the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 09 Feb 2023
Does back-flow of leaf water introduce a discrepancy in plant water source tracing through stable isotopes?
Abstract. Plant water source tracing studies often rely on differences in stable isotope composition of different water sources. However, an increasing number of studies has indicated a discrepancy between the isotopic signature of plant xylem water and the water sources assumed to be used by plants. Based on a meta-analysis we have reconfirmed this discrepancy between plant xylem water and groundwater and suggest back-flow of leaf water (BFLW), defined as a combination of (i) the Péclet effect, (ii) foliar water uptake (FWU) and (iii) hydraulic redistribution of leaf water, as a possible explanation for these observations. Using the average 2.21 ‰ 18O enrichment of xylem water compared to groundwater in our meta-analysis, we modelled the potential of BFLW to result in this observed isotopic discrepancy. With a low flow velocity of 0.052 m.h−1 and an effective path length of 2 m, the Péclet effect alone was able to account for the average offset between xylem water and groundwater. When including a realistic fraction of 5–10 % xylem water originating from FWU and tissue dehydration, 60–100 % of the average observed enrichment can be explained. By combining the Péclet effect with FWU and tissue dehydration, some of the more extreme offsets in our meta-analysis can be elucidated. These large effects are more probable during dry conditions when drought stress lowers transpiration rates, leading to a larger Péclet effect, more tissue dehydration, and a potential greater contribution of FWU.
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CC1: 'Comment on hess-2023-13', Carel Windt, 15 Feb 2023
Dear Jeroen,
Interesting manuscript! One comment: I'm surprised that phloem flow is not mentioned as a contributor to the back flow of water from leaves. Can it be neglected in this context, in your opinion? In older work, measuring water fluxes in the main stem of plants by means of MRI, I found that a significant fraction of xylem sap returns to the roots by means of the phloem (up to ~10% during the day, up to ~50% at night. Windt et al, PCE, 2006). This was measured in the main stem. This means, water used for expansion growth of shoots and young leaves did not even show up in this number (in the back flow to the roots). I therefore would expect, for individual leaves, the potential fraction of phloem backflow to be even larger than measured in the main stem.
Best, Carel
Citation: https://doi.org/10.5194/hess-2023-13-CC1 -
AC1: 'Reply on CC1', Jeroen Schreel, 16 Feb 2023
Dear Carel,
Thank you for this nice comment! Prior to publishing this preprint, we discussed the option of including phloem flow in our manuscript and had chosen to exclude this as we are considering the bias introduced in water source tracing which solely focusses on xylem water. In the current version of our manuscript, we have alluded to xylem-phloem water exchange to not leave this topic completely untouched (Line 34-35, 178, 224-225). However, based on your comment it does seem beneficial to include some more information on backflow through the phloem and we will do so in our revised manuscript.
Thanks again!Kind regards,
JeroenCitation: https://doi.org/10.5194/hess-2023-13-AC1 -
CC2: 'Reply on AC1', Carel Windt, 17 Feb 2023
Great. Thank you for the reply and best of luck with the manuscript,
Carel
Citation: https://doi.org/10.5194/hess-2023-13-CC2
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CC2: 'Reply on AC1', Carel Windt, 17 Feb 2023
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AC1: 'Reply on CC1', Jeroen Schreel, 16 Feb 2023
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RC1: 'Comment on hess-2023-13', Anonymous Referee #1, 26 May 2023
This manuscript describes numerical experiments to test a series of hypotheses about whether water exchanges at leaves may be responsible for some observed isotopic phenomena in stem xylem that are difficult to explain otherwise. The approach is to compare modeled implications of some idealized physiological conditions to a generalized empirical offset between observed and expected isotopic composition in stem xylem. This kind of investigation is of high interest in the active area of isotope ecohydrology because the lack of understanding of in-plant isotopic transformations is a major impediment to using isotopic tracers to identify plant-water relationships. However, the modeling presented here is not thorough and contains errors, with the net result that it does not reliably support conclusions. Rather than being an in-depth consideration of model results, the discussion is dominated by hypotheses that existed prior to this manuscript. For example, section 3.3, while a part of the Results and Discussion, is in essence a literature review that logically should be presented before the numerical experiments because it is completely independent of the modeling.
Major comment 1:
The choice to ignore 2H because the empirical difference between soil water and xylem water is near zero is problematic. All the modeling presented here could be done on 2H as well as 18O, and we should expect useful models to reproduce both the offsets of both 18O (~2‰) and 2H (~0‰) because neither is more valid than the other. The manuscript also does not sufficiently establish the shifts in isotopic composition (see detailed comment L36, L158).
Major comment 2:
The modeling mostly omits consideration of water volumes and assumes constant boundary conditions both upstream and downstream without justification. The dual requirements—that (1) the backflowing water to be isotopically enriched, but also that (2) there be enough water to replace water already in the stem—are in fundamental tension with each other. Ignoring this conflict is a serious omission. The one portion of the modeling that attempts mass balance makes assumptions that violate the mathematical formulation being applied—see detailed comment L137.
Major comment 3:
The modeling draws heavily from the leaf-water isotope literature and extends that theory into stem xylem mostly without justification or citation support.
Detailed comments
L14, L134 it is probably better to call it soil water instead of groundwater.
L36 I think the paragraph describing the origin of disagreement between isotopic compositions of source water vs. vegetation is too dismissive of methodological artifacts. There is currently no agreed-upon. Also, this paragraph (actually the entire manuscript) does not mention that xylem water in branches is typically more enriched in heavy isotopes than is water in lower stems. There is so much potential information in this latter phenomenon that to ignore it is puzzling.
L114-117 duplicates material from earlier in the introduction
L120 Eq 3 (given by Farquhar and Lloyd 1993 as Eq 25) is indeed the bulk mean d18O of all water within effective pathlength L in a leaf. However, in Fig 2 the bulk mean is not leaf water, it is leaf-plus-stem bulk mean. I find this confusing and wonder why not plot the point value of d18O of the enrichment point as given by F&L93 as their equation 24. That value is more intuitive and corresponds to what a wood sample would extract when extracted from point L—the analysis here confuses bulk leaf isotopic composition with composition of wood xylem water at a point. Bulk leaf d18O makes more sense when leaves are the sample, but how do you even get a “mean” sample of a leaf-to-stem combination? Perhaps more importantly, does the theory hold over the leaf-stem transition? Please see also comment L129.
L125 there are two definitions of D in this sentence; the stray needs removing.
L129 the Cernusak paper is about leaf water, not stem water, and there are important differences in the paths water takes through the two. Farquhar and Lloyd (1993) and Song et al. (2013; both cited in this manuscript) discuss the non-triviality in estimating tortuosity. What is the consequence, then, of using leaf theory to describe effective diffusivity through leaves and along stems, given that flowpaths include various membranes, pits, embolisms, etc.? Much more support is needed to justify the simplicity of the diffusion model because trees are not long, isothermal straws with no surface chemistry. Finally, all the theory is for steady state, but diffusion happens when stomata are closed, also. Nowhere are these choices justified, and the assumptions are not acknowledged.
L130 subsequent discussion does not emphasize that this is effective path length. Likewise, there is no effort in the manuscript to account for tortuosity and thus estimate linear path length along a leaf/stem: e.g., L214 seems to imply linear stem distances, neglecting tortuosity.
L137 by including the Peclet number, Eq 3 and 6 already have both diffusion and convection in them. The derivation of Eqs 7-10 is nonsensical because it attempts to describe diffusion as mixing. Eq 5 describes the diffusion of H218O into upstream xylem water, not the diffusion of evaporation-site water into upstream xylem water. The velocity-dependent distance profile in 18O upstream from the evaporation point is irreducibly diffusive and cannot be described by end-member mixing. If I understand it correctly, Fig 2b is still valid if it is relabeled “proportion similarity” and the mass-balance fallacy removed.
L150 it would be better to explain the proportional difference (Song) technique before asserting anything about it.
L158 establishing the -2.2/0 shift is crucial for this work, but substantially more clarity is needed in describing how the database was constructed and the logic of the isotopic comparisons. At least some of the cited papers do not report paired soil/groundwater-xylem isotopic composition in terms of single isotopes but rather in 2-isotope space, so it is not clear how the data for this paper were compiled.
Sec 2.3 and Eq 11 and 12 ignore diffusion. What is the justification for this?
L175-180 repeats the introduction and is not needed.
L183-202 repeats the introduction and is not needed.
L203 this sap velocity drops out of the sky in the results with no context or justification. I don’t understand why a single Peclet number is being investigated; field data surely contain a wide range.
L214 … and when diffusivity is assumed independent of morphological resistance and temperature is high
L215 L255 which smallest enrichment?
L220 in diffusion, “water” does not travel—molecules do, and “water enriched in 18O” does not travel; this statement about time to travel a certain distance is nonsensical. In the model being used here, d18O of xylem water approaches the d18O of soil water asymptotically over distance. Does the 27.7 year estimate originate in the diffusion coefficient, which is about 13.9 m2/y (27.78 m2/2y) as parameterized in Eq 5? The origin of this time estimate is not specified.
L222 “heavy water accumulating upstream” is a fundamental misrepresentation of the equations being employed. Eq 3 describes a steady-state condition, and 18O concentrations cannot change with time anywhere in the system.
L224 what is HR? What is normal cycling?
Fig 3 the caption and the axis labels seem to conflict. According to the axis labels, the plot is isotopic composition of alternative water source as a function of isotopic composition of xylem water, not as a function of difference as the caption says. When the two are equal, the proportion of alternative water is 1. I do not understand how the offset of 2.2 matters in this figure.
Sec 3.2 / Fig 4 this section on leaf surface water is difficult to defend because there are no citations to support the many “just so” stories. There are many instances of incomplete isotope physics. For example, evaporation at less than 100% humidity certainly does enrich the remaining water in 1H2H16O despite Fig 4 implying humidity must be 100% for that to happen. For another example, dew is more isotopically enriched than rainfall but is typically not more enriched than water at the point of transpiration, so that studies of the effect of dew on leaf isotopes usually find dew results in lighter leaf water, not heavier. Dew might be heavier than soil water, but it seems to me unlikely that it would be 20-42‰ heavier, as the explanation requires. Most importantly, all processes mentioned in this section require that there be enough water on the leaves, and that it be there long enough to dilute the stem xylem.
