the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 05 May 2023
Reduced transpiration without changes in root water uptake patterns in degraded trees in semi-arid afforestation ecosystems
Abstract. While reforestation in water-limited areas has increased vegetation coverage, an increasing number of studies have reported that large-scale plantations are suffering from tree degradation, which is characterized by short trees, small size, and dieback. Moreover, the changes in hydrological processes of degraded trees within the soil-plant system remain poorly understood. Here, the stable isotope method (2H, 18O, and 13C) and thermal dissipation technique were used to compare the soil water dynamics, tree transpiration, root water uptake patterns, and intrinsic water-use efficiency (WUEi) of Populus simonii under different degradation degrees (no degradation, ND; light degradation, LD; or severe degradation, SD) in the 2021 growing season. As tree degradation intensified, the root weight density decreased significantly (P<0.05) and the root proportion of the shallow layer (0–40 cm) increased. Influenced by precipitation recharge, the soil volumetric water content (SWC) of the shallow layer did not show significant differences (P>0.05) among the different degradation degrees. The SWC of the middle (40–80 cm) and deep (80–200 cm) layers were significantly (P<0.05) lower in the ND plot than the LD and SD plots. Despite the differences in SWC, the isotopic characteristics of the soil water and xylem water were similar among the ND, LD, and SD plots. Although tree transpiration in the LD and SD plots was significantly reduced (P<0.05) compared to that in the ND plot, the main depths and proportions of water uptake by the root system did not change. P. simonii trees in the ND, LD, and SD plots were able to shift the water source from shallow to deep layers in the process of soil wetting to drying. Moreover, compared to healthy trees, WUEi of degraded trees was more sensitive to SWC. Our study shows that although degraded trees alleviate the exhaustion of deep soil water reservoirs to some extent, the sustainable development of afforestation ecosystems requires appropriate stand management measures to maintain balanced forest-water relationships.
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CC1: 'Comment on hess-2023-66', Shaozhen Liu, 29 Jun 2023
In line 320, the author concluded that the low transpiration in September resulted from the soil drought. I would like to inquire whether the authors have taken into account the possibility that transpiration declines in the late growing season, even with normal soil moisture availability.
Citation: https://doi.org/10.5194/hess-2023-66-CC1 - AC1: 'Reply on CC1', Li Wang, 04 Jul 2023
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RC1: 'Comment on hess-2023-66', Anonymous Referee #1, 09 Aug 2023
Dear all,
Dai et al. present ecohydrological data of three sites varying in tree degradation with the general aim to shed light on potential differences in tree water cycling as cause for the degradation. The topic generally is very important, globally as well as in the studied region, and the data gathered are valuable to the scientific community. Before publication however, I strongly suggest major revisions to rework the paper and make the aim, outcome and implications much clearer.
I attached specific comments for clarification.
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AC2: 'Reply on RC1', Li Wang, 11 Sep 2023
We thank Reviewer #1 for thoughtfully and critically reviewing our manuscript. We greatly appreciate the detailed and thoughtful points that have certainly helped us to improve the manuscript. Overall, we agree with these suggestions and have made targeted amendments. Please see the attachment at the following link for specific responses.
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AC2: 'Reply on RC1', Li Wang, 11 Sep 2023
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RC2: 'Comment on hess-2023-66', Anonymous Referee #2, 18 Sep 2023
General comment
The focus of this manuscript is the evaluation of the water-use strategies of Populus simonii trees under different degradation conditions. The authors combined the analysis of the root system, the isotopic composition (hydrogen and oxygen) of xylem and soil water, soil water content and sap flow to test whether degraded trees used less deep soil water compared to non-degraded trees.
The topic of this manuscript is potentially interesting for the readers of Hydrology and Earth System Sciences, and the paper is generally well written, even though I recommend a revision of the English by a native speaker. Besides this, I have some comments that would require a major revision of the text. First, the authors should clearly present the general objective and novelty of their work (this should be emphasized in the discussion and the conclusions as well). Secondly, the authors should specify some more methodological details and discuss the limitations of their study (please see the specific comments). Thirdly, the authors should consider restructuring the results, by starting from the presentation of the physical characteristics of the trees and the root systems, then by describing soil moisture and sap flow dynamics, and finally by presenting the quantification of the contribution of soil water at different depths to xylem water.
Specific comments
- Section 2.3: The authors should clearly report in the text the number of samples (soil and plants) collected for isotopic analyses in each site and for each sampling date. To improve the clarity of the results, sample size (n) should be always reported in tables (e.g., Tables 1 and 2) and figures (or in their caption), and in the presentation of the results of statistical analyses.
- Line 157: More quantitative details about rainfall events (amounts, intensity, and duration), and how they were defined (time without rainfall) are needed.
- Section 2.5: The authors should clarify whether the thermal dissipation probes were calibrated or not, and add the measurement uncertainty, as well as the estimation of uncertainty in terms of daily transpiration. Furthermore, it is unclear why the authors monitored sap flow in five trees that were not used for the collection of xylem water (this should be justified). What are the main physical characteristics of the trees chosen for isotopic sampling and how did they differ from the trees selected for sap flow monitoring?
- Lines 241-250: This text can be moved to Section 3.1 or to a new section about the dynamics of soil water content.
- Section 4: This section lacks some paragraphs about the main limitations of this study. For instance, the authors have not discussed how the methodological uncertainty in the extraction of xylem and soil water (by cryogenic vacuum distillation) might have affected their results and interpretation, especially in terms of the estimated contribution of soil water at different depths to xylem water.
- The main findings of this study could be presented in a conceptual model and an accompanying figure/sketch.
- Lines 435-436: I recommend reporting the description of the hypotheses here as well.
Technical corrections
- Line 6: Please delete ‘method’ and use ‘stable isotopes’.
