Sub-daily dynamics of urban tree xylem water and ambient vapor

Ring, Ann-Marie; Tetzlaff, Dörthe; Birkel, Christian; Soulsby, Chris

doi:https://doi.org/10.5194/egusphere-2025-1444

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https://doi.org/10.5194/egusphere-2025-1444

Preprints

04 Apr 2025

| 04 Apr 2025

Sub-daily dynamics of urban tree xylem water and ambient vapor

Ann-Marie Ring, Dörthe Tetzlaff, Christian Birkel, and Chris Soulsby

Abstract. Urban vegetation is vulnerable to rising temperatures and reduced rainfall, which reduces the cooling function of urban green spaces (UGS). The sub-daily dynamics of UGS water cycling and how this changes over the growing season remains largely unexplored due to measurement constraints. The monitoring of long-term in situ high-resolution water stable isotopes can provide valuable insights into how trees internally cycle water under different conditions. In this study, we analyzed a sub-daily (~3–4 hourly) dataset of atmospheric water vapor (δ_v) and tree stem xylem water (δ_xyl) in an urban tree stand in Berlin, Germany. We compared the diurnal (24 h) patterns of water cycling in δ_v, δ_xyland ecohydrological variables during a summer drought followed by a rewetting period in 2022. Over the summer drought, water cycling was predominantly radiation driven, with highest vapor pressure deficit (VPD) rates in the afternoons and persistantly dry soils. We found systematic behaviour in both δ_v and δ_xylsignatures during the summer drought and δ_vwas characterized by a daytime depletion in heavy isotopes, driven by local evaporation and atmospheric factors (entrainment). Daytime enrichment in δ_xyl, with maximal enrichment in afternoons, was consistent with diurnal hydroclimatic cycles, limited sap flow sourced from enriched soil water and stomatal regulation of transpiration. The trees showed lower twig water potential and sap flux relative to VPD in the afternoons, as well as stagnated night-time stem swelling, but the mature trees could overall sustain their physiological functioning. During rewetting, the UGS water cycle was precipitation driven, while potential evapotranspiration (PET) rates decreased. The systematic diurnal cycling of δ_v was mostly discontinued due to lower soil and canopy evaporation. Only δ_vjust above the grassland surface (0.15 m) showed a significant daytime enrichment hinting for ET fluxes promoted by high moisture stored in soil and vegetation surfaces and transpiration of superficially enriched soil water. δ_xylwas still characterized by significant daytime enrichment, however, with sub-daily amplitudes more than halved compared to the drought period, when hydraulic conductance was restricted. Our continuous, sub-daily dataset of δ_vand δ_xylhave the potential to help constrain ecohydrological models towards prediction of climate change impacts on UGS. Urban planning should consider a “mosaic” of urban vegetation types together with resilient species composition to maximize cooling benefits throughout the 24-hour cycle.

Received: 26 Mar 2025 – Discussion started: 04 Apr 2025

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Ann-Marie Ring, Dörthe Tetzlaff, Christian Birkel, and Chris Soulsby

Status: final response (author comments only)

RC1:
'Comment on egusphere-2025-1444', Anonymous Referee #1, 30 May 2025

Dear authors,
Find my comments in the attached pdf.

Citation: https://doi.org/10.5194/egusphere-2025-1444-RC1
- AC1: 'Reply on RC1', Ann-Marie Ring, 05 Jun 2025
  