Citation: https://doi.org/10.5194/hess-2023-13-RC1 -
AC2: 'Reply on RC1', Jeroen Schreel, 28 Jun 2023
This manuscript describes numerical experiments to test a series of hypotheses about whether water exchanges at leaves may be responsible for some observed isotopic phenomena in stem xylem that are difficult to explain otherwise. The approach is to compare modeled implications of some idealized physiological conditions to a generalized empirical offset between observed and expected isotopic composition in stem xylem. This kind of investigation is of high interest in the active area of isotope ecohydrology because the lack of understanding of in-plant isotopic transformations is a major impediment to using isotopic tracers to identify plant-water relationships. However, the modeling presented here is not thorough and contains errors, with the net result that it does not reliably support conclusions. Rather than being an in-depth consideration of model results, the discussion is dominated by hypotheses that existed prior to this manuscript. For example, section 3.3, while a part of the Results and Discussion, is in essence a literature review that logically should be presented before the numerical experiments because it is completely independent of the modeling.
Thank you for your interest in our manuscript (MS). We hope that our responses clarify the comments listed below.
Major comment 1:
The choice to ignore 2H because the empirical difference between soil water and xylem water is near zero is problematic. All the modeling presented here could be done on 2H as well as 18O, and we should expect useful models to reproduce both the offsets of both 18O (~2‰) and 2H (~0‰) because neither is more valid than the other. The manuscript also does not sufficiently establish the shifts in isotopic composition (see detailed comment L36, L158).
We understand this concern, but we do not ignore ²H in this MS. Our meta-analysis indicated a shift in 18O, which we did not observe in ²H. In this particular case, water in the xylem is more enriched in 18O and not in 2H compared to groundwater, i.e., the ratio of ²H to 18O is changing. We hypothesize that this could have occurred due to back-flow of leaf water (BFLW) as water transported by BFLW is relatively more enriched in 18O. In other words, this hypothesis relies on the idea that leaf water is more enriched in 18O and not in ²H, an idea that is further pointed out in Fig. 4 and Line 230-253.
Major comment 2:
The modeling mostly omits consideration of water volumes and assumes constant boundary conditions both upstream and downstream without justification. The dual requirements—that (1) the backflowing water to be isotopically enriched, but also that (2) there be enough water to replace water already in the stem—are in fundamental tension with each other. Ignoring this conflict is a serious omission. The one portion of the modeling that attempts mass balance makes assumptions that violate the mathematical formulation being applied—see detailed comment L137.
In our MS, volumes are included. That is one of the major points we are trying to make. These volumes are included throughout the text, and are the reason why we included Eq. 7 to 10 and panel B in Fig. 2, which illustrates the link between change in isotopic composition and volume coming from leaf water (XL). This is also the basis for Fig. 3, which shows how much water should come from an alternative water source (i.e., volume; XA) to result in the observed enrichment.
Major comment 3:
The modeling draws heavily from the leaf-water isotope literature and extends that theory into stem xylem mostly without justification or citation support.
To support this transfer, we include other scientific literature in which this same extension has been suggested (Line 117-118).
Detailed comments
L14, L134 it is probably better to call it soil water instead of groundwater.
We prefer to keep using groundwater instead of soil water, because the remainder of the MS is based on groundwater data.
L36 I think the paragraph describing the origin of disagreement between isotopic compositions of source water vs. vegetation is too dismissive of methodological artifacts. There is currently no agreed-upon. Also, this paragraph (actually the entire manuscript) does not mention that xylem water in branches is typically more enriched in heavy isotopes than is water in lower stems. There is so much potential information in this latter phenomenon that to ignore it is puzzling.
We do not intend to be dismissive of methodological artifacts and clearly mention this possibility in the MS (Line 41-43).
We have briefly mentioned the enrichment of branch water (Line 56-57, 114-115, 193-195), but given your comment and the one of the other reviewer we agree that we should add more information on this topic and will do so in our revised MS and want to thank you for pointing this out.
L114-117 duplicates material from earlier in the introduction
We were merely trying to guide the reader, but given this concern, we will shorten the paragraph.
L120 Eq 3 (given by Farquhar and Lloyd 1993 as Eq 25) is indeed the bulk mean d18O of all water within effective pathlength L in a leaf. However, in Fig 2 the bulk mean is not leaf water, it is leaf-plus-stem bulk mean. I find this confusing and wonder why not plot the point value of d18O of the enrichment point as given by F&L93 as their equation 24. That value is more intuitive and corresponds to what a wood sample would extract when extracted from point L—the analysis here confuses bulk leaf isotopic composition with composition of wood xylem water at a point. Bulk leaf d18O makes more sense when leaves are the sample, but how do you even get a “mean” sample of a leaf-to-stem combination? Perhaps more importantly, does the theory hold over the leaf-stem transition? Please see also comment L129.
We agree that using Eq. 24 rather than 25 in Farquhar and Lloyd (1993) is more appropriate. We will adapt our equations and calculations accordingly. This rearrangement changes our values somewhat, but not the main premise, nor the main outcome of our discussion:
Flow is reduced to 6.6 10-6 m h-1 (equaling approximately zero flow, something that can be obtained at night or during drought). With an L of 4 m, this results in an of 54 ‰ and an XL of 4 %. With an L of 2 m this results in an of 11 ‰ and an XL of 20 % (outcome of previous simulations can be found at Line 202-211).
We do believe that this theory holds over the leaf-stem transition, something that is also mentioned by Farquhar and Lloyd (1993): “Due to lower transpiration rates will be smaller in stressed plants than in unstressed ones and may explain the enrichment in stem water observed by Flanagan et al. (1991).”
L125 there are two definitions of D in this sentence; the stray needs removing.
Our apologies for this possible confusion. There is only one definition of D used in our MS. We have calculated D based on Eq. 5 and mentioned the value of this calculation on line 125.We will make this more clear in our revised MS.
L129 the Cernusak paper is about leaf water, not stem water, and there are important differences in the paths water takes through the two. Farquhar and Lloyd (1993) and Song et al. (2013; both cited in this manuscript) discuss the non-triviality in estimating tortuosity. What is the consequence, then, of using leaf theory to describe effective diffusivity through leaves and along stems, given that flowpaths include various membranes, pits, embolisms, etc.? Much more support is needed to justify the simplicity of the diffusion model because trees are not long, isothermal straws with no surface chemistry. Finally, all the theory is for steady state, but diffusion happens when stomata are closed, also. Nowhere are these choices justified, and the assumptions are not acknowledged.
In Line 127-130, the work of Cernusak et al. (2016) is used to support our definitions of the Péclet number and effective path length, which is justified in this way.
Estimating tortuosity is indeed non-trivial, which is why we state that tortuosity is included in the path length L (Line 131) and not disentangled from the distance between point of measurement and evaporation site. We have also compared values with an effective path length of 4 and 2 m to give the reader some idea of the effect of an increase in path length. Lastly, we want to add that models are simplifications of reality and we believe that the assumptions in our model can be seen as valid in the used context. We will make the assumptions of steady vs non-steady state more explicit in our revised MS.
L130 subsequent discussion does not emphasize that this is effective path length. Likewise, there is no effort in the manuscript to account for tortuosity and thus estimate linear path length along a leaf/stem: e.g., L214 seems to imply linear stem distances, neglecting tortuosity.
L is defined as effective path length on Line 124, and 128-131. We have redefined L as effective path length on Line 203-204, something we did to avoid this confusion. The other reviewer mentioned that we shouldn’t redefine our variables. As such, we believe that L is sufficiently clear.
L137 by including the Peclet number, Eq 3 and 6 already have both diffusion and convection in them. The derivation of Eqs 7-10 is nonsensical because it attempts to describe diffusion as mixing. Eq 5 describes the diffusion of H218O into upstream xylem water, not the diffusion of evaporation-site water into upstream xylem water. The velocity-dependent distance profile in 18O upstream from the evaporation point is irreducibly diffusive and cannot be described by end-member mixing. If I understand it correctly, Fig 2b is still valid if it is relabeled “proportion similarity” and the mass-balance fallacy removed.
When a fluid diffuses, it is mixed with the fluid it diffuses into, so the isotopic signature measured will be a mix of both fluids, which validates the use of these equations.
Eq. 5 does not describe diffusion but the diffusivity (rate of diffusion) of H218O in water. Subsequently, diffusivity is used to calculate the Péclet effect (Eq. 4; Line 123): the diffusion of evaporatively enriched leaf water in the opposite direction of the bulk water flow (Line 114).
Based on these considerations, the concept of Fig. 2 is valid as it is represented, but the values will change, following the previous comment about Eq. 24 and 25 in Farquhar and Lloyd (1993).
L150 it would be better to explain the proportional difference (Song) technique before asserting anything about it.
Based on the changed equations (Eq. 24 vs 25 in Farquhar and Lloyd (1993)), this section does not longer hold and will be removed from our revised MS.
L158 establishing the -2.2/0 shift is crucial for this work, but substantially more clarity is needed in describing how the database was constructed and the logic of the isotopic comparisons. At least some of the cited papers do not report paired soil/groundwater-xylem isotopic composition in terms of single isotopes but rather in 2-isotope space, so it is not clear how the data for this paper were compiled.
A shift of 2.2118O is used as the average observed enrichment based on literature. Our models allow for a an adjustment of this value to calculate specific cases. This average value is based on a meta-analysis using a keyword-based search of published data (e.g., ecohydrological separation; Line 103-106). If data were provided in a biplot, individual datapoints were extracted using an online plot digitizer. Following this extraction, the mean and standard error of 18O and 2H were calculated and added to the dataset. We will add this additional information to our revised MS.
Sec 2.3 and Eq 11 and 12 ignore diffusion. What is the justification for this?
Section 2.3 discusses hydraulic redistribution, the redistribution of water based on a difference in water potential, and does not include diffusion.
L175-180 repeats the introduction and is not needed.
We aimed at discussing our results and putting them in the larger framework of published literature. We will check these lines for redundancy.
L183-202 repeats the introduction and is not needed.
This part aimed at making the text more fluent, but we will check for redundancy.
L203 this sap velocity drops out of the sky in the results with no context or justification. I don’t understand why a single Peclet number is being investigated; field data surely contain a wide range.
Flow velocities are mentioned throughout the text (Line 54-56, 67-68, 90, 103,…). The flow velocity mentioned is meant as an example to provide the reader with some additional insights: what are some of the values obtained by the model and how biologically relevant are they? What happens when we change L from 4 to 2 m during these same flow velocities? Instead of discussing a single Péclet number, we have calculated a range of Péclet numbers, resulting in Fig. 2. To give the readers some reference of realistic values, we discuss two of those combinations of values with an effective path length of 4 and 2 m. We will make this more clear in our revised MS.
L214 … and when diffusivity is assumed independent of morphological resistance and temperature is high
Thank you for this addition. We will add this to our revised MS. In our model we assume a leaf temperature of 25°C (Line 125).
L215 L255 which smallest enrichment?
This refers to the smallest observed enrichment in our meta-analysis. We will add this to our revised MS.
