General comments

The paper by He et al. aimed to disentangle the degree and depth limitation of deep soil desiccation and its impact on a tree’s xylem hydraulic conductivity based on the published and sampled data from China’s Loess Plateau. They argued that the degree limitation approached the permanent wilting point (PWP) and hence the PWP can be used to indicate the lower limit of root water uptake by different tree species. The depth limitation was also identified and varied between tree species and regions. Furthermore, they also reported that the loss of hydraulic conductivity was as high as 74.9-96.5% when both limitations were reached.

Overall, this paper is well-motivated and mostly well-structured and written. According to my knowledge, the research topic is less studies in the existing literature. The first conclusion seems not surprising that PWP is a good indicator of the degree limitation of deep-layer soil moisture (DSM) for the tree species examined because it is often used in this way. But I think this conclusion is interesting since PWP is usually determined using annual crops and more importantly, in that the deep-layer soil moisture reached the degree limitation while the shallow-layer soil moisture did not. Although this study has scientific merit and can be a scientific contribution potentially, I still have two major concerns about the method and discussion. My first concern is about the definition of deep soil moisture deficit (DSMD). In this definition, they mentioned background soil moisture. The authors should clarify what does background soil moisture mean here, and why the DSM in adjacent control can reflect local annual precipitation. Moreover, in my opinion, the definition of DSMD should be moved to the method part. The other concern is the discussion of the effect of limitation on xylem hydraulic conductivity. The discussion in this part seems not thorough and should be rewritten to improve the relevance.

The specific comments are given as follows.

Abstract

Line 20: Why use DSMD for analysis here?

Line 30: Do you have observational evidence of tree mortality in the study site?

Introduction

Line 35: The range of loess thickness is often used as 30-200 m in the literature.

Line 38: The citation in many parts is not mostly relevant. For instance, the citation of Gao et al., 2021 did not study the three-north shelter forest program.

Line 66: Several earlier studies have reported the dwarf aged trees. These studies should be cited here.

Lines 70-71: This sentence is not clear and should be rewritten for clarity.

Line 78: change disentangle to explore

Line 81: Does any literature support this statement about the difference of drought-resistance capacity between tree species?

Materials and methods.

Line 86: Why does the literature begin in 1999?

Line 99: Why were only apple trees and black locusts used for analysis?

Line 130: Please give the full name.

Line 136: How do you judge whether the DSM reaches the degree limitation during sampling?

Line 142: Soil moisture can be very clear at different directions around a tree. How do you address this issue?

Table 3: Table 3 should be explained in more detail. How do you classify soil texture fractions, i.e., the content of clay, silt and sand? What is the sampling depth? Are they mean values for different layers?

Line 215: The index here can not represent degree limitation of soil desiccation, but just a relative soil water content.

Line 231: The manufacture of the Canoco 5 should be added in the text.

Results

Line 235: The analysis in 3.1 seems not closely connected with the following analyses in 3.2 and 3.3.

Line 240: Please change extreme to the biggest.

Line 260: Are these data used in the section 3.2.1 from published literature or your field sampling or the combination? Please clarify.

Line 265: This sentence is not clear. Please rewrite this sentence.

Line 268: What does extreme rainfall mean here? How much precipitation?

Figure 5: The yellow line in Figure 5a is not clear. Please change it to another color.

Line 298: Soil texture can largely affect soil moisture. But, how can it lead to the degree and depth limitation of deep soil desiccation? The results here should be explained in a more sensible way.

Discussion

Line 340: Please add a citation here.

Line 347: Please delete "reference".

Line 349: Please change to soil water use.

Line 354: Keep the decimal places consistent.

Line 355: Use the full name of SPAC.

Line 378: Are the trees examined in your study site dieback?

Review of the manuscript ‘The degree and depth limitation of deep soil desiccation and its impact on xylem hydraulic conductivity in dryland tree plantations (hess-2023-12)

 General comment

This is a very interesting that focuses on the analysis of deep soil desiccation and its effect on soil moisture supply and tree hydraulic conductivity in tree plantations. This is a topic that fits the scope of this journal and that is of interest for its readership. Overall, the manuscript is well written and organized, and the motivation, scientific gaps, and objectives are clear. However, there are some confused methodological descriptions that make unclear most of the results, weakening the strength of the work. Moreover, data interpretation is not always straightforward, leaving the reader with doubts about the effective role of PWP. Please, find my main concerns and some minor corrections and suggestions below. In the end, I recommend a major revision of this work.

