Regulation of transpiration water age by plant root-rock fissure interactions in epikarst

Zheng, Xukun; Li, Yuan; Zhou, Qiuwen; Luo, Zidong; Chen, Yi; Zhang, Jun; Liu, Wenna

doi:10.5194/egusphere-2025-4366

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https://doi.org/10.5194/egusphere-2025-4366

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12 Nov 2025

| 12 Nov 2025

Status: this preprint is open for discussion and under review for Hydrology and Earth System Sciences (HESS).

Regulation of transpiration water age by plant root-rock fissure interactions in epikarst

Xukun Zheng, Yuan Li, Qiuwen Zhou, Zidong Luo, Yi Chen, Jun Zhang, and Wenna Liu

Abstract. Under climate change, rock water becomes an important source of transpiration water for plants. However, in karst regions, rock fissure water and rooting depth are often not adequately considered in existing determination systems. This study conducted monthly field sampling over a year in a rock-dominated subtropical karst region, focusing on deep-rooted trees (Ailanthus altissima and Juglans regia) and shallow-rooted trees (Zanthoxylum bungeanum and Eriobotrya japonica). It integrated stable isotope tracing, the piecewise isotope balance method (quantifying the replenishment ratio of root-zone water), the piecewise linear mixing water age model (estimating the mean residence time of water utilized by plants), and geological drilling techniques. The results showed that rock fissure and root depth regulated the root zone water recharge rate of plants: deep-rooted trees (32 %) were lower than shallow-rooted trees (44.3 %) in the rainy season, while deep-rooted trees (10.4 %) were higher than shallow-rooted trees (3.8 %) in the dry season. Differences in water recharge to the root zone affected the age of plant transpiration: deep-rooted trees (46.4 d) were higher than shallow-rooted trees (35.1 d) during the rainy season, while the opposite was true during the dry season (deep-rooted trees: 139.6 d; shallow-rooted trees: 128.5 d). In addition, geological boreholes revealed that a large number of roots were distributed in rock fissures at a depth of 1.8–3.2 m below ground. The study showed that rock fissures are not only important channels for the formation of preferential flow in the rainy season, but also interact with the root system to regulate the water recharge in the root zone of plants and thus influence the change of transpiration water age, which provides a new perspective to understand the complex hydrological processes in karst areas.

Received: 05 Sep 2025 – Discussion started: 12 Nov 2025

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.

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Xukun Zheng, Yuan Li, Qiuwen Zhou, Zidong Luo, Yi Chen, Jun Zhang, and Wenna Liu

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RC1:
'Comment on egusphere-2025-4366', Anonymous Referee #1, 24 Nov 2025 reply
This manuscript comprehensively employs multiple techniques (stable isotopes, PIB method, water age model, geological drilling, and ERT) with the aim of revealing the interaction mechanism of "rock-root-water" in the karst critical zone. The research question holds significant scientific value, and the experimental design is generally reasonable. The obtained data effectively support the main conclusion that "rock fractures are the core for regulating water supply and transpiration water age in the root zone". However, the manuscript needs further improvement in terms of logical fluency, terminological accuracy, and the depth of explanation for some core concepts.

The statements of the two scientific hypotheses in the abstract and introduction are slightly different. It is recommended to ensure that the statements of the two scientific hypotheses are completely consistent throughout the entire text (abstract, introduction, and discussion).

L47: The core indicator "transpiration water age" was directly used, but it was not defined and its ecological hydrological significance was not explained in the introduction. Before introducing the research question, define "transpiration water age" in 1-2 sentences and point out: "Understanding transpiration water age is crucial for revealing whether plants utilize current precipitation or stored 'old water' to cope with environmental changes, and it is the key to analyzing the temporal dimension of plant water use."

L99-101: The statement of Hypothesis (i) is slightly long and the expression "exploring water sources" is ambiguous. It is recommended to refine and strengthen it: make the hypothesis more direct and testable.

L232: Why is "root zone water storage" equated to "rock fissure water" to calculate the recharge rate β? This deviates from the traditional application of the PIB method in soil-dominated systems. Suggestion: Add a theoretical argument at the beginning of Section 2.4 or in the methods section.

Some paragraphs read like "data recitation" and fail to highlight the main contradictions. In Section 3.2 of the original text: "The δ²H of deep-rooted and shallow-rooted tree xylem water ranged from -65.9‰ to -57.6‰ and δ¹⁸O from -8.5‰ to -6.8‰ in the rainy season; in the dry season, they were -59.7‰ to -52.9‰ and -7.3‰ to -5.7‰ respectively."