  Response to Referee Comment 1:
  We sincerely thank the reviewer for their thorough and thoughtful evaluation of our manuscript. We are particularly grateful for the generous characterization of our work as a seminal contribution. We also appreciate the constructive suggestions provided, which we believe will help to further enhance the clarity and impact of the paper. We are confident that addressing the comments will be straightforward and will serve to better communicate the core message of our study. Below, we respond to each of the reviewer’s points in detail and offer clarifications where appropriate.
  Sincerely,
  Ann-Marie Ring (on behalf of all co-authors)
  ############################################
  Monitoring sub-daily dynamics in stable water isotopic signatures in plant xylem (𝛿𝑥𝑦𝑙) and atmospheric water vapor (𝛿𝑣) reveal marked diurnal patterns in water cycling. In their study, the authors relate observed dynamics in isotopic signatures to several environmental drivers (radiation, vapor pressure deficit, soil moisture, ...). The main insights are summarized in figure 11, highlighting that dry season conditions mark a period of large 𝛿𝑥𝑦𝑙 differences between day and night, i.e., a 38‰ daytime enrichment. Diurnal differences in 𝛿𝑥𝑦𝑙 also manifest during the wet period, but to a lower extent (approx. half the observed dry period differences). Atmospheric isotope concentrations tell an opposite story, with daytime depletion in 𝛿𝑣 (~26‰) during the dry period, and mostly no differences observed during the wet season.
  This is a very clear and convincing study providing an interesting and timely contribution to the body of research unveiling overlooked complexities in the use of stable water isotopes to assess water cycling and source contributions in plants and atmosphere. The study is very well executed. I am confident that this will be a seminal paper and will pave the way for many new and exciting research opportunities. I do have a few comments (listed below) that concern making the work more concise, diverting the current narrative to a broader one, and reducing the number of figures in the main text, which all stands to benefit the reader.
  **Thank you for this positive evaluation of our manuscript!
  Main comments
  The link between monitoring stable water isotopes in plant and atmosphere to the cooling effect and insights sought herein to inform urban green space planning must be strengthened or dropped altogether. The narrative of urban green spaces (UGS) and their vulnerability and cooling function with respect to climate change, feels rather tangential.
  ** we will clarify this in the revision.
  It remains unclear how observed patterns (diurnal and dry-wet) in stable water isotopic signatures in xylem and atmospheric exactly relate to the cooling potential of urban trees and green spaces. The mechanistic link is not presented, nor is it apparent how using an isotope assessment strategy exceeds those simply relying on directly monitoring air humidity, thermal cooling, and/or plant transpiration rates (which would be logistically easier, cheaper, and can cover a higher temporal and spatial resolution). Consequently, the UGS narrative distracts from the fascinating and likely much broader insights this study holds for plant physiology, ecology, and stable water isotope assessment. I suggest presenting a very clear and mechanistic link between the obtained insights and how this informs UGS planning and functionalities (be very concrete, although I argue that following such narrative might undercut your key findings), or the authors could tone down/step away from such narrative and focus more on the potential causes and implications/opportunities of the observed dynamics for the broader scientific community.
  ** We thank the reviewer for raising this important point regarding the unclear connection between our findings and the urban green space (UGS) narrative. We acknowledge that the mechanistic link between stable isotope dynamics and the cooling potential of urban vegetation was both over-emphasised and not clearly articulated in the initial version. We will include a more concise clarification regarding the conceptual and mechanistic framework that connects our isotopic observations with ecosystem cooling processes in urban areas, while emphasising the scientific focus on plant physiological responses and isotopic dynamics.
  