L220 in diffusion, “water” does not travel—molecules do, and “water enriched in 18O” does not travel; this statement about time to travel a certain distance is nonsensical. In the model being used here, d18O of xylem water approaches the d18O of soil water asymptotically over distance. Does the 27.7 year estimate originate in the diffusion coefficient, which is about 13.9 m2/y (27.78 m2/2y) as parameterized in Eq 5? The origin of this time estimate is not specified.
Thank you for pointing this out. We will adapt our phrasing from ‘water’ to ‘water molecules’ as that is indeed what we meant.
Also, thank you for pointing out we forgot to include our calculations. 27.7 years is based on the equation of diffusion time: t L2/(2D) with t the time, L the distance (2 m) and D the diffusion coefficient calculated in Eq. 5.
L222 “heavy water accumulating upstream” is a fundamental misrepresentation of the equations being employed. Eq 3 describes a steady-state condition, and 18O concentrations cannot change with time anywhere in the system.
In this case, we are not referring to a steady-state because a steady-state can be valid for a short time period, but will not hold for a longer time period like 27.7 years. We will adjust our phrasing accordingly to make this distinction more clear in the revised MS.
L224 what is HR? What is normal cycling?
HR refers to hydraulic redistribution. This clarification has probably been removed during one of our revisions, and we sincerely apologies. We will add it to the revised MS.
With ‘normal cycling’ we refer to traditional sap flow. We will clarify this part and add some more information on this to our revised MS based on the community comment by Carel Windt.
Fig 3 the caption and the axis labels seem to conflict. According to the axis labels, the plot is isotopic composition of alternative water source as a function of isotopic composition of xylem water, not as a function of difference as the caption says. When the two are equal, the proportion of alternative water is 1. I do not understand how the offset of 2.2 matters in this figure.
The axis labels in Fig. 3 were checked and are correct. We understand that the caption might be confusing and will therefore adapt it to reflect the following:
Fig. 3 illustrates possible combinations of the isotopic composition and volume of an alternative water source to result in the observed offset of 2.21. In other words, if the offset would be smaller, the needed volume of alternative water and/or its required isotopic enrichment would decrease resulting in a different gradient in Fig. 3.
Sec 3.2 / Fig 4 this section on leaf surface water is difficult to defend because there are no citations to support the many “just so” stories. There are many instances of incomplete isotope physics. For example, evaporation at less than 100% humidity certainly does enrich the remaining water in 1H2H16O despite Fig 4 implying humidity must be 100% for that to happen. For another example, dew is more isotopically enriched than rainfall but is typically not more enriched than water at the point of transpiration, so that studies of the effect of dew on leaf isotopes usually find dew results in lighter leaf water, not heavier. Dew might be heavier than soil water, but it seems to me unlikely that it would be 20-42‰ heavier, as the explanation requires. Most importantly, all processes mentioned in this section require that there be enough water on the leaves, and that it be there long enough to dilute the stem xylem.
To take away the “just so” stories concern, we are happy to add additional references to the revised MS, such as:
Berry et al., 2017. The two water worlds hypothesis: addressing multiple working hypothesis and proposing a way forward
Gat, 1996. Oxygen and hydrogen isotopes in the hydrological cycle.
We do not suggest that all 1H216O and 1H2H16O evaporate when RH < 100%. However, based on these references it becomes clear that enrichment is stronger in terms of 1H218O during conditions with RH < 100. In other words, we do not rule out enrichment of the remaining water in 1H2H16O, but enrichment in 1H218O should be more pronounced.
As suggested, dew might be heavier than soil water and we suggest that lighter isotopologues will continue to evaporate at a higher rate compared to heavy isotopologues (Line 246-249). This enriched water layer can subsequently be absorbed by leaves and redistributed to the xylem. Furthermore, this is only part of larger concept of BFLW which includes the Péclet effect, hydraulic redistribution and FWU. As such, dew water does not need to explain the occurrence of enriched water on its own and does not need to be 20-42 ‰ heavier.
Thank you for your detailed review of our work.
Yours sincerely,
Jeroen Schreel and co-authors.
Citation: https://doi.org/10.5194/hess-2023-13-AC2
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AC2: 'Reply on RC1', Jeroen Schreel, 28 Jun 2023
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RC2: 'Comment on hess-2023-13', Anonymous Referee #2, 31 May 2023
Schreel et al. test whether the back-flow of enriched leaf water can explain the observed discrepancies between the d18O values of the source water and the d18O values of the xylem water. Back-flow of leaf water is defined as a combined effect of the Péclet effect, water uptake by leaves, and hydraulic redistribution of leaf water.
The topic is very interesting and fits within the scope of HESS. One major constrain is that the model was not tested for 2H. Numerous studies have also shown discrepancies between source and xylem water for 2H, including, for instance, the Chen et al. (2020) study cited here.
Further comments:
- Problematically, the entire study appears to be based on the assumption that ground water = source water, which is not true for many ecosystems and species. Therefore, much of the observed isotopic differences are also likely due to the unknown "true" water source. This point needs to be adequately discussed.
- The statistical analyses related to the d18O discrepancy and Fig S1 and S2 need to be explained and justified in mor detail. The whole study is based on this value of “2.21 per mille”.
- An enrichment in branch water relative to the stem xylem water is well known. See e.g. Martín‐Gómez et al., 2017, Vega‐Grau et al., 2021. This effect was not discussed at all.
- Vega‐Grau, A.M., McDonnell, J., Schmidt, S., Annandale, M. & Herbohn, J. (2021) Isotopic fractionation from deep roots to tall shoots: a forensic analysis of xylem water isotope composition in mature tropical savanna trees. Science of the Total Environment, 795, 148675. Available from: https://doi.org/10.1016/j.scitotenv.2021.148675
- Martín‐Gómez, P., Serrano, L. & Ferrio, J.P. (2017) Short‐term dynamics of evaporative enrichment of xylem water in woody stems: implications for ecohydrology. Tree Physiology, 37, 511–522. Available from: https://doi.org/10.1093/treephys/tpw115
- In this context, it would be good to point out already in the introduction that many studies sample branches (i.e., closer to the leaf water) for xylem water and not the stem, which is particularly relevant for your idea. Also this information (what was sampled?) would be important to know for the data you use (see comment on l109)
- Also, this publication may be relevant: https://nph.onlinelibrary.wiley.com/doi/full/10.1111/nph.18113
- I am wondering how large the amount of leaf water is relative to the amount of stem water. It would be great to have some estimates for the ratio of these pools from literature. What is a realistic estimate of the leaf water for the whole plant water pool or relative to the stem xylem water? You only mentioned an estimate for foliar water uptake.
- The enrichment also depends on the leaf water turn overtime, which is controlled by the stomatal conductance and the leaf water content. How does leaf water content influence the isotopic differences between xylem and ground water? This point would be interesting to discuss, in particular in 3.3.
- Section 3 (Results and Discussion) does not relate the results of the study well to those found in literature.
- The steady- vs. non-steady state effect should be shortly discussed. The whole approach is based on steady-state conditions which do not always apply.
Line-by-line comments:
l15: remove dot between m h-1 (check entire text)
l37: introduce d2H and d18O
l39: Brooks et al. did not sample ground water. Also, it would be worth to state something like: “While d18O values of xylem water were relatively enriched to stream water d18O values, d2H values of xylem water were relatively depleted”.
l63: I would consider writing the full term here again, and introduce FWU here
l71: start new sentence starting from Kagawa.
l77: add also the “timing of leaf wetting event”
l82: how about d2H? there is numerous studies that also point out differences between xylem and source water d2H.
l91: phrasing
l94: “The isotopic compositions of...” is or more precise: “The isotopic compositions of ... are...”
l102: phrasing
l109: so you only included studies where ground water data were available? Or did you also include studies where ground water was not sampled? Also, it would be important to report how the studies sampled xylem water and what techniques they used for water extraction. In addition, not every plant has access to ground water… A table summarizing this information (plant material, extraction technique etc.) and also indicating the geographical region would be good.
l116: water isotopic composition; isotopic composition of water contained in...
l117: effect on branch and stem water
l118: isotopic steady state of what?
l118: plants do not only rely on ground water as source water (if at all)
l119: it would be worth mentioning that this equation actually applies to leaf water and you transfer it to the xylem water level
l126: consider starting a new sentence; D is defined.... and give only one value
l128: L was already introduced, should be also used see line 130
l142: better "ground- and leaf water"
l149: XS? you mean XG?
l150: this part needs more explanation
l158-159: the model should also be tested for d2H
l163: represent
l184: … xylem water is generally more enriched (also the values are enriched, not the water itself, check entire document for this formulation)
l202: “– the average… –” (missing space, comes later again)
l203: delete dot between m h-1
l203-204: why do you introduce the abbreviations again; also already in line 188
l205: estimates from literature available for XL of different plants?
l209: phrasing
l2010: “for instance, when reducing”
l224: HR was not introduced
l224: phrasing, “normal water cycling”
l219-227: please use references for this paragraph
l265: this sentence seems out of place
l280: be more precise here, how does it affect V and L
L262-283: It would be good to first start with your results and then refer it to the drought aspect.
l308-3011: I would not end your conclusion with this section.
Please check the text for consistent use of the terms ratio, composition and signature.
Figures
Figure 1: blue and green lines are not dashed. explain boxplots. Please add number of studies here (n = x).
Figure 2: no capital letter in “Isotopic”
Figure 3: as a function of d18OX or d18OXG?
Figure 4: add explanation of MWL to legend.
Figure S1 and S2: Please add number of studies here (n = x).
Citation: https://doi.org/10.5194/hess-2023-13-RC2 -
AC3: 'Reply on RC2', Jeroen Schreel, 28 Jun 2023
Schreel et al. test whether the back-flow of enriched leaf water can explain the observed discrepancies between the d18O values of the source water and the d18O values of the xylem water. Back-flow of leaf water is defined as a combined effect of the Péclet effect, water uptake by leaves, and hydraulic redistribution of leaf water.
The topic is very interesting and fits within the scope of HESS. One major constrain is that the model was not tested for 2H. Numerous studies have also shown discrepancies between source and xylem water for 2H, including, for instance, the Chen et al. (2020) study cited here.
We understand this concern, which was also made by the other reviewer. We will make this point more clear in the revised manuscript (MS). In this particular case, water in the xylem is more enriched in 18O (Fig. S1) and not in 2H (Fig. S2) compared to groundwater, i.e., the ratio of ²H to 18O is changing. We hypothesize that this change in the ratio of ²H to 18O could have occurred due to back-flow of leaf water (BFLW) as water transported by BFLW is relatively more enriched in 18O, an idea that is further pointed out in Fig. 4 and Line 230-253. Based on our equations we were able to model the enrichment and volume of an alternative water source to explain this observation. We briefly discussed these results in context of ²H discrepancies (Line 284-290).