The results section contains too much explanatory content. For example, in Section 3.3, "Overall, the hydrological connectivity of deep-rooted trees in the rainy season was lower than that of shallow-rooted trees, while the opposite was true in the dry season, which may be related to the depth of the plant roots and the water utilization strategies." It is recommended to strictly distinguish between "results" and "discussion". The results section should objectively state the discovered data and phenomena, while moving all explanations, comparisons, and mechanism analyses to the discussion section. This will make the manuscript structure clearer and the argument more powerful.

Lack of clear statistical information. It is recommended to specify the statistical test methods: indicate in the methods section or figure captions what statistical tests were used (e.g., one-way ANOVA with Tukey's HSD post hoc test).

L385: What does "hydrological disconnection" mean?

L454-472: The author's speculation and description of root depth in this paragraph are too rough and the logic is confusing. It is suggested to further clarify the context and present a clearer expression.

L522-529: "However, within the context of climate change—particularly in regions with limited water storage capacity like karst areas—this "old water" sustains transpiration during dry seasons, thereby creating a critical survival window for plants to await subsequent precipitation recharge." The discussion about "old water" and the vulnerability of the ecosystem needs to be balanced.

At the end of the discussion, there is no strong review and confirmation of whether the two scientific hypotheses have been verified. It is suggested to add a summary paragraph: In Section 4.3, it is clearly stated: "The results of this study fully support the two scientific hypotheses initially proposed. ... " 。 This jointly proves that the interaction between rock fissures and root depth is a regulatory factor for the water utilization of karst plants...

L557-560: The manuscript mentions that the study area is dominated by rocks (with a soil thickness of approximately 40 cm). I do not recommend that the author stratify such a shallow soil. Instead, the soil sampling description should be adjusted to an integrated single-layer approach, represented as soil or overlying soil. This is because when quantifying plant water sources through the Bayesian mixing model, excessive division of water sources would weaken the importance of rock fissure water emphasized in the manuscript.

Make sure that the text in the charts (especially the axis labels and legends) is of appropriate size and legible in the PDF version. For instance, can the axes and images in Figure 9 be presented more clearly through other inversion software?

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Citation: https://doi.org/10.5194/egusphere-2025-4366-RC1
- CC2: 'Reply on RC1', Yuan Li, 19 Jan 2026 reply
  
  We sincerely appreciate the valuable comments and revision suggestions provided by the reviewer. We will respond to each point in the revised manuscript and provide explanations, and compile them into a PDF document.
  
  Reply
  
  Citation: https://doi.org/10.5194/egusphere-2025-4366-CC2
CC1:
'Comment on egusphere-2025-4366', su yuming, 19 Jan 2026 reply

This study focuses on how the interaction between plant roots and rock fissures in the karst epikarst regulates root-zone water replenishment and thereby influences transpiration water age—a question of significant ecohydrological importance in the context of climate change. However, I believe several issues remain:

1、Throughout the manuscript, superscript and subscript formatting is inconsistent. This is particularly noticeable in isotope notation (e.g., δ²H (δ²H), δ¹⁸O (δ¹⁸O)), where many characters are not properly formatted as superscripts/subscripts (e.g., line 49).

2、The manuscript contains excessive and incorrect usage of long dashes (—). These often appear in unnecessary or inappropriate positions in scientific writing. Please carefully revise all dash usage (e.g., line 31).

3、Table 1: Are the four plant species of the same age? Please supplement with crown width data. Currently, the DBH of the four species shows significant variation—does this reflect a distinction between trees and shrubs? Additionally, when is the leaf-fall period for the two deciduous species? During sampling in December and January, were leaves still present on the plants?

4、Regarding “2.2.2 Experimental Sample Collection,” the collection procedures for different sample types could be further subdivided for clearer readability.

5、Lines 188–191: The authors only state that no isotopic shift occurs when the extraction volume exceeds 1.5 mL. Is there evidence to support this? If any shift remains, correction should be applied.

6、Line 302: What does “SD” stand for? It is recommended to define this in the Methods section.

7、Figure 9 only describes using ERT to invert subsurface water content but does not specify which software was used. Please include this in the Methods section.

8、Unify the format for citing figures throughout the manuscript (e.g., line 400 vs. line 406).

9、Some sentences are overly long and could be broken down. For example, “the depth of the roots further determines the seasonal variation of the water replenishment rate (β) in the root zone.” could be revised to “Root depth is a key factor in regulating the seasonal variation of water supply rates within the root zone.”

The overall quality of this paper meets the standards of high-level academic achievements, and it is recommended for acceptance after revisions.