  Minor comments
  [Introduction] Several of the paragraphs in the introduction can be removed or shortened when the authors step away from the UGS narrative, making the introduction more concise.
  **Yes, we will shorten parts of the UGS narrative in the introduction for more conciseness related to plant physiology and water isotopic assessment.
  [r24] The PET abbreviation is not necessary since it is not repeated in the abstract.
  ** We will remove the abbreviation.
  [r26] Evapotranspiration should be written in full. Providing an ET abbreviation is not necessary since it is not repeated in the abstract.
  ** agreed, we will write evapotranspiration in full.
  [r44-45] Tree xylem water is the sap moving through the xylem tissue of a plant. Because multiple water flow paths exist within a plant (i.e., xylem, phloem and in-and out water storage tissue), the provided description is imprecise.
  ** thank you for this suggestion, we will include a more precise description.
  [r46] ET has been introduced in line 40, no need to reintroduce it here.
  ** We will remove this.
  [r50] Be more concrete on what is understood under “water fluxes”. Specifically, does this concern temporal patters, source water partitioning, … This is important because the stable water isotope analysis is generally used to inform on water uptake depth and the contribution of different water sources. Quantifying the amount of water lost to the atmosphere can more easily be obtained using other, logistically more convenient tools (i.e., porometer, gas exchange monitor such as a Licor or Ciras, flux towers, …).
  ** We will add a more specific explanation of sub-daily water fluxes within the urban soil-plant-atmosphere continuum informed by stable water isotope assessment.
  [r73] Given the study approach presented in this study, it is important to credit the work of Volkmann et al. (2016) who pioneered in the development of the borehole technique.
  **thank you for pointing this out, we will credit the work of Volkmann et al. (2016).
  [r103-104] The study currently does not live up to this claim. The true mechanisms underlying the emergence of these patterns and how these mechanistically link to UGS functionalities are not clear. Forecasting or trying to curtail the impact of a changing environment on UGS is therefore speculative at most, especially because the monitored trees do not show strong proof of being subject to water limitations (see comment below). Such goals require monitoring under clear water limitations as further climate change expects to acerbate droughts. Following my main comment, the authors should thus very clearly explain how their insight informs concrete UGS guidelines or should step away from this narrative.
  **We will rephrase our main research goal to emphasize broader ecological and physiological implications, while also briefly mentioning urban applications as a potential avenue for future research. We can see in retrospect that the over-emphasis on the broader context of the work in UGS has distracted from the main objectives and findings.
  [r118-119] The authors should use the more recent reference (DWD, 2023) used in their proceeding study (Ring et al. 2024).
  **We will change this.
  [Table 1] With readers only skimming the paper in mind, full names of each parameter abbreviations should be provided in the table or its caption (i.e., GRnet, WS, …). For consistency within the table, provide the units for the stable water isotopes (‰) behind the respective parameters, and remove it from the subtitle. Where relevant, consistently write: A. platanoides and B. pendula. (i.e., at ‘and’ between species, and family can be abbreviated). Finally, consider moving the table to the supplementary to make the paper more concise.
  **that’s a great suggestion. We will implement your formatting suggestions and will consistently write “A. platanoides” and “B. pendula” throughout the text. Also, we will move the table to the supplementary material.
  [r190] Provide values for a, b, and c in the supplementary for completeness and reproducibility of the study.
  **We will add this information in the supplementary.
  [Fig 1 & 2] These figures can be combined.
  ** We will combine figures 1 and 2.
  [r214] Given that samples are obtained on the same tree, and present repeated measurements, assuming independence and randomness of observations is inappropriate. A Friedman test might be more appropriate.
  **Thank you for the suggestion. We agree, a Friedman test is an appropriate alternative for more than 2 observations. We will reexplore our statistical assumptions and correct the data analysis chapter based on your comment.
  [r220] Provide the values for a and b here for completeness and reproducibility of the study.
  **We will provide the values for a and b.
  [r235] Since this specific data has been published before, best practice requires to cite that the figure is adapted from Ring et al. (2024).
  **You are correct, apologies, we will include the missing information for best practice.
  [Fig4 & 5] These are excellent figures, clearly showing the main insight from this study. Can a similar figure be provided in the supplementary for 18O? (supplementary figure S3 suggests that such data might be available).
  **Thank you, yes, we will provide figures including
  [r269] Consider adding this information to the abstract as this hammers down a very important insight which has huge implications for the interpretation of stable water isotope assessments based on point sampling. This is an excellent argument why the community should shift toward in-situ, high resolution measurements (i.e., like Volkmann et al. 2016; Kühnhammer et al. 2021, 2023), or to try to adopt a more adequate sampling protocol and/or interpretation strategy (i.e., similar to Magh et al. 2020; De Deurwaerder et al. 2020). (for the latter citation, note that part of the author’s name, i.e., ‘De’, is missing in [r465]).
  **Thank you for your suggestion; we will add this important finding to the abstract. We will further expand our discussion on the importance of in-situ high resolution measurements and adequate sampling protocols. We apologise for the miss-spelling of Mr. De Deurwaerders name and will revise this throughout the manuscript.
  [Fig 6 & Fig 7]
  - The x-axis is not great and should be redone.
  - What is the black dark line in the VPD graph? (if this is simply to highlight zero, a similar marking should be considered in all other panels, although that would confound with the soil depth line of 70cm and the maple growth line)
  **We will improve the x-axis in both figures 6 and 7, including improved VPD panels.
  - Redirect the left panels (boxplots) to the supplementary