Further comments:
- Problematically, the entire study appears to be based on the assumption that ground water = source water, which is not true for many ecosystems and species. Therefore, much of the observed isotopic differences are also likely due to the unknown "true" water source. This point needs to be adequately discussed.
We agree that other water sources such as soil water will be at play. Therefore we have used the generic term ‘alternative water source’ in section 3.2 and Fig. 3 (Line 231-232). However, the contribution of groundwater to a plant’s water budget does increase during dry conditions (Line100-101), which is why we have focused section 3.3 on the fact that the increased use of groundwater coincides with more favorable conditions for BFLW (Line 262-297). This will be better explained in the revised MS.
- The statistical analyses related to the d18O discrepancy and Fig S1 and S2 need to be explained and justified in more detail. The whole study is based on this value of “2.21 per mille”.
The statistical analysis is based on a weighted linear regression (Line 109-112). Inspired by your comment and the comment of the other reviewer we will add some more explanation to our meta-analysis. A shift in 18O of 2.21 is used as the average observed enrichment based on literature, however, our models allow for an adjustment of this value to calculate specific cases.
- An enrichment in branch water relative to the stem xylem water is well known. See e.g. Martín‐Gómez et al., 2017, Vega‐Grau et al., 2021. This effect was not discussed at all.
- Vega‐Grau, A.M., McDonnell, J., Schmidt, S., Annandale, M. & Herbohn, J. (2021) Isotopic fractionation from deep roots to tall shoots: a forensic analysis of xylem water isotope composition in mature tropical savanna trees. Science of the Total Environment, 795, 148675. Available from: https://doi.org/10.1016/j.scitotenv.2021.148675
- Martín‐Gómez, P., Serrano, L. & Ferrio, J.P. (2017) Short‐term dynamics of evaporative enrichment of xylem water in woody stems: implications for ecohydrology. Tree Physiology, 37, 511–522. Available from: https://doi.org/10.1093/treephys/tpw115
We agree that our brief mentioning of enrichment of branch water (Line 56-57, 114-115, 193-195) is too limited, and we will add more information on this topic in our revised MS. Thank you for the suggestion of these interesting references.
- In this context, it would be good to point out already in the introduction that many studies sample branches (i.e., closer to the leaf water) for xylem water and not the stem, which is particularly relevant for your idea. Also this information (what was sampled?) would be important to know for the data you use (see comment on l109)
Thank you for this suggestion. We will include this information in the introduction of our revised MS.
- Also, this publication may be relevant: https://nph.onlinelibrary.wiley.com/doi/full/10.1111/nph.18113
Thank you for the suggestion of this relevant reference. We will include this in our revised MS.
- I am wondering how large the amount of leaf water is relative to the amount of stem water. It would be great to have some estimates for the ratio of these pools from literature. What is a realistic estimate of the leaf water for the whole plant water pool or relative to the stem xylem water? You only mentioned an estimate for foliar water uptake.
We agree that this would be very useful. However, the availability of this data is very limited and will vary significantly between different species, development stages, seasons and even on a daily scale.
- The enrichment also depends on the leaf water turn overtime, which is controlled by the stomatal conductance and the leaf water content. How does leaf water content influence the isotopic differences between xylem and ground water? This point would be interesting to discuss, in particular in 3.3.
This is a very interesting point. A change in leaf water content is linked to a non-steady state. When leaf water content decreases due to transpiration, the remaining leaf water is more enriched compared to initial conditions as lighter isotopologues preferably evaporate (Fig. 4). When leaf water content increases due to water supply by the xylem, the remaining leaf water will be less enriched compared to initial conditions due to mixing of enriched leaf water with xylem water. However, some of the enriched water molecules in the leaf will move towards the xylem, e.g., due to the Péclet effect. As most of our MS discusses steady state rather than non-steady state, changes in water content are largely (but not completely) unconnected to our discussion. We will include some text on steady vs non-steady state, and hence this interesting point, to our revised MS.
- Section 3 (Results and Discussion) does not relate the results of the study well to those found in literature.
Thank you for pointing this out. We will add some references, e.g., in section 3.2 as requested by the other reviewer, and will try to relate section 3 better to our results and those found in literature in our revised MS.
- The steady- vs. non-steady state effect should be shortly discussed. The whole approach is based on steady-state conditions which do not always apply.
The majority of our MS discusses steady state, but, for example, Line 219-220 only holds during non-steady state. We will therefore include some additional discussion on steady vs non-steady state in our revised MS to make this more clear.
Line-by-line comments:
l15: remove dot between m h-1 (check entire text)
We will adjust our MS accordingly.
l37: introduce d2H and d18O
We will introduce these concepts in our revised MS.
l39: Brooks et al. did not sample ground water. Also, it would be worth to state something like: “While d18O values of xylem water were relatively enriched to stream water d18O values, d2H values of xylem water were relatively depleted”.
Brooks et al. (2010) did not measure groundwater, as you say, but they indicate that the mobile water pool in their hypothesis reflects the isotopic composition of groundwater. We will adjust our text accordingly and include your suggested connotation.
l63: I would consider writing the full term here again, and introduce FWU here
We will introduce foliar water uptake (FWU) as suggested.
l71: start new sentence starting from Kagawa.
We will separate these sentences in our revised MS.
l77: add also the “timing of leaf wetting event”
We will add this information.
l82: how about d2H? there is numerous studies that also point out differences between xylem and source water d2H.
We would like to refer here to our replies to previous comments.
l91: phrasing
We will rephrase this as “Third, these data were used to model the required enrichment of an alternative stem water source (e.g., hydraulic redistribution originating from FWU and/or tissue dehydration) to account for the observed enrichment in xylem water.”
l94: “The isotopic compositions of...” is or more precise: “The isotopic compositions of ... are...”
We will rephrase: “The isotopic compositions of H and O in water samples are…”
l102: phrasing
We will rephrase: “During these same dry conditions, BFLW is expected to be larger due to tissue dehydration and low flow velocities resulting in a stronger Péclet effect (see below).”
l109: so you only included studies where ground water data were available? Or did you also include studies where ground water was not sampled? Also, it would be important to report how the studies sampled xylem water and what techniques they used for water extraction. In addition, not every plant has access to ground water… A table summarizing this information (plant material, extraction technique etc.) and also indicating the geographical region would be good.
Indeed, we only included studies that sampled groundwater and xylem water. We will add this information to our revised MS, along with the suggested Table to our Appendix.
l116: water isotopic composition; isotopic composition of water contained in...
We will adjust our phrasing accordingly.
l117: effect on branch and stem water
We will adapt this sentence in our revised MS.
l118: isotopic steady state of what?
A steady state in water flow. We will rephrase this accordingly.
l118: plants do not only rely on ground water as source water (if at all)
We would like to refer to the answer on your second remark: the contribution of water to a plant’s water budget does increase during dry conditions, references:
Barbeta and Peñuelas (2017); Evaristo and McDonnell (2017)
l119: it would be worth mentioning that this equation actually applies to leaf water and you transfer it to the xylem water level
We will add this connotation, and add that we are not the first to suggest this transfer as mentioned in Line 117-118.
l126: consider starting a new sentence; D is defined.... and give only one value
We will adjust this based on your comment and that of the other reviewer. We have calculated D based on Eq. 5 and mentioned the value of this calculation on Line 125. We will make this clear in our revised MS.
l128: L was already introduced, should be also used see line 130
We prefer to use the full notation on several occasions, especially as the other reviewer pointed out the potential confusion between effective path length and distance between point of measurement and evaporation site.
l142: better "ground- and leaf water"
We will adapt our phrasing accordingly.
l149: XS? you mean XG?
Thank you for pointing this out, you are correct! We will change this in our revised MS.
l150: this part needs more explanation
Based on the suggestion of the other reviewer, we will adapt Eq. 3. As a result, this statement does not longer hold and will be removed from our revised MS.
l158-159: the model should also be tested for d2H
Based on our meta-analysis no enrichment in 2H was observed (Fig. S2), explaining that the source water has a similar 2H signature as the xylem water. The model is reduced to source equals sample in terms of 2H. This will be highlighted in the revised MS.
l163: represent
Thank you for your correction, we will change this accordingly.
l184: … xylem water is generally more enriched (also the values are enriched, not the water itself, check entire document for this formulation)
The other reviewer had similar concerns regarding formulation, we will adapt our MS to meet this concern.
l202: “– the average… –” (missing space, comes later again)
We will adjust this.
l203: delete dot between m h-1
We will check our entire MS for this type of phrasing and adapt accordingly.
l203-204: why do you introduce the abbreviations again; also already in line 188
Some guidelines suggest reintroducing abbreviations in new major sections. We will remove in the revised MS.
l205: estimates from literature available for XL of different plants?
To the best of our knowledge, no data on this are available.
l209: phrasing
We will adjust our phrasing: “…L can be smaller than 4 m, or herbaceous plant species can be used in this assessment.”
l210: “for instance, when reducing”
We will adjust our phrasing accordingly.
l224: HR was not introduced
You are correct. HR refers to hydraulic redistribution, we will add the introduction of this abbreviation to our revised MS.
l224: phrasing, “normal water cycling”
With ‘normal cycling’ we refer to traditional sap flow. We will clarify this part and add some more information on this based on the community comment by Carel Windt.
l219-227: please use references for this paragraph
We will include some references to this paragraph, e.g.:
Nadezhdina et al., 2010. Trees never rest: the multiple facets of hydraulic redistribution
We will also add some information on phloem-xylem exchange with the appropriate references to this paragraph, based on the community comment by Carel Windt.
l265: this sentence seems out of place
We will rephrase this sentence: “A reduced connection between roots and soil might explain why deep-rooted trees typically use deeper groundwater sources during dry conditions (Barbeta and Peñuelas, 2017).
l280: be more precise here, how does it affect V and L
When V is low, the distance travelled by a diffusing water molecule due to the Péclet effect, during a given amount of time, increases. We will add this information to our revised MS.
L262-283: It would be good to first start with your results and then refer it to the drought aspect.
We prefer to keep the current buildup of this section which starts with the effects of drought and explains how these effects are coupled to BFLW.
l308-311: I would not end your conclusion with this section.
We will remove this paragraph in our revised MS.
Please check the text for consistent use of the terms ratio, composition and signature.
We will go through our text and correct these inconsistencies in our revised MS.
Figures
Figure 1: blue and green lines are not dashed. explain boxplots. Please add number of studies here (n = x).
You are right, these lines are not dashed, we will adjust our caption accordingly and include (n = 25).
Figure 2: no capital letter in “Isotopic”
This is the first letter of a sentence, as such, we prefer to keep the capital letter.
Figure 3: as a function of d18OX or d18OXG?
