. Next time we would use a soil with better known isotopic composition. On the other hand, since the “old” water wasn´t completely replaced we found some interesting results regarding the plant water uptake.
L130
We changed the sentences to: “During the first labelling, we split the total applied amount of label water into two rounds of irrigation to be able to better monitor the arrival in soil water content”
We hope that the sentences is clear now.
212
We added the relevant information.
L280
Partly yes. A college of us, Simon Haberstroh, works a lot with cork oak and sap flow sensors. He told us that likely the tree would inactivate damaged sapwood tissue which somehow would be a sensor failure. He observed partly similar behaviour for his trees. However, we didn´t really find a paper which did publish data to sensor failure. Unfortunately, talking about non-working experiments is not very popular… We are also aware that the result fits partly with the general inactivity of the oak tree. However, we also measured photosynthesis and vapour conductance (not mentioned in the paper) and both values were more or less normal. Summarized, we did not really consider the cork oak for our results but instead of excluding the tree completely one can learn about possible difficulties and potential improvements in future..
Figure 3
Yes, it would be helpful. However, the second label is far above the limit of the x-axis which would cause quite some white space. We did add the number as text.
L341-344
we rephrased the sentences to: “when comparing in situ isotope data with destructive measurements, the later showed a wider spread of isotope measurements within”
L378
the sentence was removed…
L374-385
the paragraph was rephrased.
L388-392
Thank you!
L396
We followed your general comments and rephrased the beginning of the discussion.
L403
Yes, thank you for mentioning. We changed it to depths
L408
we added the sentence: “This could be caused by temporal trapped water or different flow path lengths and velocities or potential isotopic fractioning effects reduced the isotopic tracer signal”
L416
Yes. We added Barbeta et al. 2020 here.
L457
Thank you for your important comment. We are aware of the related problems of using a Mediterranean species in Germany and added the sentence “it should be mentioned that Quercus suber is a Mediterranean species and does not naturally grow in Germany.”
L478
We added: “, e. g. slower decrease in VWC after irrigation (Fig. 4)” to support the statement
L480-484
This paragraph was rephrased
L486
This paragraph was rephrased
L502-506
This paragraph was rephrased
L509
This paragraph was rephrased
L511
Thank you for your question. We did not test for significant differences here because of the limited number of datapoints. We rephrased the sentence to make this clear.: For instance, we found, in general, that δ–values for both isotopes were mostly more positive for the destructive measurements than for in situ measurements.
L531-543
This paragraph was deleted.
L549-550
We joined the sentences to another paragraph.
L551-568
We did connect the results by Millar with our study by adding the following sentences:
“However, in our experiment we found that most δ-values of cryogenic vacuum extraction were more negative than from the water vapour equilibration method (Fig. 6). Nevertheless, we are aware of our small destructive sampling size and therefore focus less on differences between both destructive measurement methods.”
L536-656
Sorry, we don´t understand your comment. Also, the line numbers might be wrong / misleading.
L568
Thank you for your valid comment and your argumentation to move this section into the method section. However, we would like to keep this section in the discussion part to get higher reader attention regarding future possible experimental set-ups. Consequently, we here want to discuss and present our thoughts why we did our experiment the way we did it.
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RC2: 'Comment on hess-2020-674', Anonymous Referee #2, 09 Mar 2021
In this paper, Mennekes et al. present an application of in-situ, high-frequency measurements of stable water isotopes in soil and tree xylem to a semi-controlled water labelling experiment involing three tree individual from three different species. From this, they derive water travel time from soil to the xylem, analyse tree water source depth, and compare this method to results brought by destructive isotope sampling and sap flow measurements. From the consistency between in-situ- and sapflow-derived travel times, and the higher robustness of isotopic signatures across in-situ measurements as compared to the variability found here (as in other studies) with destructive sampling, the authors underline the potential of this in-situ methodology to better infer water pathways and associated travel times in the soil-vegetation continuum.
The topic addressed by this study is highly relevant to the ecohydrological research community, and the paper outlines promising potential for this methodology to address the current need for higher temporal resolution in isotopic measurements to understand which water are plants using, and when. I found the paper pleasant to read, with a clear description of this very interesting experimental protocol. However, it seems to me that some more effort needs to be devoted to the text itself, in particular the discussion of the results, after which it will be suitable for publication in HESS.
Overall, I agree with the other Reviewer that the Discussion could be shortened, and some repetitions could be avoided. As several points are being discussed there, a better management of logical connections between paragraphs is generally needed as well to help the reader to grasp take-home messages.
Specific comments
- L39-41: This is quite a strong statement, all the more that there is a growing body of literature reporting fractionation -somewhere between the soil and evaporation- (e.g. Vargas et al., 2017; Barbeta et al., 2019, Poca et al., 2019). Maybe you should simply state that no fractionation is one of your working hypothesis/assumption for this study? Such an assumption could also be discussed, albeit briefly, somewhere in the discussion, as this novel data set enables looking for preferential isotope uptake along the across soil depth and xylem heights.