  ** We will redirect the panels displaying antecedent conditions of Figure 6 and 7 to the supplementary.
  - Combine all lines belonging to a specific category (general ecohydrology, soil depths, sap flow & growth, and water isotopes). There is no reason to have temperature as a standalone panel.
  ** We will update the panel layout.
  - Redirect the lc-excess panel to supplementary and provide the delta 18O in the supplementary for completeness. These adjustments will support larger panels, which benefit improved readability as this figure contains a lot of information. In addition, there are interesting patterns in the figures that have not received much attention in the manuscript: (i) Figure 6 shows stronger fluctuations in 𝛿𝑥𝑦𝑙 amplitude at 1.5m compared to 2.5m, suggestive of a dampening effect at 2.5m. When looking at Figure 7, (ii) this amplitude dampening seems to be gone, however, now a consistent shift in isotope depletion establishes at 2.5m. While this setup might not support characterizing what exactly underlies these patterns, they are interesting and deserve some attention in the document.
  **These are great suggestions, thank you! We will redirect the lc-excess panel to supplementary and provide a panel displaying d¹⁸O in the supplementary for completeness. We will give more reference and include a discussion on the particularly interesting amplitudes displayed in the specific sub-daily resolved time-series plots.
  [r325-326] This is very interesting. Any reasons why this might be, and how the proposed hypothesis should be verified?
  **As the location of B. pendula was closer to a building and growing on a slope, we assume precipitation did not percolate and soil replenishment was potentially composed of more fractionated waters. We will add this relevant information to the text.
  [Fig 8.] While interesting and important, this figure could be moved to the supplementary materials. In addition, do the same observations also hold for 18O? Provide this analysis for 18O in the supplement.
  **We will move this figure to the supplementary materials and add a Figure 8b including an analysis of d18O.
  [Fig 9 & 10] These figures can be combined. Similarly to the comment on figure 8, while an important visualization, this paper can be provided as a supplement, and the analysis should be repeated with ¹⁸O.
  **Yes, we will move figures 9 and 10 to the supplementary materials and add figure 9b and 10b including an analysis of d18O.
  [Fig 11] This is an excellent and beautiful summary of the findings. Make certain that all the abbreviations are clarified in the caption (i.e., define 𝛿𝑥𝑦𝑙 , 𝛿𝑣 , and 𝛿2𝐻) to benefit readers skimming the paper.
  **We highly appreciate the positive and constructive feedback. We will clarify all abbreviations.
  [r383-388] That midday water potential at the twig level is more negative than the morning leaf water potential is expected from a plant physiological perspective. In the morning, the driving force of water movement through the plant is low, as stomates are generally closed with no photosynthetic activity as there is no light. At midday, the need for water is greater because sunlight allows photosynthesis. This might suggest water limitations if leaf (or twig) water potentially drops below the P50 value, which could trigger stomatal closure. However, having a minimal twig water potential around -1.2MPa, and no clear reduction in sapflow activity (10L/h), there seems to be no obvious indication that the trees suffer water limitation. The observed isotopic signature dynamics suggest that the plant water needs are met by tapping different soil water sources. Similarly, a shift between radiation, sapflow, and VPD have been observed in the plant physiological research and are not sufficient proof that a tree experiences drought stress. Additionally, it is unclear how the provided reference (Kraemer and Kabisch, 2022) is in any way supporting such a statement, as that study does not monitor sapflow. In conclusion, the presented physiological/mechanistic drivers of the observed isotopic trends within the trees are not well laid out, obscuring a clear link between observations and how this might inform UGS decision making. This paragraph should be made stronger and provided arguments should be supported with plant physiological theory.
  **Thank you for this clear explanation and summary. We will rephrase this paragraph, including evidence from plant physiological theory, remove inadequate discussions on drought stress and provide adequate reference.
  [paragraph 5.3] Following my main comment, it remains unclear how the authors envision their presented observations to guide UGS planning and management efforts. For instance, in [r594] the authors’ state:’ Our findings provide novel insights on the cooling potential of urban vegetation.’. What specific insights are implied here, and, how exactly do the presented observations link to USG cooling capacity? This sentence remains rather cryptic. Besides, restricting the importance of this study in support of UGS planning almost feels like an injustice to the likely broader importance of the observations for the fields of stable water isotope assessments, and plant physiology and ecology. As such, I suggest refocusing the storyline to reflect a broader picture, which would (i) attract a wider scientific interest, and (ii) would strengthen conveying the key observations to the reader.
  ** We agree that the current phrasing in the manuscript (e.g., in [r594]) may have overstated the direct applicability of our observations to UGS cooling potential, without providing a sufficiently detailed mechanistic link. While isotopic signatures themselves do not directly quantify cooling, they provide valuable information on plant water use dynamics, which underpin vegetation-driven cooling processes. This connection will be described more explicit and appropriately cautious in tone throughout the manuscript. We will reduce the emphasis on immediate implications for UGS planning, in line with the reviewer’s suggestion. Instead, we briefly highlight potential future applications of isotopic monitoring in urban environments as a complementary tool to assess water source dynamics, especially under heterogeneous soil and microclimatic conditions typical of urban settings. We will expand the discussion for a broader scientific context to underscore the relevance of our results for understanding plant hydraulic functioning, ecohydrological partitioning, and stable isotope method development. This broader framing will align with the reviewer’s insight that the study’s primary contributions lie in advancing knowledge in plant physiology and isotope ecology.
  