As a function of d18OX. This was also pointed out by the other reviewer and we will adjust our caption to make this more clear and reflect the following: Fig. 3 illustrates possible combinations of the isotopic composition and volume of an alternative water source to result in the observed offset of 2.21 ‰.
Figure 4: add explanation of MWL to legend.
We will add ‘MWL = Meteoric Water Line’ to our caption.
Figure S1 and S2: Please add number of studies here (n = x).
We will add (n = 25) to both captions.
Thank you for the comprehensive review of our MS.
Yours sincerely,
Jeroen Schreel and co-authors.
Citation: https://doi.org/10.5194/hess-2023-13-AC3
Status: closed
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CC1: 'Comment on hess-2023-13', Carel Windt, 15 Feb 2023
Dear Jeroen,
Interesting manuscript! One comment: I'm surprised that phloem flow is not mentioned as a contributor to the back flow of water from leaves. Can it be neglected in this context, in your opinion? In older work, measuring water fluxes in the main stem of plants by means of MRI, I found that a significant fraction of xylem sap returns to the roots by means of the phloem (up to ~10% during the day, up to ~50% at night. Windt et al, PCE, 2006). This was measured in the main stem. This means, water used for expansion growth of shoots and young leaves did not even show up in this number (in the back flow to the roots). I therefore would expect, for individual leaves, the potential fraction of phloem backflow to be even larger than measured in the main stem.
Best, Carel
Citation: https://doi.org/10.5194/hess-2023-13-CC1 -
AC1: 'Reply on CC1', Jeroen Schreel, 16 Feb 2023
Dear Carel,
Thank you for this nice comment! Prior to publishing this preprint, we discussed the option of including phloem flow in our manuscript and had chosen to exclude this as we are considering the bias introduced in water source tracing which solely focusses on xylem water. In the current version of our manuscript, we have alluded to xylem-phloem water exchange to not leave this topic completely untouched (Line 34-35, 178, 224-225). However, based on your comment it does seem beneficial to include some more information on backflow through the phloem and we will do so in our revised manuscript.
Thanks again!Kind regards,
JeroenCitation: https://doi.org/10.5194/hess-2023-13-AC1 -
CC2: 'Reply on AC1', Carel Windt, 17 Feb 2023
Great. Thank you for the reply and best of luck with the manuscript,
Carel
Citation: https://doi.org/10.5194/hess-2023-13-CC2
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CC2: 'Reply on AC1', Carel Windt, 17 Feb 2023
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AC1: 'Reply on CC1', Jeroen Schreel, 16 Feb 2023
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RC1: 'Comment on hess-2023-13', Anonymous Referee #1, 26 May 2023
This manuscript describes numerical experiments to test a series of hypotheses about whether water exchanges at leaves may be responsible for some observed isotopic phenomena in stem xylem that are difficult to explain otherwise. The approach is to compare modeled implications of some idealized physiological conditions to a generalized empirical offset between observed and expected isotopic composition in stem xylem. This kind of investigation is of high interest in the active area of isotope ecohydrology because the lack of understanding of in-plant isotopic transformations is a major impediment to using isotopic tracers to identify plant-water relationships. However, the modeling presented here is not thorough and contains errors, with the net result that it does not reliably support conclusions. Rather than being an in-depth consideration of model results, the discussion is dominated by hypotheses that existed prior to this manuscript. For example, section 3.3, while a part of the Results and Discussion, is in essence a literature review that logically should be presented before the numerical experiments because it is completely independent of the modeling.
Major comment 1:
The choice to ignore 2H because the empirical difference between soil water and xylem water is near zero is problematic. All the modeling presented here could be done on 2H as well as 18O, and we should expect useful models to reproduce both the offsets of both 18O (~2‰) and 2H (~0‰) because neither is more valid than the other. The manuscript also does not sufficiently establish the shifts in isotopic composition (see detailed comment L36, L158).
Major comment 2:
The modeling mostly omits consideration of water volumes and assumes constant boundary conditions both upstream and downstream without justification. The dual requirements—that (1) the backflowing water to be isotopically enriched, but also that (2) there be enough water to replace water already in the stem—are in fundamental tension with each other. Ignoring this conflict is a serious omission. The one portion of the modeling that attempts mass balance makes assumptions that violate the mathematical formulation being applied—see detailed comment L137.
Major comment 3:
The modeling draws heavily from the leaf-water isotope literature and extends that theory into stem xylem mostly without justification or citation support.
Detailed comments
L14, L134 it is probably better to call it soil water instead of groundwater.
L36 I think the paragraph describing the origin of disagreement between isotopic compositions of source water vs. vegetation is too dismissive of methodological artifacts. There is currently no agreed-upon. Also, this paragraph (actually the entire manuscript) does not mention that xylem water in branches is typically more enriched in heavy isotopes than is water in lower stems. There is so much potential information in this latter phenomenon that to ignore it is puzzling.
L114-117 duplicates material from earlier in the introduction
L120 Eq 3 (given by Farquhar and Lloyd 1993 as Eq 25) is indeed the bulk mean d18O of all water within effective pathlength L in a leaf. However, in Fig 2 the bulk mean is not leaf water, it is leaf-plus-stem bulk mean. I find this confusing and wonder why not plot the point value of d18O of the enrichment point as given by F&L93 as their equation 24. That value is more intuitive and corresponds to what a wood sample would extract when extracted from point L—the analysis here confuses bulk leaf isotopic composition with composition of wood xylem water at a point. Bulk leaf d18O makes more sense when leaves are the sample, but how do you even get a “mean” sample of a leaf-to-stem combination? Perhaps more importantly, does the theory hold over the leaf-stem transition? Please see also comment L129.
L125 there are two definitions of D in this sentence; the stray needs removing.
L129 the Cernusak paper is about leaf water, not stem water, and there are important differences in the paths water takes through the two. Farquhar and Lloyd (1993) and Song et al. (2013; both cited in this manuscript) discuss the non-triviality in estimating tortuosity. What is the consequence, then, of using leaf theory to describe effective diffusivity through leaves and along stems, given that flowpaths include various membranes, pits, embolisms, etc.? Much more support is needed to justify the simplicity of the diffusion model because trees are not long, isothermal straws with no surface chemistry. Finally, all the theory is for steady state, but diffusion happens when stomata are closed, also. Nowhere are these choices justified, and the assumptions are not acknowledged.
L130 subsequent discussion does not emphasize that this is effective path length. Likewise, there is no effort in the manuscript to account for tortuosity and thus estimate linear path length along a leaf/stem: e.g., L214 seems to imply linear stem distances, neglecting tortuosity.
L137 by including the Peclet number, Eq 3 and 6 already have both diffusion and convection in them. The derivation of Eqs 7-10 is nonsensical because it attempts to describe diffusion as mixing. Eq 5 describes the diffusion of H218O into upstream xylem water, not the diffusion of evaporation-site water into upstream xylem water. The velocity-dependent distance profile in 18O upstream from the evaporation point is irreducibly diffusive and cannot be described by end-member mixing. If I understand it correctly, Fig 2b is still valid if it is relabeled “proportion similarity” and the mass-balance fallacy removed.
L150 it would be better to explain the proportional difference (Song) technique before asserting anything about it.
L158 establishing the -2.2/0 shift is crucial for this work, but substantially more clarity is needed in describing how the database was constructed and the logic of the isotopic comparisons. At least some of the cited papers do not report paired soil/groundwater-xylem isotopic composition in terms of single isotopes but rather in 2-isotope space, so it is not clear how the data for this paper were compiled.
Sec 2.3 and Eq 11 and 12 ignore diffusion. What is the justification for this?
L175-180 repeats the introduction and is not needed.
L183-202 repeats the introduction and is not needed.
L203 this sap velocity drops out of the sky in the results with no context or justification. I don’t understand why a single Peclet number is being investigated; field data surely contain a wide range.
L214 … and when diffusivity is assumed independent of morphological resistance and temperature is high
L215 L255 which smallest enrichment?
L220 in diffusion, “water” does not travel—molecules do, and “water enriched in 18O” does not travel; this statement about time to travel a certain distance is nonsensical. In the model being used here, d18O of xylem water approaches the d18O of soil water asymptotically over distance. Does the 27.7 year estimate originate in the diffusion coefficient, which is about 13.9 m2/y (27.78 m2/2y) as parameterized in Eq 5? The origin of this time estimate is not specified.
L222 “heavy water accumulating upstream” is a fundamental misrepresentation of the equations being employed. Eq 3 describes a steady-state condition, and 18O concentrations cannot change with time anywhere in the system.
L224 what is HR? What is normal cycling?
Fig 3 the caption and the axis labels seem to conflict. According to the axis labels, the plot is isotopic composition of alternative water source as a function of isotopic composition of xylem water, not as a function of difference as the caption says. When the two are equal, the proportion of alternative water is 1. I do not understand how the offset of 2.2 matters in this figure.
Sec 3.2 / Fig 4 this section on leaf surface water is difficult to defend because there are no citations to support the many “just so” stories. There are many instances of incomplete isotope physics. For example, evaporation at less than 100% humidity certainly does enrich the remaining water in 1H2H16O despite Fig 4 implying humidity must be 100% for that to happen. For another example, dew is more isotopically enriched than rainfall but is typically not more enriched than water at the point of transpiration, so that studies of the effect of dew on leaf isotopes usually find dew results in lighter leaf water, not heavier. Dew might be heavier than soil water, but it seems to me unlikely that it would be 20-42‰ heavier, as the explanation requires. Most importantly, all processes mentioned in this section require that there be enough water on the leaves, and that it be there long enough to dilute the stem xylem.
Citation: https://doi.org/10.5194/hess-2023-13-RC1 -
AC2: 'Reply on RC1', Jeroen Schreel, 28 Jun 2023
This manuscript describes numerical experiments to test a series of hypotheses about whether water exchanges at leaves may be responsible for some observed isotopic phenomena in stem xylem that are difficult to explain otherwise. The approach is to compare modeled implications of some idealized physiological conditions to a generalized empirical offset between observed and expected isotopic composition in stem xylem. This kind of investigation is of high interest in the active area of isotope ecohydrology because the lack of understanding of in-plant isotopic transformations is a major impediment to using isotopic tracers to identify plant-water relationships. However, the modeling presented here is not thorough and contains errors, with the net result that it does not reliably support conclusions. Rather than being an in-depth consideration of model results, the discussion is dominated by hypotheses that existed prior to this manuscript. For example, section 3.3, while a part of the Results and Discussion, is in essence a literature review that logically should be presented before the numerical experiments because it is completely independent of the modeling.
Thank you for your interest in our manuscript (MS). We hope that our responses clarify the comments listed below.