- Figure 5: The isotopic concentrations scale are quite narrow and make it hard to tease out the breakthrough dynamics. I'd suggest to split the figure in two panels (one isotopic, one for cumulated sap flow), arranged vertically to see the synchronicity (or lack thereof) between isotopic dynamics in soil/xylem and sap flow.
- L480: I did not understand how the author can directly derive this conclusion from Fig.4, could you expand?
- L480-481: If my understanding is correct, in case of complete replacement, i.e. if δsoil ~ δlabel, Fig. 7c would be the "transpose" of Fig. 7a, since Fig. 7c would then plot δ2Hlabel-X15 against δ18Olabel-X15
- L485-486: I am not sure to understand the logical connection leading to / coming after this sentence, maybe move it somewhere else?
- L549-550: Again, the logical flow seems to be interrupted with this two-line sentence.
- Conclusion: The conclusion mostly repeats the results and discussion sections, which makes it somewhat redundant, in my view. Considering merging it with the revised Discussion. The Conclusion could be the place for a high-level perspective on this study and its implications, for example using a slightly more compact version of what is currently the "Future implications" subsection of the Discussion.
Technical comments
- L165: Custom-made?
- L267: It seems that Fig. 4 is described before Fig. 3 in the main text, maybe switch the order of the two?
- L481-484: I would suggest rephrasing for clarity, for example: "Furthermore, Figs. 5, 7b and 7d all suggest that the tracer arrival in X150 was less pronounced than in X15 [...]. This attenuation was stronger for Pinus than for Quercus, while for Alnus no sufficient data were available."
References
- Barbeta, A., Jones, S. P., Clavé, L., Wingate, L., Gimeno, T. E., Fréjaville, B., Wohl, S., and Ogée, J.: Unexplained hydrogen isotope offsets complicate the identification and quantification of tree water sources in a riparian forest, Hydrol. Earth Syst. Sci., 23, 2129?2146, https://doi.org/10.5194/hess-23-2129-2019, 2019.
- Poca, M., Coomans, O., Urcelay, C. et al. Isotope fractionation during root water uptake by Acacia caven is enhanced by arbuscular mycorrhizas. Plant Soil 441, 485?497, https://doi.org/10.1007/s11104-019-04139-1, 2019.
- Vargas, A. I., Schaffer, B., Yuhong, L., and Sternberg, L. da S. L.: Testing plant use of mobile vs immobile soil water sources using stable isotope experiments, New Phytol., 215, 582?594, https://doi.org/10.1111/nph.14616, 2017.
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AC2: 'Reply on RC2', David Mennekes, 16 Apr 2021
We thank referee #2 for the valuable comments, which helped us to improve the manuscript. Please find the answers to each comment below.
(comments by RC are bold, answers are normal font)
General comments
We generally shortened the discussion and added more logical connections to our results. Overall we also improved the understanding to find some clear take home messages, especially regarding the results section.
Specific comments
L39-41
Thank you for your comment. We changed this part to better highlight the recently discovered fractionation processes.
Furthermore, in our discussion we talk about possible effects of fractionation occurring in our experiment when we discuss Fig. 7 in section “Differences between tree species”.
Figure 5
Thank you for your suggestion. We agree that the way we chose the y2-axis makes it difficult to see the dynamic of the isotope data (because dynamics is damped). To improve this, we changed (i.e., stretched) the y2-axis to better show the dynamics of the isotope data. However, we think that plotting isotope data and cumulated sap flow data into the same figure allows a better comparison of the timing of isotope break through and the sap flow based ecohydrological travel time (i.e., the time it took sap to travel from the roots to 15 cm and 150 cm tree hight, respectively). This is what we try to emphasize with the brown, green and gray horizontal bar.
L480,
Indeed, this was confusing. We rephrased this section and we think it should be clearer now.
L485-486
We change the paragraph regarding Fig. 7. We hope it will be clearer now.
L485-486
As mentioned above, we changed the discussion part. We hope it is understandable now.
L549-550
We removed the two-line paragraphs.
Conclusion
Thank you for your idee. We appreciated it and transformed the conclusion to a section called “conclusion and future implications”. Consequently, we also hope that we avoid too many redundant information and everything becomes shorter and more precise.
Technical comments
L165
Thanks a lot for pointing out this typo! We changed it to custom-made
L267
Thank you for this technical comment. We are aware of this.
In the revised version, we would like to have the two graphs on one page if possible. In this case we wanted to have the more interesting result on top. If the final edited version won´t be like this, we will change the order.
L481-484
We rephrased this section and we think it should be clearer now. See comments above and comments, too.
David Mennekes et al.
David Mennekes et al.
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