  Citation: https://doi.org/10.5194/egusphere-2025-1444-AC1
RC2:
'Comment on egusphere-2025-1444', Anonymous Referee #2, 07 Jun 2025

Dear authors, please find my comments in the pdf attached.

Citation: https://doi.org/10.5194/egusphere-2025-1444-RC2
- AC2:
  'Reply on RC2', Ann-Marie Ring, 16 Jun 2025
  Response to Referee Comment 2:
  We thank the reviewer for their thoughtful and constructive feedback. We appreciate the positive recognition of the diverse results and acknowledge the concern regarding the clarity of the overall narrative. We will revise the manuscript to improve focus and flow, while acknowledging the limitations of our study, including reorganizing and shortening the Materials and Methods section as suggested. Below, we address the reviewer’s comments in detail and provide clarification where needed.
  Sincerely,
  Ann-Marie Ring (on behalf of all co-authors)
  ############################################
  The here presented study by Ring et al. assesses the impact of plant water use and other environmental drivers on the dynamics of atmospheric water vapor isotope signatures in an urban landscape, comparing two distinct periods: a drought and a rewetting period.
  The study was well conducted and comprehensive, including many interesting and different results, which make it challenging to follow the story’s “red string”. Based on figure 11 (which is a really nice summary), I suggest streamlining your whole story: what is needed in the main text to understand the patterns observed in figure 11.
  The link to urban green spaces is interesting, but the relevance of isotope measurements for assessing the cooling impact of vegetation seems a bit far-fetched. Other parameters, such as actual transpiration flux, provide better means to determine the actual cooling effect on the studied area.
  I recommend reorganizing and revising your manuscript based on the Fig. 11 so that you focus primarily on results that explain or support your findings there. It seems that large parts of the Material and Method section are based on the author’s previous studies, resulting in unexplained acronyms throughout the main text and paragraphs not optimally organized. I suggest rewriting and reorganizing the Material and Method section to make it shorter and more concise, ensuring that important information is included in the main text while moving or deleting unnecessary details.
  11 figures in the main text is too much, consider merging or moving some to the supplement. Moreover, you only measured two trees. This limitation should be mentioned. Given the limited representative of two single trees and a patch of grass, caution needs to be taken to transfer this to larger green spaces. This point needs to be raised as well (see e.g., line 600).
  All in all, a very nice study! Please find below my line-by-line comments.
  ** Thank you for the constructive and encouraging review! We agree that reorganizing the manuscript based on Figure 11 will strengthen the clarity and coherence of the narrative, and we will streamline the main text accordingly. We also acknowledge the need to revise the Data and Methods section for conciseness and clarity, and will ensure all acronyms are introduced properly and relevant information is prioritized. While we recognize the limitations of our small sample size of two trees, we will address this explicitly in the discussion to clarify the scope and transferability of our findings. We appreciate your suggestion to refocus the link between isotopic data and vegetation cooling and will revise this interpretation to more clearly delineate the scope and implications of our approach.
  
  Line-by-line comments
  Title
  I would suggest here a better link to the "water stable isotope" topic, as this is the focus
  isotope dynamics?
  **We will amend the title to include a link to water stable isotopes.
  Abstract
  L15: Specify "natural summer drought." How long was the drought? When did the rewetting occur?
  
  **We will add this information to the abstract. Like included in the Data and Methods section.
  L18: "dv values were characterized."
  