Reply

Citation: https://doi.org/10.5194/egusphere-2025-4366-CC1
- CC3: 'Reply on CC1', Yuan Li, 19 Jan 2026 reply
  
  Thank you for your constructive suggestions. We will respond to them and provide explanations in the revised manuscript and compile it into a PDF file.
  
  Reply
  
  Citation: https://doi.org/10.5194/egusphere-2025-4366-CC3
RC2:
'Comment on egusphere-2025-4366', Anonymous Referee #2, 28 Jan 2026 reply

The manuscript entitled Regulation of transpiration water age by plant root–rock fissure interactions in epikarst investigates the root-zone water recharge ratio and transpiration water age in regions where bedrock fissure water serves as an important plant water source, and further compares water-age differences between deep-rooted and shallow-rooted plant species. The topic is potentially novel. However, the current study has substantial weaknesses in experimental design, lacks key methodological descriptions, and provides limited and insufficient data analysis. In addition, the Introduction does not align with the results presented, which makes it difficult to evaluate and accept the current Discussion. I recommend major revision. Specific comments are provided below.

Major Concerns
(1) The Introduction states that plant access to bedrock fissure water is influenced by bedrock weathering degree, fissure density, and fissure depth. The author also repeatedly emphasizes the regulation effect of the interactions between root distribution and rock fissure on root-zone water recharge and transpiration water age. However, the manuscript provides neither root distribution data for the studied species nor structural parameters of the weathered bedrock. More importantly, there is no statistical analysis testing interaction or regulation effects, making the claimed “interaction” unsupported. Consequently, the results are seriously disconnected from the Introduction. In addition, the two stated scientific hypotheses appear problematic: for hypothesis (i), plant root water uptake is a consumption term of the root zone water, and should not be framed as a key process controlling root-zone recharge; for hypothesis (ii), testing this hypothesis is unnecessary because the root-zone recharge ratio (β) is a key parameter used to calculate transpiration water age, and therefore is inherently expected to be a dominant driver.

(2) The study treats bedrock fissure water as a plant water source. However, based on the sampling distribution shown in Fig. 2b, plant sampling sites and fissure-water sampling sites are far apart. From the map scale, the farthest fissure-water site appears to be more than 1 km from the plant sampling locations. Given the likely horizontal spatial heterogeneity in fissure-water isotopes, this design can lead to a serious mismatch between plant water and fissure water, making the inferred plant water source contributions unreliable.

(3) The appendix provides sap flow data, yet the Methods lack any description of sap-flow measurement and processing. Moreover, the study includes four plant species, but sap flow is reported for only one species. The appendix also includes MixSIAR-based source contribution results, but the manuscript does not describe model settings, nor does it explain how isotopic data from different sources were aggregated for model input. In addition, Fig. 2c shows the eddy-covariance tower and the caption mentions evapotranspiration measurements, but the manuscript does not present corresponding results. Finally, Fig. 9 uses ERT inversion to characterize soil and bedrock water storage; however, the Methods do not specify which of the four survey line correspond to which locations on the sampling map, nor how the two survey lines per plot were selected (what criteria or principle guided the survey line layout).

(4) There is no dedicated “Statistical analysis” subsection in the Methods, and the Results largely read as a compilation of observations. For both root-zone recharge ratio and transpiration water age, the manuscript does not conduct significance tests among species, nor does it analyze how these parameters respond to environmental drivers—therefore the subsequent Discussion is not well supported. Additionally, the current PIB modeling framework considers only precipitation and plant xylem water as end-members, while treating soil as a “black box.” This creates a key logical gap: parameters with strong vertical heterogeneity (e.g., root distribution, soil water storage, and fissure-water occurrence) may be spatially mismatched with the estimated root-zone recharge ratio and transpiration water age. The authors must clarify and justify this issue explicitly in the Methods.

Other concerns
(1) Line 45: “However” is used as a transition, but there is no logical contrast between the two sentences.
(2) Fig. 5: Scatter points for different plant water sources and plant water overlap heavily, and no clear pattern is visible.
(3) Fig. 6: The soil moisture panel lacks a y-axis. Also, according to the Methods, soil-water data should be discrete, yet the figure shows continuous curves—what statistical basis supports this representation?
(4) Fig. 8: A significance test is reported, but the specific test method is not stated.
(5) Line 501–502: Preferential flow is a concept developed for soils; applying “preferential flow” directly to fissure flow is questionable.
(6) Line 503–504: Attributing β = 0 to low rainfall and high evapotranspiration lacks supporting evidence.
(7) Line 504–506: The manuscript does not quantify water age of water at different depths in the root zone; therefore, the claim that deep-rooted species can utilize “older water” is not supported by the presented analyses.