Major comment 1:
The choice to ignore 2H because the empirical difference between soil water and xylem water is near zero is problematic. All the modeling presented here could be done on 2H as well as 18O, and we should expect useful models to reproduce both the offsets of both 18O (~2‰) and 2H (~0‰) because neither is more valid than the other. The manuscript also does not sufficiently establish the shifts in isotopic composition (see detailed comment L36, L158).
We understand this concern, but we do not ignore ²H in this MS. Our meta-analysis indicated a shift in 18O, which we did not observe in ²H. In this particular case, water in the xylem is more enriched in 18O and not in 2H compared to groundwater, i.e., the ratio of ²H to 18O is changing. We hypothesize that this could have occurred due to back-flow of leaf water (BFLW) as water transported by BFLW is relatively more enriched in 18O. In other words, this hypothesis relies on the idea that leaf water is more enriched in 18O and not in ²H, an idea that is further pointed out in Fig. 4 and Line 230-253.
Major comment 2:
The modeling mostly omits consideration of water volumes and assumes constant boundary conditions both upstream and downstream without justification. The dual requirements—that (1) the backflowing water to be isotopically enriched, but also that (2) there be enough water to replace water already in the stem—are in fundamental tension with each other. Ignoring this conflict is a serious omission. The one portion of the modeling that attempts mass balance makes assumptions that violate the mathematical formulation being applied—see detailed comment L137.
In our MS, volumes are included. That is one of the major points we are trying to make. These volumes are included throughout the text, and are the reason why we included Eq. 7 to 10 and panel B in Fig. 2, which illustrates the link between change in isotopic composition and volume coming from leaf water (XL). This is also the basis for Fig. 3, which shows how much water should come from an alternative water source (i.e., volume; XA) to result in the observed enrichment.
Major comment 3:
The modeling draws heavily from the leaf-water isotope literature and extends that theory into stem xylem mostly without justification or citation support.
To support this transfer, we include other scientific literature in which this same extension has been suggested (Line 117-118).
Detailed comments
L14, L134 it is probably better to call it soil water instead of groundwater.
We prefer to keep using groundwater instead of soil water, because the remainder of the MS is based on groundwater data.
L36 I think the paragraph describing the origin of disagreement between isotopic compositions of source water vs. vegetation is too dismissive of methodological artifacts. There is currently no agreed-upon. Also, this paragraph (actually the entire manuscript) does not mention that xylem water in branches is typically more enriched in heavy isotopes than is water in lower stems. There is so much potential information in this latter phenomenon that to ignore it is puzzling.
We do not intend to be dismissive of methodological artifacts and clearly mention this possibility in the MS (Line 41-43).
We have briefly mentioned the enrichment of branch water (Line 56-57, 114-115, 193-195), but given your comment and the one of the other reviewer we agree that we should add more information on this topic and will do so in our revised MS and want to thank you for pointing this out.
L114-117 duplicates material from earlier in the introduction
We were merely trying to guide the reader, but given this concern, we will shorten the paragraph.
L120 Eq 3 (given by Farquhar and Lloyd 1993 as Eq 25) is indeed the bulk mean d18O of all water within effective pathlength L in a leaf. However, in Fig 2 the bulk mean is not leaf water, it is leaf-plus-stem bulk mean. I find this confusing and wonder why not plot the point value of d18O of the enrichment point as given by F&L93 as their equation 24. That value is more intuitive and corresponds to what a wood sample would extract when extracted from point L—the analysis here confuses bulk leaf isotopic composition with composition of wood xylem water at a point. Bulk leaf d18O makes more sense when leaves are the sample, but how do you even get a “mean” sample of a leaf-to-stem combination? Perhaps more importantly, does the theory hold over the leaf-stem transition? Please see also comment L129.
We agree that using Eq. 24 rather than 25 in Farquhar and Lloyd (1993) is more appropriate. We will adapt our equations and calculations accordingly. This rearrangement changes our values somewhat, but not the main premise, nor the main outcome of our discussion:
Flow is reduced to 6.6 10-6 m h-1 (equaling approximately zero flow, something that can be obtained at night or during drought). With an L of 4 m, this results in an of 54 ‰ and an XL of 4 %. With an L of 2 m this results in an of 11 ‰ and an XL of 20 % (outcome of previous simulations can be found at Line 202-211).
We do believe that this theory holds over the leaf-stem transition, something that is also mentioned by Farquhar and Lloyd (1993): “Due to lower transpiration rates will be smaller in stressed plants than in unstressed ones and may explain the enrichment in stem water observed by Flanagan et al. (1991).”
L125 there are two definitions of D in this sentence; the stray needs removing.
Our apologies for this possible confusion. There is only one definition of D used in our MS. We have calculated D based on Eq. 5 and mentioned the value of this calculation on line 125.We will make this more clear in our revised MS.
L129 the Cernusak paper is about leaf water, not stem water, and there are important differences in the paths water takes through the two. Farquhar and Lloyd (1993) and Song et al. (2013; both cited in this manuscript) discuss the non-triviality in estimating tortuosity. What is the consequence, then, of using leaf theory to describe effective diffusivity through leaves and along stems, given that flowpaths include various membranes, pits, embolisms, etc.? Much more support is needed to justify the simplicity of the diffusion model because trees are not long, isothermal straws with no surface chemistry. Finally, all the theory is for steady state, but diffusion happens when stomata are closed, also. Nowhere are these choices justified, and the assumptions are not acknowledged.
In Line 127-130, the work of Cernusak et al. (2016) is used to support our definitions of the Péclet number and effective path length, which is justified in this way.
Estimating tortuosity is indeed non-trivial, which is why we state that tortuosity is included in the path length L (Line 131) and not disentangled from the distance between point of measurement and evaporation site. We have also compared values with an effective path length of 4 and 2 m to give the reader some idea of the effect of an increase in path length. Lastly, we want to add that models are simplifications of reality and we believe that the assumptions in our model can be seen as valid in the used context. We will make the assumptions of steady vs non-steady state more explicit in our revised MS.
L130 subsequent discussion does not emphasize that this is effective path length. Likewise, there is no effort in the manuscript to account for tortuosity and thus estimate linear path length along a leaf/stem: e.g., L214 seems to imply linear stem distances, neglecting tortuosity.
L is defined as effective path length on Line 124, and 128-131. We have redefined L as effective path length on Line 203-204, something we did to avoid this confusion. The other reviewer mentioned that we shouldn’t redefine our variables. As such, we believe that L is sufficiently clear.
L137 by including the Peclet number, Eq 3 and 6 already have both diffusion and convection in them. The derivation of Eqs 7-10 is nonsensical because it attempts to describe diffusion as mixing. Eq 5 describes the diffusion of H218O into upstream xylem water, not the diffusion of evaporation-site water into upstream xylem water. The velocity-dependent distance profile in 18O upstream from the evaporation point is irreducibly diffusive and cannot be described by end-member mixing. If I understand it correctly, Fig 2b is still valid if it is relabeled “proportion similarity” and the mass-balance fallacy removed.
When a fluid diffuses, it is mixed with the fluid it diffuses into, so the isotopic signature measured will be a mix of both fluids, which validates the use of these equations.
Eq. 5 does not describe diffusion but the diffusivity (rate of diffusion) of H218O in water. Subsequently, diffusivity is used to calculate the Péclet effect (Eq. 4; Line 123): the diffusion of evaporatively enriched leaf water in the opposite direction of the bulk water flow (Line 114).
Based on these considerations, the concept of Fig. 2 is valid as it is represented, but the values will change, following the previous comment about Eq. 24 and 25 in Farquhar and Lloyd (1993).
L150 it would be better to explain the proportional difference (Song) technique before asserting anything about it.
Based on the changed equations (Eq. 24 vs 25 in Farquhar and Lloyd (1993)), this section does not longer hold and will be removed from our revised MS.
L158 establishing the -2.2/0 shift is crucial for this work, but substantially more clarity is needed in describing how the database was constructed and the logic of the isotopic comparisons. At least some of the cited papers do not report paired soil/groundwater-xylem isotopic composition in terms of single isotopes but rather in 2-isotope space, so it is not clear how the data for this paper were compiled.
A shift of 2.2118O is used as the average observed enrichment based on literature. Our models allow for a an adjustment of this value to calculate specific cases. This average value is based on a meta-analysis using a keyword-based search of published data (e.g., ecohydrological separation; Line 103-106). If data were provided in a biplot, individual datapoints were extracted using an online plot digitizer. Following this extraction, the mean and standard error of 18O and 2H were calculated and added to the dataset. We will add this additional information to our revised MS.
Sec 2.3 and Eq 11 and 12 ignore diffusion. What is the justification for this?
Section 2.3 discusses hydraulic redistribution, the redistribution of water based on a difference in water potential, and does not include diffusion.
L175-180 repeats the introduction and is not needed.
We aimed at discussing our results and putting them in the larger framework of published literature. We will check these lines for redundancy.
L183-202 repeats the introduction and is not needed.
This part aimed at making the text more fluent, but we will check for redundancy.
L203 this sap velocity drops out of the sky in the results with no context or justification. I don’t understand why a single Peclet number is being investigated; field data surely contain a wide range.
Flow velocities are mentioned throughout the text (Line 54-56, 67-68, 90, 103,…). The flow velocity mentioned is meant as an example to provide the reader with some additional insights: what are some of the values obtained by the model and how biologically relevant are they? What happens when we change L from 4 to 2 m during these same flow velocities? Instead of discussing a single Péclet number, we have calculated a range of Péclet numbers, resulting in Fig. 2. To give the readers some reference of realistic values, we discuss two of those combinations of values with an effective path length of 4 and 2 m. We will make this more clear in our revised MS.
L214 … and when diffusivity is assumed independent of morphological resistance and temperature is high
Thank you for this addition. We will add this to our revised MS. In our model we assume a leaf temperature of 25°C (Line 125).
L215 L255 which smallest enrichment?
This refers to the smallest observed enrichment in our meta-analysis. We will add this to our revised MS.
L220 in diffusion, “water” does not travel—molecules do, and “water enriched in 18O” does not travel; this statement about time to travel a certain distance is nonsensical. In the model being used here, d18O of xylem water approaches the d18O of soil water asymptotically over distance. Does the 27.7 year estimate originate in the diffusion coefficient, which is about 13.9 m2/y (27.78 m2/2y) as parameterized in Eq 5? The origin of this time estimate is not specified.
Thank you for pointing this out. We will adapt our phrasing from ‘water’ to ‘water molecules’ as that is indeed what we meant.
Also, thank you for pointing out we forgot to include our calculations. 27.7 years is based on the equation of diffusion time: t L2/(2D) with t the time, L the distance (2 m) and D the diffusion coefficient calculated in Eq. 5.
L222 “heavy water accumulating upstream” is a fundamental misrepresentation of the equations being employed. Eq 3 describes a steady-state condition, and 18O concentrations cannot change with time anywhere in the system.