  ** We will change this.
  L19: Consider "i.e., entrainment" (perhaps better phrasing).
  
  ** We will improve the phrasing.
  L19: add "values" or similar when using delta abbreviations (e.g., "enrichment dxyl values")
  
  **We will include this throughout the manuscript.
  L20: "enriched soil water at the topsoil" ?
  
  ** Thank you, we will specify this accordingly.
  L24: How was PET during the summer drought? You only mention it for the rewetting period.
  
  ** We will add this information about PET during the summer drought to the abstract.
  L26: ET was not introduced yet (only PET).
  
  **We will change this.
  The abstract could highlight more the main results.
  **We will amend the abstract to better highlight the main results based on Figure 11.
  Main text
  L73: See also publications from Till Volkmann and Markus Weiler (see references below).
  
  **We will add these important publication references as suggested.
  L93: It would be more interesting if it was the hottest summer in Berlin.
  
  **We will specify the information as for the weather conditions during summer in Berlin, 2022, were among the top 5 hottest since recording began (2024 was hottest).
  L97: here it is written again xylem and water vapor isotope, I would stick to dxyl and dv values once introduced (especially in the same section)
  
  **We will change this and stick to the introduced abbreviations.
  L101: would also talk about sub-daily values here
  
  ** We will rephrase this accordingly.
  L110: Add the full name for CRDS.
  
  ** We will include the full name cavity ring-down spectroscopy.
  L112: Write the full name for SE.
  
  **We will write southeast in full.
  L121: Compare the amount of rain received during the study period to the long-term mean.
  
  **We will include the long-term mean of precipitation amount according to the German weather service.
  Lines 124-129: Merge these paragraphs with lines 113-116, as they both discuss the study site.
  
  ** We will merge these paragraphs.
  L145: maybe add the distance from the rooftop to your site.
  
  **The distance from rooftop to the study site will be added.
  L167: Introduce CRDS once, including the full name, company, etc.
  
  **We will add the information to introduce cavity ring-down spectroscopy (CRDS; L2130-i, PICARRO, INC., Santa Clara, CA).
  Table 1: Explain the acronyms.
  **As per suggestion of reviewer 1, we will move this table to the supplementary to make the paper more concise. Full names of each parameter abbreviation will also be provided then.
  
  Figure 2: Consider writing "Atmosphere Tubing" to avoid confusion, as technically, xylem tubing also samples vapor. Mention standards in the caption.
  **We will improve the naming in the legend of Figure 2 and mention the standards in the caption.
  L159: Provide details on how you accounted for temperature dependencies and corrected them (e.g., Wassenaar 2008, Haberstroh 2024).
  
  **We will include this information based on the suggested studies.
  L198: Include the spatial resolution of the sampling (e.g., depths).
  
  ** We will include information on the spatial resolution of soil sampling.
  L206: Introduce SPAC
  
  ** We introduced the SPAC in the introduction of this manuscript (L57) and will provide more information about it in sub-chapter 3.5.
  L218: "the line..."
  
  **We will correct this.
  L240: Compare to long-term mean.
  
  **We will provide this information.
  L241: mm in summer? and provide the long-term mean.
  
  ** We will provide this information.
  The order is confusing: first, it mentions dryness with some numbers, then introduces temperature and PET values, and finally mentions precipitation values in l245 (which is an indicator for dryness). Consider reorganizing for clarity.
  