- Line 57: Techniques do not mature; I suggest rephrasing the current unclear sentence.
- Line 70: Please delete ‘technology’ and use ‘stable isotopes’.
- Line 111: Please use ‘plots’ instead of ‘quadrats’.
- Line 118: Please replace ‘inferior’ with ‘lower’.
- Line 147: Please replace ‘rate’ with ‘efficiency’.
- Lines 256 and 259: Please replace ‘scatter points’ with another term.
- Line 304: Please replace ‘middle’ with the specific soil depth.
Citation: https://doi.org/10.5194/hess-2023-66-RC2 -
AC3: 'Reply on RC2', Li Wang, 12 Oct 2023
We thank Reviewer #2 for thoughtfully and critically reviewing our manuscript. We greatly appreciate the detailed and thoughtful points that have certainly helped us to improve the manuscript. Please see the attachment at the following link for specific responses.
Status: closed
-
CC1: 'Comment on hess-2023-66', Shaozhen Liu, 29 Jun 2023
In line 320, the author concluded that the low transpiration in September resulted from the soil drought. I would like to inquire whether the authors have taken into account the possibility that transpiration declines in the late growing season, even with normal soil moisture availability.
Citation: https://doi.org/10.5194/hess-2023-66-CC1 - AC1: 'Reply on CC1', Li Wang, 04 Jul 2023
-
RC1: 'Comment on hess-2023-66', Anonymous Referee #1, 09 Aug 2023
Dear all,
Dai et al. present ecohydrological data of three sites varying in tree degradation with the general aim to shed light on potential differences in tree water cycling as cause for the degradation. The topic generally is very important, globally as well as in the studied region, and the data gathered are valuable to the scientific community. Before publication however, I strongly suggest major revisions to rework the paper and make the aim, outcome and implications much clearer.
I attached specific comments for clarification.
-
AC2: 'Reply on RC1', Li Wang, 11 Sep 2023
We thank Reviewer #1 for thoughtfully and critically reviewing our manuscript. We greatly appreciate the detailed and thoughtful points that have certainly helped us to improve the manuscript. Overall, we agree with these suggestions and have made targeted amendments. Please see the attachment at the following link for specific responses.
-
AC2: 'Reply on RC1', Li Wang, 11 Sep 2023
-
RC2: 'Comment on hess-2023-66', Anonymous Referee #2, 18 Sep 2023
General comment
The focus of this manuscript is the evaluation of the water-use strategies of Populus simonii trees under different degradation conditions. The authors combined the analysis of the root system, the isotopic composition (hydrogen and oxygen) of xylem and soil water, soil water content and sap flow to test whether degraded trees used less deep soil water compared to non-degraded trees.
The topic of this manuscript is potentially interesting for the readers of Hydrology and Earth System Sciences, and the paper is generally well written, even though I recommend a revision of the English by a native speaker. Besides this, I have some comments that would require a major revision of the text. First, the authors should clearly present the general objective and novelty of their work (this should be emphasized in the discussion and the conclusions as well). Secondly, the authors should specify some more methodological details and discuss the limitations of their study (please see the specific comments). Thirdly, the authors should consider restructuring the results, by starting from the presentation of the physical characteristics of the trees and the root systems, then by describing soil moisture and sap flow dynamics, and finally by presenting the quantification of the contribution of soil water at different depths to xylem water.
Specific comments
- Section 2.3: The authors should clearly report in the text the number of samples (soil and plants) collected for isotopic analyses in each site and for each sampling date. To improve the clarity of the results, sample size (n) should be always reported in tables (e.g., Tables 1 and 2) and figures (or in their caption), and in the presentation of the results of statistical analyses.
- Line 157: More quantitative details about rainfall events (amounts, intensity, and duration), and how they were defined (time without rainfall) are needed.
- Section 2.5: The authors should clarify whether the thermal dissipation probes were calibrated or not, and add the measurement uncertainty, as well as the estimation of uncertainty in terms of daily transpiration. Furthermore, it is unclear why the authors monitored sap flow in five trees that were not used for the collection of xylem water (this should be justified). What are the main physical characteristics of the trees chosen for isotopic sampling and how did they differ from the trees selected for sap flow monitoring?
- Lines 241-250: This text can be moved to Section 3.1 or to a new section about the dynamics of soil water content.
- Section 4: This section lacks some paragraphs about the main limitations of this study. For instance, the authors have not discussed how the methodological uncertainty in the extraction of xylem and soil water (by cryogenic vacuum distillation) might have affected their results and interpretation, especially in terms of the estimated contribution of soil water at different depths to xylem water.
- The main findings of this study could be presented in a conceptual model and an accompanying figure/sketch.
- Lines 435-436: I recommend reporting the description of the hypotheses here as well.
Technical corrections
- Line 6: Please delete ‘method’ and use ‘stable isotopes’.
- Line 57: Techniques do not mature; I suggest rephrasing the current unclear sentence.
- Line 70: Please delete ‘technology’ and use ‘stable isotopes’.
- Line 111: Please use ‘plots’ instead of ‘quadrats’.
- Line 118: Please replace ‘inferior’ with ‘lower’.
- Line 147: Please replace ‘rate’ with ‘efficiency’.
- Lines 256 and 259: Please replace ‘scatter points’ with another term.
- Line 304: Please replace ‘middle’ with the specific soil depth.
Citation: https://doi.org/10.5194/hess-2023-66-RC2 -
AC3: 'Reply on RC2', Li Wang, 12 Oct 2023
We thank Reviewer #2 for thoughtfully and critically reviewing our manuscript. We greatly appreciate the detailed and thoughtful points that have certainly helped us to improve the manuscript. Please see the attachment at the following link for specific responses.
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