the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Root water uptake patterns are controlled by tree species interactions and soil water variability
Gökben Demir
Andrew J. Guswa
Janett Filipzik
Johanna Clara Metzger
Christine Römermann
Anke Hildebrandt
Abstract. Throughfall is the largest source of water entering the soil in forests, and its spatial distribution depends on several biotic and abiotic factors. It is well documented that the distribution of throughfall results in reoccurring higher and lower water inputs at certain locations. However, the role of horizontal root water uptake patterns in understanding the effects of throughfall patterns on subsurface water dynamics remains unresolved. Therefore, here we investigate root water uptake patterns by considering spatial patterns of throughfall and soil water patterns in addition to soil and neighboring tree characteristics. We conducted weekly intensive throughfall sampling at locations paired with soil moisture sensors during the 2019 growing season. We employed a linear mixed effects model to understand controlling factors for root water uptake patterns. Our results show that soil water patterns and interactions among neighbouring trees are the most significant factors regulating root water uptake patterns. Temporally stable throughfall patterns did not influence root water uptake patterns. Similarly, soil properties were unimportant for spatial patterns of root water uptake. We found that wetter locations (rarely associated with throughfall hotspots) promoted greater root water uptake. Root water uptake in monitored soil layers also increased with neighbourhood species richness. Ultimately our findings suggest that complementarity mechanisms within the forest stand, in addition to soil water variability and availability, govern root water uptake patterns.
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Gökben Demir et al.
Status: open (until 27 Jun 2023)
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RC1: 'Comment on hess-2023-91', Anonymous Referee #1, 30 May 2023
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MS#: hess-2023-91
Title: Root water uptake patterns are controlled by tree species interactions and soil water variability
It has been a pleasure reading through this contribution. This research underlines the lack of research on how throughfall patterns influence root water uptake patterns. The authors propose to close this knowledge gap by examining the role of throughfall patterns, soil water variability, soil properties, and biotic factors on root water uptake patterns using a statistical model.
I will organize my comments following the structure, per section, of the manuscript.
INTRODUCTION
The introduction provides a detailed background on below-canopy precipitation, specifically focusing on throughfall. It covers previous research about throughfall, its spatial distribution, and its impact on soil moisture patterns.
- Line 62: Consider rephrasing the phrase "Previously proposed explanations" to "Previous studies have suggested". This would be more direct.
- Line 86: The term "water scarcity" is introduced without any context or explanation. A brief explanation or definition would enhance understanding. In fact, this is the only part of the manuscript where "water scarcity" is mentioned.
MATERIALS AND METHODS
The Materials and Methods section is well-written. Some areas that could be improved to make it clearer and easier to understand follow:
- Line 174: Provide units for variables, such as λ (latent heat of vaporization).
- Line 196-197: Elaborate why only 56 sensor locations provided high-quality data, and why only 34 of these provided data for root water uptake estimation. What qualifies as ‘high-quality data’?
- Linear Mixed Effects Model: Explain the terminology used (like 'random effects', 'fixed effects') for readers unfamiliar with these statistical methods.
- Linear Mixed Effects Model: Add a few sentences as justification or rationale for including each of the factor (fixed and random) and their interactions (for fixed) in the model. The reader may be able to identify the rationale by referring to the texts in Introduction. But it pays to be redundant in the Methods so that it is clear to the reader which factors were included, and more importantly ‘why’.
DISCUSSION
This discussion section is generally well-written, but there are some areas for improvement to enhance clarity and readability:
- Structure and Organization: The text is divided into several subtopics, which makes it easier to follow. However, it could benefit from a clearer outline or roadmap at the start of the discussion section that provides an overview of what will be discussed.
- Data Presentation: Some results are mentioned without an explicit description of how they were obtained. For example, in lines 477 to 479, the authors state they found that bulk density of the monitored soil layer did not affect local water uptake, but there is no explanation of how this conclusion was reached. Providing a more detailed explanation would enhance the credibility of the findings.
- Clearer takeaways: The section could benefit from more direct conclusions or ‘takeaway’ after discussing each main point. For instance, after discussing the influence of tree species richness on root water uptake patterns (lines 504 to 533), a one-sentence conclusion summarizing the main takeaway could be beneficial.
- Hypotheses and Expectations: It might be useful to explicitly state the original hypotheses or expectations before explaining how the results confirm or contradict them. This can provide the reader with a clearer understanding of the study's purpose and significance. Perhaps, these hypotheses can be explicitly stated towards the end of Introduction. The authors can then ‘revisit’ these hypotheses in Discussion following their presentation of results.
- Broader context of the literature. This study appeared broadly consistent with the finding of Knighton, Singh, Evaristo (2019, DOI: 10.1029/2019GL085937), which showed that monoculture catchments dense with trees reliant on shallow soil water exhibited reduced transpiration losses compared to deep‐rooted and mixed‐species forests. It is an important confirmation to make considering that this study is based purely on a statistical framework whilst that of Knighton et al. (2019) was based on the Budyko framework.
- Broader context of the literature. Demir et al. may find use in placing their finding within the larger context of the topic (root water uptake studies) that used other techniques, particularly stable H and O isotopes in water. That is, the hours-long timescales used in this study for estimating transpiration losses are orders of magnitude shorter than what stable isotopes would show. See, for example, the study by Evaristo et al. (2019, DOI: 10.1029/2018WR023265), which showed that transpiration water was between 17 and 62 days. How do the timescales of Demir et al. (and their findings) compare and contrast to the timescales (and findings) of studies using tracers? A few sentences that place Demir et al. within the larger context of the topic would be useful for future researchers to recognize.
Citation: https://doi.org/10.5194/hess-2023-91-RC1
Gökben Demir et al.
Gökben Demir et al.
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