  **We will reorganise this to improve the clarity.
  L268: Specify which tree.
  
  **We will specify this.
  One possible reason for the enrichment in the xylem during the day could be water loss through the bark (see e.g., Lintunen et al. 2021). Do you have xylem/leaf water data from grassland vegetation? Can you rule out that the daily cycle in dD is not affected or driven by temperature/concentration changes in your isotope measurements? How much did the concentration change during the day in the trees? Was full saturation always reached? (You can check this by calculating the saturation point for the respective temperature and comparing it to your value.)
  The isotopic signature of transpiration, which significantly influences deltav values, can deviate substantially from the xylem isotopic signature due to stomatal regulation and non-steady state transpiration (see Simonin et al. 2013; Dubbert et al. 2017; Kübert et al. 2023). During drought and periods of high VPD, stomates may be closed. Since the isotopic signature of transpiration was not measured directly, discuss the possible deviation between xylem and transpiration isotopic patterns.
  **Thank you for this clear explanation and concise methodological suggestion. Unfortunately, we did not measure plant stable water isotopes of the grassland vegetation. To correct for isotopic offsets and vapour concentration dependency, we calculated for all the xylem data the values of temperature dependent equilibrium fractionation from vapor to liquid with the correction formulated by Majoube (1971). Temperatures were measured within the boreholes. During the automated calibration, water vapor concentrations and isotopic compositions of known standards in the headspace of the glass containers were measured and linear regressions of temperature dependency slopes added. We also included regular checking for stable values of both standards to avoid headspace depletion. We have checked the sub-daily water vapor concentration change and calculated the saturation point for the respective temperatures. Water vapor concentration was always close to full saturation during the measurements. Thus, we can rule out evaporation effects and kinetic fractionation of measurements – we will add this insight to the method section. Further, we will give reference to water loss through the bark as a possible reason for xylem water enrichment and further explain the possible deviation between xylem and transpiration isotopic patterns, especially during drought, in the discussion chapter.
  L278: Add p-value.
  
  **We will add the p-value.
  Figure 6: VWC of 5-7% is very low; were the sensors calibrated? Why is the VWC so low between 6 and 20 cm? What happened on 28.7. at 00:00?
  ** Yes, the values are very low, which is typical for sandy soils during drought conditions. The upper 5 cm lose more water through ET processes during the chosen timeframe. In Figure 7, you can see the typical patterns, when precipitation changes soil moisture in the different depths. The soil moisture sensors were calibrated in the factory with 3% precision (included in Table 1). The drop in the lc-excess values after 28.7. at 00:00 can be explained by stronger entrainment processes during that night, meaning an intensified turbulent flux of water vapor that occurred between the relatively dry air in the free troposphere above and the moister air within the surface boundary layer (cf. Lai & Ehleringer, 2011; Lee et al., 2006).
  -> Lai, C.-T. and Ehleringer, J. R.: Deuterium excess reveals diurnal sources of water vapor in forest air, Oecologia, 165, 213–223, doi:10.1007/s00442-010-1721-2, 2011.
  -> Lee, X., Smith, R. and Williams, J.: Water vapour 18O/16O isotope ratio in surface air in New England, USA. Tellus B, 58: 293-304. https://doi.org/10.1111/j.1600-0889.2006.00191.x, 2006.
  L287: Note that you do not show radiation. Provide numbers for this "drive."
  
  ** We will provide numbers for the amounts of radiation input.
  L289: Did you do a zero lcorrection for sap flow? Unit missing for lc-excess.
  
  ** We provide the following information in the methods section: “Sap flow rates were assessed via the monitored sap velocity (heat ratio method by Marshall (1958) with the softwares implexx (SFM-4 meters, UGT) and Sap Flow Tool (SFM1, ICT International) including data of sap wood, heart wood and bark depths from drilled tree cores.” We will provide the missing unit for lc-excess throughout the text and Figures.
  Consider merging Figure 6 and 7 to save space, as the legend can be used for both. Also Figure 9 and 10.
  **Thank you for these important suggestions for figure improvements. We will consider merging Figures 6 and 7. As per suggestion of reviewer 1, we will improve the x-axis of both figures, including improved VPD panels and also redirect the panels displaying antecedent conditions of Figure 6 and 7 to the supplementary. Another recommendation of reviewer 1, which follows your suggestions, is that we will move Figures 8, 9 and 10 to the supplementary material and also add our analysis of d18O there to make the paper more concise. In this context, we will try to merge Figures 9 and 10.
  L321-326: This paragraph is very descriptive; could you add some numbers?
  
  ** We will add numbers as evidence for the descriptive part for the paragraph
  Figure 11: Consider summarizing the day-night change. How was day/night defined? Add information to the legend.
  **We will add information about our definition of day (8 am – 8 pm) and night (8 pm – 8 am) to the legend and we will thoroughly summarize the day-night change including numbers in the paragraph 5.1 which introduces Figure 11 to make the findings clearer.
  L381: that is very general, maybe: "Sub-daily changes in isotopic signatures of... and … "
  
  **Yes, we will improve this sentence.
  L384: Midday LWP is usually more negative than in the morning, which is not necessarily related to drought.
  
