the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 01 Sep 2025
The role of rock fractures on tree water use of water stored in bedrock: Mixing and residence times
Abstract. The processes of tree water uptake in karst environments are poorly understood. One of the main challenges to improved understanding is the complex interaction between soil water and bedrock water, especially in systems characterized by rock fractures. While some studies have highlighted the potential importance of fractured bedrock as a water source for plants, few have quantitatively assessed the sources, residence times, and seasonal dynamics of tree water uptake from both soil and rock fracture compartments. Here, we combine stable isotope tracing, a Bayesian mixing model (MixSIAR), and hydrometric monitoring to quantify the contributions and mean residence times (MRT) of soil and rock water accessed by trees across seasons. We use a four-compartment sampling framework that distinguishes between soil water (mobile and bulk) and rock water (fracture and infilled fracture). Our results reveal clear seasonal shifts in plant water sourcing: during the peak rainy season, mobile soil water (mean MRT = 88 days) dominates uptake (mean contribution 41 %), whereas in late growing season, trees increasingly rely on bulk soil water (mean MRT = 95 days, mean contribution 55 %). Strikingly, in early spring, trees in fracture-rich areas exhibit the highest reliance on rock water (mean MRT = 113 days, mean contribution 69 %). During the subsequent early growing season, large trees derive up to 85 % of their water from rock, particularly from soil-filled fractures with apertures >10 mm, which act as transitional reservoirs capable of retaining precipitation for extended periods (MRT = 84–303 days). Trees preferentially access short-MRT sources under wet conditions and shift to longer-MRT pools during dry periods, reinforcing the concept of ecohydrological separation between tightly bound and dynamically recharged water pools. However, this separation is attenuated during periods of high precipitation due to increased hydraulic connectivity and water mixing. This work advances our understanding of vegetation resilience in structurally complex and hydrologically dynamic karst landscapes with important insights for sustainable water resource management under changing climatic conditions.
- Preprint
(3137 KB) - Metadata XML
- BibTeX
- EndNote
Status: final response (author comments only)
-
RC1: 'Comment on egusphere-2025-3937', Anonymous Referee #1, 13 Oct 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-3937/egusphere-2025-3937-RC1-supplement.pdfCitation: https://doi.org/
10.5194/egusphere-2025-3937-RC1 -
RC2: 'Comment on egusphere-2025-3937', Anonymous Referee #2, 30 Dec 2025
This manuscript presents a valuable contribution to understanding the role of rock fractures in the seasonal dynamics of tree water use in karst environments. By using stable isotope analysis and a Bayesian mixing model, the authors provide significant insights into how different water sources, including mobile and bulk soil water and rock fracture water, support tree transpiration under varying seasonal conditions. The findings hold potential value for ecohydrological modeling and karst ecosystem restoration. However, the manuscript requires substantial revisions to meet the publication standards of Hydrology and Earth System Sciences (HESS). The current version lacks clarity in several sections, and the analysis, while promising, needs to be more integrated and quantitatively robust. Below are the detailed comments:
1. Title and Abstract
Title: The current title, which mentions “mixing and residence times,” is somewhat vague. It would be more appropriate to focus on the fracture characteristics (e.g., aperture, infill) and their influence on plant water uptake, which is the core of the study.
Abstract: The abstract should have consistent terminology for seasonal changes (e.g., "rain season," "growing season," "spring"). Additionally, the abstract focuses on seasonal shifts in water sourcing, but the title and introduction emphasize fracture characteristics, leading to a disconnect between the summary and the actual study focus. The abstract should reflect the true focus of the research more clearly.
2. Introduction
MRT and MTT: The introduction lacks a comprehensive review of the research progress related to Mean Residence Time (MRT) and Mean Transit Time (MTT) methods. The authors should discuss these methods more thoroughly, including their advantages, disadvantages, and how they have been applied in similar contexts, particularly in complex karst environments. This will help readers understand the relevance of the methods used in this study.
Line 40-50: The second paragraph of the introduction lacks clear connections to the main research questions. The significance of ecohydrological separation should be more explicitly linked to the current study. The authors should clarify how their work addresses existing gaps in ecohydrological separation, particularly in karst ecosystems.
Line 77-79: The introduction suggests the manuscript’s innovation is focused on fracture characteristics affecting plant water uptake, but the results and discussion primarily focus on seasonal shifts. The link between fracture characteristics and plant water uptake is not clearly demonstrated. The authors should emphasize the role of fractures more directly in their introduction and rewrite this section to align with the results.