In this case, we are not referring to a steady-state because a steady-state can be valid for a short time period, but will not hold for a longer time period like 27.7 years. We will adjust our phrasing accordingly to make this distinction more clear in the revised MS.
L224 what is HR? What is normal cycling?
HR refers to hydraulic redistribution. This clarification has probably been removed during one of our revisions, and we sincerely apologies. We will add it to the revised MS.
With ‘normal cycling’ we refer to traditional sap flow. We will clarify this part and add some more information on this to our revised MS based on the community comment by Carel Windt.
Fig 3 the caption and the axis labels seem to conflict. According to the axis labels, the plot is isotopic composition of alternative water source as a function of isotopic composition of xylem water, not as a function of difference as the caption says. When the two are equal, the proportion of alternative water is 1. I do not understand how the offset of 2.2 matters in this figure.
The axis labels in Fig. 3 were checked and are correct. We understand that the caption might be confusing and will therefore adapt it to reflect the following:
Fig. 3 illustrates possible combinations of the isotopic composition and volume of an alternative water source to result in the observed offset of 2.21. In other words, if the offset would be smaller, the needed volume of alternative water and/or its required isotopic enrichment would decrease resulting in a different gradient in Fig. 3.
Sec 3.2 / Fig 4 this section on leaf surface water is difficult to defend because there are no citations to support the many “just so” stories. There are many instances of incomplete isotope physics. For example, evaporation at less than 100% humidity certainly does enrich the remaining water in 1H2H16O despite Fig 4 implying humidity must be 100% for that to happen. For another example, dew is more isotopically enriched than rainfall but is typically not more enriched than water at the point of transpiration, so that studies of the effect of dew on leaf isotopes usually find dew results in lighter leaf water, not heavier. Dew might be heavier than soil water, but it seems to me unlikely that it would be 20-42‰ heavier, as the explanation requires. Most importantly, all processes mentioned in this section require that there be enough water on the leaves, and that it be there long enough to dilute the stem xylem.
To take away the “just so” stories concern, we are happy to add additional references to the revised MS, such as:
Berry et al., 2017. The two water worlds hypothesis: addressing multiple working hypothesis and proposing a way forward
Gat, 1996. Oxygen and hydrogen isotopes in the hydrological cycle.
We do not suggest that all 1H216O and 1H2H16O evaporate when RH < 100%. However, based on these references it becomes clear that enrichment is stronger in terms of 1H218O during conditions with RH < 100. In other words, we do not rule out enrichment of the remaining water in 1H2H16O, but enrichment in 1H218O should be more pronounced.
As suggested, dew might be heavier than soil water and we suggest that lighter isotopologues will continue to evaporate at a higher rate compared to heavy isotopologues (Line 246-249). This enriched water layer can subsequently be absorbed by leaves and redistributed to the xylem. Furthermore, this is only part of larger concept of BFLW which includes the Péclet effect, hydraulic redistribution and FWU. As such, dew water does not need to explain the occurrence of enriched water on its own and does not need to be 20-42 ‰ heavier.
Thank you for your detailed review of our work.
Yours sincerely,
Jeroen Schreel and co-authors.
Citation: https://doi.org/10.5194/hess-2023-13-AC2
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AC2: 'Reply on RC1', Jeroen Schreel, 28 Jun 2023
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RC2: 'Comment on hess-2023-13', Anonymous Referee #2, 31 May 2023
Schreel et al. test whether the back-flow of enriched leaf water can explain the observed discrepancies between the d18O values of the source water and the d18O values of the xylem water. Back-flow of leaf water is defined as a combined effect of the Péclet effect, water uptake by leaves, and hydraulic redistribution of leaf water.
The topic is very interesting and fits within the scope of HESS. One major constrain is that the model was not tested for 2H. Numerous studies have also shown discrepancies between source and xylem water for 2H, including, for instance, the Chen et al. (2020) study cited here.
Further comments:
- Problematically, the entire study appears to be based on the assumption that ground water = source water, which is not true for many ecosystems and species. Therefore, much of the observed isotopic differences are also likely due to the unknown "true" water source. This point needs to be adequately discussed.
- The statistical analyses related to the d18O discrepancy and Fig S1 and S2 need to be explained and justified in mor detail. The whole study is based on this value of “2.21 per mille”.
- An enrichment in branch water relative to the stem xylem water is well known. See e.g. Martín‐Gómez et al., 2017, Vega‐Grau et al., 2021. This effect was not discussed at all.
- Vega‐Grau, A.M., McDonnell, J., Schmidt, S., Annandale, M. & Herbohn, J. (2021) Isotopic fractionation from deep roots to tall shoots: a forensic analysis of xylem water isotope composition in mature tropical savanna trees. Science of the Total Environment, 795, 148675. Available from: https://doi.org/10.1016/j.scitotenv.2021.148675
- Martín‐Gómez, P., Serrano, L. & Ferrio, J.P. (2017) Short‐term dynamics of evaporative enrichment of xylem water in woody stems: implications for ecohydrology. Tree Physiology, 37, 511–522. Available from: https://doi.org/10.1093/treephys/tpw115
- In this context, it would be good to point out already in the introduction that many studies sample branches (i.e., closer to the leaf water) for xylem water and not the stem, which is particularly relevant for your idea. Also this information (what was sampled?) would be important to know for the data you use (see comment on l109)
- Also, this publication may be relevant: https://nph.onlinelibrary.wiley.com/doi/full/10.1111/nph.18113
- I am wondering how large the amount of leaf water is relative to the amount of stem water. It would be great to have some estimates for the ratio of these pools from literature. What is a realistic estimate of the leaf water for the whole plant water pool or relative to the stem xylem water? You only mentioned an estimate for foliar water uptake.
- The enrichment also depends on the leaf water turn overtime, which is controlled by the stomatal conductance and the leaf water content. How does leaf water content influence the isotopic differences between xylem and ground water? This point would be interesting to discuss, in particular in 3.3.
- Section 3 (Results and Discussion) does not relate the results of the study well to those found in literature.
- The steady- vs. non-steady state effect should be shortly discussed. The whole approach is based on steady-state conditions which do not always apply.
Line-by-line comments:
l15: remove dot between m h-1 (check entire text)
l37: introduce d2H and d18O
l39: Brooks et al. did not sample ground water. Also, it would be worth to state something like: “While d18O values of xylem water were relatively enriched to stream water d18O values, d2H values of xylem water were relatively depleted”.
l63: I would consider writing the full term here again, and introduce FWU here
l71: start new sentence starting from Kagawa.
l77: add also the “timing of leaf wetting event”
l82: how about d2H? there is numerous studies that also point out differences between xylem and source water d2H.
l91: phrasing
l94: “The isotopic compositions of...” is or more precise: “The isotopic compositions of ... are...”
l102: phrasing
l109: so you only included studies where ground water data were available? Or did you also include studies where ground water was not sampled? Also, it would be important to report how the studies sampled xylem water and what techniques they used for water extraction. In addition, not every plant has access to ground water… A table summarizing this information (plant material, extraction technique etc.) and also indicating the geographical region would be good.
l116: water isotopic composition; isotopic composition of water contained in...
l117: effect on branch and stem water
l118: isotopic steady state of what?
l118: plants do not only rely on ground water as source water (if at all)
l119: it would be worth mentioning that this equation actually applies to leaf water and you transfer it to the xylem water level
l126: consider starting a new sentence; D is defined.... and give only one value
l128: L was already introduced, should be also used see line 130
l142: better "ground- and leaf water"
l149: XS? you mean XG?
l150: this part needs more explanation
l158-159: the model should also be tested for d2H
l163: represent
l184: … xylem water is generally more enriched (also the values are enriched, not the water itself, check entire document for this formulation)
l202: “– the average… –” (missing space, comes later again)
l203: delete dot between m h-1
l203-204: why do you introduce the abbreviations again; also already in line 188
l205: estimates from literature available for XL of different plants?
l209: phrasing
l2010: “for instance, when reducing”
l224: HR was not introduced
l224: phrasing, “normal water cycling”
l219-227: please use references for this paragraph
l265: this sentence seems out of place
l280: be more precise here, how does it affect V and L
L262-283: It would be good to first start with your results and then refer it to the drought aspect.
l308-3011: I would not end your conclusion with this section.
Please check the text for consistent use of the terms ratio, composition and signature.
Figures
Figure 1: blue and green lines are not dashed. explain boxplots. Please add number of studies here (n = x).
Figure 2: no capital letter in “Isotopic”
Figure 3: as a function of d18OX or d18OXG?
Figure 4: add explanation of MWL to legend.
Figure S1 and S2: Please add number of studies here (n = x).
Citation: https://doi.org/10.5194/hess-2023-13-RC2 -
AC3: 'Reply on RC2', Jeroen Schreel, 28 Jun 2023
Schreel et al. test whether the back-flow of enriched leaf water can explain the observed discrepancies between the d18O values of the source water and the d18O values of the xylem water. Back-flow of leaf water is defined as a combined effect of the Péclet effect, water uptake by leaves, and hydraulic redistribution of leaf water.
The topic is very interesting and fits within the scope of HESS. One major constrain is that the model was not tested for 2H. Numerous studies have also shown discrepancies between source and xylem water for 2H, including, for instance, the Chen et al. (2020) study cited here.
We understand this concern, which was also made by the other reviewer. We will make this point more clear in the revised manuscript (MS). In this particular case, water in the xylem is more enriched in 18O (Fig. S1) and not in 2H (Fig. S2) compared to groundwater, i.e., the ratio of ²H to 18O is changing. We hypothesize that this change in the ratio of ²H to 18O could have occurred due to back-flow of leaf water (BFLW) as water transported by BFLW is relatively more enriched in 18O, an idea that is further pointed out in Fig. 4 and Line 230-253. Based on our equations we were able to model the enrichment and volume of an alternative water source to explain this observation. We briefly discussed these results in context of ²H discrepancies (Line 284-290).
Further comments:
- Problematically, the entire study appears to be based on the assumption that ground water = source water, which is not true for many ecosystems and species. Therefore, much of the observed isotopic differences are also likely due to the unknown "true" water source. This point needs to be adequately discussed.
We agree that other water sources such as soil water will be at play. Therefore we have used the generic term ‘alternative water source’ in section 3.2 and Fig. 3 (Line 231-232). However, the contribution of groundwater to a plant’s water budget does increase during dry conditions (Line100-101), which is why we have focused section 3.3 on the fact that the increased use of groundwater coincides with more favorable conditions for BFLW (Line 262-297). This will be better explained in the revised MS.
- The statistical analyses related to the d18O discrepancy and Fig S1 and S2 need to be explained and justified in more detail. The whole study is based on this value of “2.21 per mille”.