 Specific comments

- As far as I know, the approach of mixing data extracted from the literature with those collected in the field is quite unusual. Still, I welcome this approach because the two distinct datasets can corroborate each other. However, the presentation of the two dataset is quite confusing and it was hard for me to understand which results refer to data collected in the field and which were extracted by the literature. This holds true for the tables, the figures, and the results presented in the text. All this creates some confusions. I think it’s important to well explain this difference for the sake of clarity and to allow the reader understanding where the findings come from.

- Moreover, there are some unclear explanations and definitions that hamper a complete understanding of the work and of it’s results. See below the specific comments.

- Sometimes, there are too many acronyms. Some are used several times in the text and become familiar to the reader, and some are very well knows. However, some are not, and an excessive use of acronyms can fragmentate the reading flow and hamper understanding. Moreover, sometimes acronyms are used for the entire term and sometimes not: please be consistent throughout the entire manuscript.

- The Authors insist a lot on the role of the PWP as an indicator of the degree of moisture limitation for plants. However, the PWP was not directly measured, it can be very variable according to different locations and depth, and DSM is used as a proxy. I suggest making the connection between PWP and DSM clearer and stronger. As far as I understand, the main message of the manuscript lays on this aspect which should be crystal clear to the reader.  

- The language is sometime not appropriate, some sentences are not linear and a bit hard to follow. There are sparce grammar mistakes. I made a few suggestions only (below). I suggest a language review by a native (or very proficient) English-speaker.

81. What is the drought-resistance related to? Water use strategy? Root depth? It is important to define this here, I believe.

89. What kind of bias? Please explain.

111. What does the Authors mean hear with “random errors”, and how can they be avoided? Please, explain.

141. This sounds unbelievable to me. I have used an hand auger to get soil samples down to a couple of meters, and only occasionally to 4-5 through a driller. Here the Authors report 25 m! How is that possible? Please, explain.

Fig. 2. This is supposed to be an important figure for the “story” the Authors tell but it is not very clear to me. What is the response ratio? I did not find any definition. Without understanding the RR meaning it’s difficult to grasp the importance of the figure. What do negative and positive values of RR mean? In the caption: what are the “effect sizes”. Please, clarify all this.

Paragraph 3.2. I have some doubts about the definition of SMD and DSMD. I assume that SM is highly variable in space…so, how can we use a control tree as a real control? Should we measure the SM in several control trees? Can you use a more pedological definition of deficit, such as the difference between the actual water content and saturation? The Authors have the soil samples and therefore could perform a lab analysis to determine the porosity of the samples. Moreover, I intuitively understand the difference between SMD and DSMD but this difference was not analytically define. This rises some issue on the use of this metric and the interpretation of the data subsequently presented. Please, clarify well this part.

Fig. 8. Is this based on the results of this study or is a more general conceptual figure deriving from what we already know? This is not clear to me. Please, specify.

Minor comments and technical corrections

18. I’d add “water” or “moisture” before “limitation”.

21. “minimize the effect…”: this is not clear without reading the manuscript. Please explain it or skip it.

16. Remove “there”.

45. Move “in drylands” at line above, after “tree planting”.

74. Typo/language issues.

105. Why “intuitively”? Please, explain.

143. Replace “was” into “were”.

Table 2, caption: Why only those, and not all? Please, explain.

225-228. “significance”: do you mean the possibly significant difference? Please explain. In general, these three lines are not very clear, please explain better.

231. Canoco 5. What is this ? Please, explain.

240. Which plantation? Only one of them or both?

274. What do the Authors mean by “randomness”? Please, clarify.

Fig. 4, 5, 6 and 7. Are these figures based on field data or extracted data only? Please, specify.

307. Here again, “limitations” is vague…I would add “water” or “moisture”.

320-321. Language issues.