Line 86: The manuscript focuses on a single research site, which limits its applicability. The authors should provide a justification for why this site can be considered representative of karst ecosystems or acknowledge its limitations as a case study.
3. Materials and Methods
Sampling Design: Key details regarding the sampling design should be provided in a table (e.g., tree species, number of individuals, DBH, root zone depth, and sampling replications). More details on how the sampling protocol accounts for site heterogeneity are needed.
Water Sampling Procedures: The differentiation between "mobile" and "bulk" soil water needs further clarification, particularly regarding suction ranges used in lysimeters. The method for distinguishing between "rock fracture water" and "infilled rock fracture water" should be better explained. A schematic of the sampling process would be helpful.
Precipitation and Sensor Data: The method for collecting precipitation isotope data is missing, and details on the replication and installation of soil moisture sensors are absent.
Analytical Protocols: The use of two different laboratories (Tianjin and Saskatchewan) requires justification. The authors should demonstrate cross-validation of results between labs to ensure no systematic bias.
Assumptions of MRT Method: The assumptions behind the MRT method are not discussed in the manuscript. In a complex karst system, using such methods may introduce significant uncertainties. The authors need to explain the assumptions clearly and discuss the potential sources and magnitude of uncertainty in their methodology.
Isotopic Fitting for Different Water Types: The MRT and MTT methods rely on fitting isotopic data (e.g., for precipitation, soil water, and plant water) to sinusoidal functions. However, the manuscript lacks a discussion of the isotopic fitting for different water types over the study period, and there is no assessment of the fitting quality (e.g., R-squared values). The authors should include this information to evaluate the reliability and applicability of these methods.
4. Results
The Results section is overly descriptive, listing isotopic values and MRTs without synthesizing them to test core hypotheses. The manuscript should minimize overly detailed, descriptive statistics and focus on synthesizing the major findings in a more concise manner. For example:
Does fracture aperture or infill correlate with water MRT or its proportional contribution to plant xylem water across different seasons? This should be clearly analyzed and presented in the results.
Figure 6 (Source Partitioning): This figure currently presents preliminary data. It should specify the year(s) of data and include uncertainty estimates, such as standard deviations from MixSIAR posteriors. The data should be grouped by hydrological season or fracture density (high vs. low) rather than showing monthly bars for individual sites. This would offer more insights into the data.
MRT Clarification: In section 3.2, it is unclear whether the reported MRT values are for the same depth or different depths. This should be clarified to ensure the results are presented unambiguously.
MRT and MTT Methods: The manuscript calculates both MRT and MTT, but the authors do not explain why both methods are necessary or what the differences between the two are. The values of MRT and MTT differ significantly in some cases, but the manuscript does not provide any explanation of how these differences relate to underlying hydrological processes. Furthermore, the presence of negative MTT values needs to be addressed—why do they occur, and are they physiologically meaningful?
5. Discussion
The discussion should place more emphasis on the ecohydrological implications of the study, particularly the role of fracture properties in plant water uptake. The authors should better integrate their findings with existing literature on karst hydrology and ecohydrological separation. There should be more attention paid to how fracture aperture sizes and infill affect water availability and uptake, especially during dry or transitional periods.
6. Study Design Limitations and Confounding Factors
Site and Species Confounding: The study involves different tree species at each site. It is possible that differences in water sourcing patterns could be driven by species-specific rooting strategies rather than fracture characteristics. This potential confounding factor should be addressed more clearly.
Pseudoreplication: The study design lacks true replication of fracture types across sites, which limits the ability to generalize findings about the role of fractures in plant water uptake. This limitation should be acknowledged and discussed.
Representativeness: The conclusions are drawn from a single study site. Given the extreme heterogeneity of karst landscapes, the authors should frame their findings as a case study rather than assuming general applicability to all karst environments.
Citation: https://doi.org/10.5194/egusphere-2025-3937-RC2
Viewed
| HTML | XML | Total | BibTeX | EndNote | |
|---|---|---|---|---|---|
| 2,302 | 171 | 25 | 2,498 | 32 | 32 |
- HTML: 2,302
- PDF: 171
- XML: 25
- Total: 2,498
- BibTeX: 32
- EndNote: 32
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1