Water in Wood: A Review of Current Understanding and Knowledge Gaps
Round 1
Reviewer 1 Report
The relationship between wood and water is very important to wood drying, wood modification and wood utilization. This review describes the current understanding of water in wood based on available data from experimental and computational methods. Until now, these existing review paper related to wood and water are not as comprehensive as this paper, and this paper will provide much more information than the existing research material, and researchers could find almost all the knowledge detail about water in wood in this paper. I suggest the authors add some wood drying outlook in summary and outlook section because wood drying is stronger correlation to water in wood than other wood research fields.
Author Response
COMMENT: The relationship between wood and water is very important to wood drying, wood modification and wood utilization. This review describes the current understanding of water in wood based on available data from experimental and computational methods. Until now, these existing review paper related to wood and water are not as comprehensive as this paper, and this paper will provide much more information than the existing research material, and researchers could find almost all the knowledge detail about water in wood in this paper. I suggest the authors add some wood drying outlook in summary and outlook section because wood drying is stronger correlation to water in wood than other wood research fields.
REPLY: Thank you for the very positive review of our manuscript. Since the manuscript is devoted to understanding the fundamental aspects of water in wood, we have chosen not to include wood drying in Section 9. Otherwise, we would have to include a range of other important topics where water in wood is relevant, which would lengthen the section and dilute the important messages we would like to confer, we think. Instead, we have added the following sentence to the introduction: “Additionally, transport of water in wood is important for example within the field of wood drying.“ (lines 42-43).
Author Response File: 
Author Response.docx
Reviewer 2 Report
The manuscript entitled “Water in wood: a review of current understanding and knowledge gaps” has summarized the existing knowledge of water in wood through seven relevant aspects (location of water in wood, equilibrium moisture content, fibre saturation point and maximum cell wall moisture content, dry shrinkage and wet swelling, sorption hysteresis, water transport in wood and kinetics of water vapour sorption in the cell wall). In general, this manuscript is not well-written and has several sever deficiencies. So, this manuscript requires detailed revisions before it can be considered for publication in Forests. A few of my comments are provided below.
1. There are too many Keywords, up to 11 and unreasonable. In my opinion, “sorption” and “sorption isotherm” are subordinate; “thermodynamics” is not the focus of this reviewï¼›”models” are too broad in scope and very unprofessional; “maximum cell wall moisture content” is only a short paragraph in the manuscript and is used to distinguish the fibre saturation point, so it does not make sense as a keyword.
2. This review missing the database query research statement, and the range of years for references.
3. In section 3 Wood-water equilibrium states, the relationship between the equilibrium moisture content and the relative humidity of the air should not only be shown as an image, but the detailed equations in the Wood handbook should also be listed.
4. The authors named Section 4 as “Fiber saturation point and maximum cell wall moisture content”, but maximum cell wall moisture content is only a short paragraph in the manuscript and is used to distinguish the fibre saturation point.
5. Overall, the author's summary is rather broad and more like a popular science than a review paper. The focus is not prominent enough and lacks authors own insights/comments.
6. The authors wrote that this review summarizes our current understanding of the fundamentals of water in wood and highlights significant knowledge gaps that need to be addressed to advance this area of wood science. But in section 9 Summary and outlooks, the authors do not summaries the major knowledge gaps that need to be addressed.
Author Response
COMMENT: The manuscript entitled “Water in wood: a review of current understanding and knowledge gaps” has summarized the existing knowledge of water in wood through seven relevant aspects (location of water in wood, equilibrium moisture content, fibre saturation point and maximum cell wall moisture content, dry shrinkage and wet swelling, sorption hysteresis, water transport in wood and kinetics of water vapour sorption in the cell wall). In general, this manuscript is not well-written and has several sever deficiencies. So, this manuscript requires detailed revisions before it can be considered for publication in Forests. A few of my comments are provided below.
REPLY: Thank you for considering that our manuscript covers the relevant aspects of water in wood. We have responded to the specific criticism below.
COMMENT: 1. There are too many Keywords, up to 11 and unreasonable. In my opinion, “sorption” and “sorption isotherm” are subordinate; “thermodynamics” is not the focus of this reviewï¼›”models” are too broad in scope and very unprofessional; “maximum cell wall moisture content” is only a short paragraph in the manuscript and is used to distinguish the fibre saturation point, so it does not make sense as a keyword.
REPLY: We have deleted the keywords “sorption isotherm”, “thermodynamics”, and “models”. Regarding “maximum cell wall moisture content” this is discussed as an important property of wood, hence we keep it in the keywords. See further justification in the Reply to Comment 4.
COMMENT: 2. This review missing the database query research statement, and the range of years for references.
REPLY: This review is conducted based on our combined knowledge of the literature on water in wood and not a specific database query. Furthermore, the manuscript is mainly focused on the most recent work improving our understanding of water in wood. Therefore, we disagree that the range of years of used references is important for the reader.
COMMENT: 3. In section 3 Wood-water equilibrium states, the relationship between the equilibrium moisture content and the relative humidity of the air should not only be shown as an image, but the detailed equations in the Wood handbook should also be listed.
REPLY: Mathematical description of the moisture sorption isotherm is already described in Section 3.3 of the manuscript. It is important to note that the Wood Handbook equations are based on an undocumented mixture of experimental sorption data for various wood species with extrapolated and interpolated data, and several methods for establishing equilibrium (absorption, desorption, or a mixture of both) have been used. Further details can be found in Reference [100], which is cited in Section 3.3 of the manuscript. For this reason we have not added the equations from the USDA Wood Handbook as suggested by the reviewer.