The statistical analysis is based on a weighted linear regression (Line 109-112). Inspired by your comment and the comment of the other reviewer we will add some more explanation to our meta-analysis. A shift in 18O of 2.21 is used as the average observed enrichment based on literature, however, our models allow for an adjustment of this value to calculate specific cases.
- An enrichment in branch water relative to the stem xylem water is well known. See e.g. Martín‐Gómez et al., 2017, Vega‐Grau et al., 2021. This effect was not discussed at all.
- Vega‐Grau, A.M., McDonnell, J., Schmidt, S., Annandale, M. & Herbohn, J. (2021) Isotopic fractionation from deep roots to tall shoots: a forensic analysis of xylem water isotope composition in mature tropical savanna trees. Science of the Total Environment, 795, 148675. Available from: https://doi.org/10.1016/j.scitotenv.2021.148675
- Martín‐Gómez, P., Serrano, L. & Ferrio, J.P. (2017) Short‐term dynamics of evaporative enrichment of xylem water in woody stems: implications for ecohydrology. Tree Physiology, 37, 511–522. Available from: https://doi.org/10.1093/treephys/tpw115
We agree that our brief mentioning of enrichment of branch water (Line 56-57, 114-115, 193-195) is too limited, and we will add more information on this topic in our revised MS. Thank you for the suggestion of these interesting references.
- In this context, it would be good to point out already in the introduction that many studies sample branches (i.e., closer to the leaf water) for xylem water and not the stem, which is particularly relevant for your idea. Also this information (what was sampled?) would be important to know for the data you use (see comment on l109)
Thank you for this suggestion. We will include this information in the introduction of our revised MS.
- Also, this publication may be relevant: https://nph.onlinelibrary.wiley.com/doi/full/10.1111/nph.18113
Thank you for the suggestion of this relevant reference. We will include this in our revised MS.
- I am wondering how large the amount of leaf water is relative to the amount of stem water. It would be great to have some estimates for the ratio of these pools from literature. What is a realistic estimate of the leaf water for the whole plant water pool or relative to the stem xylem water? You only mentioned an estimate for foliar water uptake.
We agree that this would be very useful. However, the availability of this data is very limited and will vary significantly between different species, development stages, seasons and even on a daily scale.
- The enrichment also depends on the leaf water turn overtime, which is controlled by the stomatal conductance and the leaf water content. How does leaf water content influence the isotopic differences between xylem and ground water? This point would be interesting to discuss, in particular in 3.3.
This is a very interesting point. A change in leaf water content is linked to a non-steady state. When leaf water content decreases due to transpiration, the remaining leaf water is more enriched compared to initial conditions as lighter isotopologues preferably evaporate (Fig. 4). When leaf water content increases due to water supply by the xylem, the remaining leaf water will be less enriched compared to initial conditions due to mixing of enriched leaf water with xylem water. However, some of the enriched water molecules in the leaf will move towards the xylem, e.g., due to the Péclet effect. As most of our MS discusses steady state rather than non-steady state, changes in water content are largely (but not completely) unconnected to our discussion. We will include some text on steady vs non-steady state, and hence this interesting point, to our revised MS.
- Section 3 (Results and Discussion) does not relate the results of the study well to those found in literature.
Thank you for pointing this out. We will add some references, e.g., in section 3.2 as requested by the other reviewer, and will try to relate section 3 better to our results and those found in literature in our revised MS.
- The steady- vs. non-steady state effect should be shortly discussed. The whole approach is based on steady-state conditions which do not always apply.
The majority of our MS discusses steady state, but, for example, Line 219-220 only holds during non-steady state. We will therefore include some additional discussion on steady vs non-steady state in our revised MS to make this more clear.
Line-by-line comments:
l15: remove dot between m h-1 (check entire text)
We will adjust our MS accordingly.
l37: introduce d2H and d18O
We will introduce these concepts in our revised MS.
l39: Brooks et al. did not sample ground water. Also, it would be worth to state something like: “While d18O values of xylem water were relatively enriched to stream water d18O values, d2H values of xylem water were relatively depleted”.
Brooks et al. (2010) did not measure groundwater, as you say, but they indicate that the mobile water pool in their hypothesis reflects the isotopic composition of groundwater. We will adjust our text accordingly and include your suggested connotation.
l63: I would consider writing the full term here again, and introduce FWU here
We will introduce foliar water uptake (FWU) as suggested.
l71: start new sentence starting from Kagawa.
We will separate these sentences in our revised MS.
l77: add also the “timing of leaf wetting event”
We will add this information.
l82: how about d2H? there is numerous studies that also point out differences between xylem and source water d2H.
We would like to refer here to our replies to previous comments.
l91: phrasing
We will rephrase this as “Third, these data were used to model the required enrichment of an alternative stem water source (e.g., hydraulic redistribution originating from FWU and/or tissue dehydration) to account for the observed enrichment in xylem water.”
l94: “The isotopic compositions of...” is or more precise: “The isotopic compositions of ... are...”
We will rephrase: “The isotopic compositions of H and O in water samples are…”
l102: phrasing
We will rephrase: “During these same dry conditions, BFLW is expected to be larger due to tissue dehydration and low flow velocities resulting in a stronger Péclet effect (see below).”
l109: so you only included studies where ground water data were available? Or did you also include studies where ground water was not sampled? Also, it would be important to report how the studies sampled xylem water and what techniques they used for water extraction. In addition, not every plant has access to ground water… A table summarizing this information (plant material, extraction technique etc.) and also indicating the geographical region would be good.
Indeed, we only included studies that sampled groundwater and xylem water. We will add this information to our revised MS, along with the suggested Table to our Appendix.
l116: water isotopic composition; isotopic composition of water contained in...
We will adjust our phrasing accordingly.
l117: effect on branch and stem water
We will adapt this sentence in our revised MS.
l118: isotopic steady state of what?
A steady state in water flow. We will rephrase this accordingly.
l118: plants do not only rely on ground water as source water (if at all)
We would like to refer to the answer on your second remark: the contribution of water to a plant’s water budget does increase during dry conditions, references:
Barbeta and Peñuelas (2017); Evaristo and McDonnell (2017)
l119: it would be worth mentioning that this equation actually applies to leaf water and you transfer it to the xylem water level
We will add this connotation, and add that we are not the first to suggest this transfer as mentioned in Line 117-118.
l126: consider starting a new sentence; D is defined.... and give only one value
We will adjust this based on your comment and that of the other reviewer. We have calculated D based on Eq. 5 and mentioned the value of this calculation on Line 125. We will make this clear in our revised MS.
l128: L was already introduced, should be also used see line 130
We prefer to use the full notation on several occasions, especially as the other reviewer pointed out the potential confusion between effective path length and distance between point of measurement and evaporation site.
l142: better "ground- and leaf water"
We will adapt our phrasing accordingly.
l149: XS? you mean XG?
Thank you for pointing this out, you are correct! We will change this in our revised MS.
l150: this part needs more explanation
Based on the suggestion of the other reviewer, we will adapt Eq. 3. As a result, this statement does not longer hold and will be removed from our revised MS.
l158-159: the model should also be tested for d2H
Based on our meta-analysis no enrichment in 2H was observed (Fig. S2), explaining that the source water has a similar 2H signature as the xylem water. The model is reduced to source equals sample in terms of 2H. This will be highlighted in the revised MS.
l163: represent
Thank you for your correction, we will change this accordingly.
l184: … xylem water is generally more enriched (also the values are enriched, not the water itself, check entire document for this formulation)
The other reviewer had similar concerns regarding formulation, we will adapt our MS to meet this concern.
l202: “– the average… –” (missing space, comes later again)
We will adjust this.
l203: delete dot between m h-1
We will check our entire MS for this type of phrasing and adapt accordingly.
l203-204: why do you introduce the abbreviations again; also already in line 188
Some guidelines suggest reintroducing abbreviations in new major sections. We will remove in the revised MS.
l205: estimates from literature available for XL of different plants?
To the best of our knowledge, no data on this are available.
l209: phrasing
We will adjust our phrasing: “…L can be smaller than 4 m, or herbaceous plant species can be used in this assessment.”
l210: “for instance, when reducing”
We will adjust our phrasing accordingly.
l224: HR was not introduced
You are correct. HR refers to hydraulic redistribution, we will add the introduction of this abbreviation to our revised MS.
l224: phrasing, “normal water cycling”
With ‘normal cycling’ we refer to traditional sap flow. We will clarify this part and add some more information on this based on the community comment by Carel Windt.
l219-227: please use references for this paragraph
We will include some references to this paragraph, e.g.:
Nadezhdina et al., 2010. Trees never rest: the multiple facets of hydraulic redistribution
We will also add some information on phloem-xylem exchange with the appropriate references to this paragraph, based on the community comment by Carel Windt.
l265: this sentence seems out of place
We will rephrase this sentence: “A reduced connection between roots and soil might explain why deep-rooted trees typically use deeper groundwater sources during dry conditions (Barbeta and Peñuelas, 2017).
l280: be more precise here, how does it affect V and L
When V is low, the distance travelled by a diffusing water molecule due to the Péclet effect, during a given amount of time, increases. We will add this information to our revised MS.
L262-283: It would be good to first start with your results and then refer it to the drought aspect.
We prefer to keep the current buildup of this section which starts with the effects of drought and explains how these effects are coupled to BFLW.
l308-311: I would not end your conclusion with this section.
We will remove this paragraph in our revised MS.
Please check the text for consistent use of the terms ratio, composition and signature.
We will go through our text and correct these inconsistencies in our revised MS.
Figures
Figure 1: blue and green lines are not dashed. explain boxplots. Please add number of studies here (n = x).
You are right, these lines are not dashed, we will adjust our caption accordingly and include (n = 25).
Figure 2: no capital letter in “Isotopic”
This is the first letter of a sentence, as such, we prefer to keep the capital letter.
Figure 3: as a function of d18OX or d18OXG?
As a function of d18OX. This was also pointed out by the other reviewer and we will adjust our caption to make this more clear and reflect the following: Fig. 3 illustrates possible combinations of the isotopic composition and volume of an alternative water source to result in the observed offset of 2.21 ‰.
Figure 4: add explanation of MWL to legend.
We will add ‘MWL = Meteoric Water Line’ to our caption.
Figure S1 and S2: Please add number of studies here (n = x).
We will add (n = 25) to both captions.
Thank you for the comprehensive review of our MS.
Yours sincerely,
Jeroen Schreel and co-authors.
Citation: https://doi.org/10.5194/hess-2023-13-AC3
Data sets
Isotopic data of xylem water and groundwater, extracted from literature. Jeroen D. M. Schreel, Kathy Steppe, Adam B. Roddy, and María Poca http://doi.org/10.6084/m9.figshare.21940955
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