Reply

Citation: https://doi.org/10.5194/egusphere-2025-4366-RC2
- AC1: 'Reply on RC2', Yuan Li, 29 Jan 2026 reply
  
  We sincerely appreciate the valuable comments and revision suggestions provided by the reviewer. We will respond to each point in the revised manuscript and provide explanations, and compile them into a PDF document.
  
  Reply
  
  Citation: https://doi.org/10.5194/egusphere-2025-4366-AC1
RC3:
'Comment on egusphere-2025-4366', Anonymous Referee #3, 28 Jan 2026 reply

This MS explores how root-rock fissure interactions in epikarst regulate root-zone recharge and transpiration water age, combining stable isotopes with mixing models and supporting drilling/ERT observations. The research question and dataset are valuable; however, several issues related to internal consistency, metric/definition mismatch, and reproducibility may directly weaken the robustness of the main conclusions.
Specific comments:
L26–L30 (Abstract) / L546–L549 (Conclusion): The text states “the opposite was true during the dry season”, but the numbers in parentheses still indicate water age for deep-rooted trees > shallow-rooted trees (e.g., 139.6 d vs 128.5 d), i.e., not a reversal. Please verify and make the statement consistent throughout the manuscript: if the intended meaning is that deep-rooted trees still have older water age in the dry season, revise/remove “opposite”; if a reversal is truly intended, the numbers, statistical tests, and discussion logic must be updated accordingly.
L228–L229 (Section 2.3) / L L248 (Section 2.4): The reported analytical precision for δ18O and the “error threshold” used for setting β to zero are inconsistent, which may affect β classification. Section 2.3 reports δ18O precision of ±1‰, while Section 2.4 uses “δ18O error ±0.3‰” as the β=0 threshold. Please check whether these values are misreported or swapped (e.g., δ2H vs δ18O).
L232–L265 (Section 2.4, Eq. 2–4) / L398–L403 (Discussion) / L556–L562 (Appendix A, Fig. A1): MixSIAR and the PIB (β) framework do not report the same type of “ratio”. β is closer to the fraction by which recent precipitation updates the plant-available storage in the root zone, whereas MixSIAR outputs the proportional contributions of different water pools to plant uptake; these are not equivalent. Please clarify β’s physical meaning, end-member choices, units/truncation rules, and explain why “precipitation-dominated updating (recharge)” and “fissure-water-dominated uptake (source)” can both hold in karst systems. Also provide minimal MixSIAR methodological details (source grouping and temporal aggregation, priors, MCMC settings, convergence diagnostics, and credible intervals), and state whether the MixSIAR input time window is consistent with the β/water-age segmentation used in the PIB framework.
L279–L286: Incorrect figure citations make it difficult for readers to trace the results. Section 3.1 cites relative humidity and soil moisture as Fig. 2b/2c, but Fig. 4(b) corresponds to ET and relative humidity, and Fig. 4(c) corresponds to soil moisture. Please check and correct the panel citations in Fig. 2 vs Fig. 4.
L323–L324 (Section 3.3): The text mentions “walnut, willow and loquat”, but “willow” is not among the study species, suggesting a typo.
L427–L439 (Fig. 9 caption): The caption mixes terminology such as “electrical conductivity profiles” and “low resistivity anomaly”, and directly interprets colors as “higher/lower water content” or “reservoirs”. Please unify the terminology (choose either resistivity or conductivity), add units and a color bar, and provide the basis and uncertainty for inferring water content/“reservoirs” from electrical properties (or explicitly state that the figure is only a qualitative/relative indicator rather than quantitative water content).
L524 (Section 4.3): Citation format needs standardization. “Roberts and Hanan et al., 2025” (L523) should be revised to “Roberts and Hanan, 2025” (or “Roberts et al., 2025”, depending on the actual author list).

Reply

Citation: https://doi.org/10.5194/egusphere-2025-4366-RC3
- AC2: 'Reply on RC3', Yuan Li, 29 Jan 2026 reply
  
  We sincerely appreciate the valuable comments and revision suggestions provided by the reviewer. We will respond to each point in the revised manuscript and provide explanations, and compile them into a PDF document.
  
  Reply
  
  Citation: https://doi.org/10.5194/egusphere-2025-4366-AC2

Xukun Zheng, Yuan Li, Qiuwen Zhou, Zidong Luo, Yi Chen, Jun Zhang, and Wenna Liu

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Short summary

In rocky landscapes with thin soils, plants have developed clever strategies for finding water. We found that rock fissures not only regulate the root zone recharge state of plants with different root depths, but their stored water is also an important source of water for plant transpiration. This reveals an important and often overlooked water source for maintaining vegetation in rocky environments, providing new insights for ecological restoration studies.


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