  **This is an important comment. Reviewer 1 added similar suggestions to this part. We realize, that we have to rephrase this paragraph, including evidence from plant physiological theory, remove inadequate discussions on drought stress and provide adequate reference.
  L385: "Indicating stomatal control which..."
  
  **We will include this.
  L565: Add "direct measurement of transpiration."
  
  **We will add this.
  L575: "which did not infiltrate"
  
  **We will correct this.
  L582: maximum
  
  **We will change this.
  L600: Include "and sample size," as the studied area and the number of plants were quite limited.
  
  **Thank you, we will add this important aspect.
  General comments:
  Check for British vs. American English spelling, e.g., "vapor" vs. "vapour."
  
  Write in situ or in-situ
  
  Consistently use "water stable isotopes" (see l160, "stable water isotopes").
  
  Define delta notation.
  
  Use "value" or a similar term with deltav or deltaxyl
  
  lc-excess unit is missing
  
  **Thank you for these important general comments. We will provide consistent spelling according to HESS regulations, use the term “value” for the istotopologues and add the missing lc-excess unit.
  References
  Dubbert, M., Kübert, A., and Werner, C.: Impact of Leaf Traits on Temporal Dynamics of Transpired Oxygen Isotope Signatures and Its Impact on Atmospheric Vapor, Front. Plant Sci., 8, 2017.
  Haberstroh, S., Kübert, A., and Werner, C.: Two common pitfalls in the analysis of water-stable isotopologues with cryogenic vacuum extraction and cavity ring-down spectroscopy, Anal. Sci. Adv., 5, 2300053, https://doi.org/10.1002/ansa.202300053, 2024.
  Kübert, A., Dubbert, M., Bamberger, I., Kühnhammer, K., Beyer, M., van Haren, J., Bailey, K., Hu, J., Meredith, L. K., Nemiah Ladd, S., and Werner, C.: Tracing plant source water dynamics during drought by continuous transpiration measurements: An in-situ stable isotope approach, Plant Cell Environ., 46, 133–149, https://doi.org/10.1111/pce.14475, 2023.
  Simonin, K. A., Roddy, A. B., Link, P., Apodaca, R., Tu, K. P., Hu, J., Dawson, T. E., and Barbour, M. M.: Isotopic composition of transpiration and rates of change in leaf water isotopologue storage in response to environmental variables, Plant Cell Environ., 36, 2190–2206, https://doi.org/10.1111/pce.12129, 2013.
  Volkmann, T. H. M. and Weiler, M.: Continual in situ monitoring of pore water stable isotopes in the subsurface, Hydrol. Earth Syst. Sci., 18, 1819–1833, https://doi.org/10.5194/hess-18-1819-2014, 2014.
  Volkmann, T. H. M., Haberer, K., Gessler, A., and Weiler, M.: High-resolution isotope measurements resolve rapid ecohydrological dynamics at the soil–plant interface, New Phytol., 210, 839–849, https://doi.org/10.1111/nph.13868, 2016.
  Wassenaar, L. I., Hendry, M. J., Chostner, V. L., and Lis, G. P.: High Resolution Pore Water δ 2 H and δ 18 O Measurements by H 2 O (liquid) −H 2 O (vapor) Equilibration Laser Spectroscopy, Environ. Sci. Technol., 42, 9262–9267, https://doi.org/10.1021/es802065s, 2008.
  
  Citation: https://doi.org/10.5194/egusphere-2025-1444-AC2

Ann-Marie Ring, Dörthe Tetzlaff, Christian Birkel, and Chris Soulsby

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https://doi.org/10.5194/egusphere-2025-1444-supplement

Ann-Marie Ring, Dörthe Tetzlaff, Christian Birkel, and Chris Soulsby

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Short summary

During summer drought, a clear sub-daily cycling of atmospheric water vapour isotopes (δ_v) and plant xylem water isotopes (δ_xyl) was observed. δ_vdaytime depletion was driven by evaporation and local atmospheric factors (entrainment). δ_xyldaytime enrichment was consistent with limited sap flow and stomatal regulation of transpiration. Water limitations during drought in urban trees are visible in δ_xyland ecohydrological data. This sub-daily dataset can help constrain ecohydrological models.


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