COMMENT: 4. The authors named Section 4 as “Fiber saturation point and maximum cell wall moisture content”, but maximum cell wall moisture content is only a short paragraph in the manuscript and is used to distinguish the fibre saturation point.
REPLY: We disagree with the reviewer on the importance of maximum cell wall moisture content as a fundamental property of wood. It is discussed not only in a short paragraph (Section 4.3) but also in regard to methods in Section 4.2. This term is helpful in bringing clarity to a topic with a lack of consensus in the literature. Therefore, we would like to highlight the importance of the maximum cell wall moisture content by keeping it in the header.
COMMENT: 5. Overall, the author's summary is rather broad and more like a popular science than a review paper. The focus is not prominent enough and lacks authors own insights/comments.
REPLY: This is a difficult comment to address without specific examples. Our review paper contains 258 references. It features not only a listing of findings contained within these 258 references, but a detailed discussion of how these findings fit together to provide a coherent understanding of water in wood, as well as where gaps remain in current understanding.
COMMENT: 6. The authors wrote that this review summarizes our current understanding of the fundamentals of water in wood and highlights significant knowledge gaps that need to be addressed to advance this area of wood science. But in section 9 Summary and outlooks, the authors do not summaries the major knowledge gaps that need to be addressed.
REPLY: We do not understand this comment. Section 9 describes gaps about fundamental mechanisms in lines 973-976 as well as flaws in existing theoretical models described in lines 977-979. In lines 979-981 we describe where focus should be put to address knowledge gaps, and in lines 982-986 we highlight how this could be done with experimental techniques and chemical modification. Thus, nearly all of Section 9 is devoted to describing important knowledge gaps and how we think these can be addressed.
Author Response File: Author Response.docx
Reviewer 3 Report
The topic of the research work and manuscript is really interesting, while the manuscript is well-prepared and summarizes relevant information/knowledge. However there are some issues to be addressed towards its quality improvement before publication.
On line 26, mafe "of" (not "from"). In line 32, the parenthesis " (e.g. when exposed to rain or in the production of biofuels)" needs improvement, to clarify the meaning. In line 40, please specify more the "high moisture contents". In line 51, I would substitute "must" with "should". In the paragraph of lines 57-65, a reference should be incorporated in the text. There are no doubts that it is a detailed work based on a big number of references, though it looks like more to a book analyzing these relations, while it could be turned more to the analysis of recent findings concerning this topic, especially adding to the literature a valuable source of paper reviewing as well the water-wood relations in wood after thermal treatment, which is the most eco-friendly and of low energy consumption. Please, incorporate as well the relevant study entitled "Effect of thermal treatment on colour and hygroscopic properties of poplar wood". That would be indeed something novel and extremely useful to the academic and research world. RH should be explained in first time used, as well as all the acronyms. Please, try to change your writing approach which resembles to a book. In my opinion, a review paper should avoid repeating the already well accepted knowledge found in all relevant academic books (for example definitions) and try to gather and highlight all the singificant relevant findings of the last years or decades. Since there can be found previous review papers summarizing the well-established core knowledge.
Author Response
COMMENT: The topic of the research work and manuscript is really interesting, while the manuscript is well-prepared and summarizes relevant information/knowledge. However there are some issues to be addressed towards its quality improvement before publication.
REPLY: Thank you for the positive review of our manuscript. We have tried to improve the manuscript based on the specific comments received.
COMMENT: On line 26, mafe "of" (not "from"). In line 32, the parenthesis " (e.g. when exposed to rain or in the production of biofuels)" needs improvement, to clarify the meaning. In line 40, please specify more the "high moisture contents". In line 51, I would substitute "must" with "should". In the paragraph of lines 57-65, a reference should be incorporated in the text.
REPLY: We have changed the manuscript according to the reviewer comment. However, we have not added a definition of “high moisture contents” as this comes later in the manuscript.
COMMENT: There are no doubts that it is a detailed work based on a big number of references, though it looks like more to a book analyzing these relations, while it could be turned more to the analysis of recent findings concerning this topic, especially adding to the literature a valuable source of paper reviewing as well the water-wood relations in wood after thermal treatment, which is the most eco-friendly and of low energy consumption. Please, incorporate as well the relevant study entitled "Effect of thermal treatment on colour and hygroscopic properties of poplar wood". That would be indeed something novel and extremely useful to the academic and research world.
REPLY: We do not understand why special focus should be paid to thermal modification. The review concerns water in wood and how this can be investigated. Whether a given treatment is the “most eco-friendly and of low energy consumption” has no relevance in the selection of methods for studies of the fundamental mechanisms of water in wood. Therefore, we have chosen not to include the mentioned reference, since we also refer to thermal modification in line 553.
COMMENT: RH should be explained in first time used, as well as all the acronyms.
REPLY: RH is defined in line 145 when it is first used. We have moved the definition of NMR to the first use in line 118.
COMMENT: Please, try to change your writing approach which resembles to a book. In my opinion, a review paper should avoid repeating the already well accepted knowledge found in all relevant academic books (for example definitions) and try to gather and highlight all the singificant relevant findings of the last years or decades. Since there can be found previous review papers summarizing the well-established core knowledge.
REPLY: Thank you for your opinion. Given the overall positive feedback from reviewers concerning the content and writing style, we have chosen not to follow this comment. In particular, we would like to stress that not all readers, e.g. early stage researchers, are aware of all definitions and terminology described in our manuscript. Therefore, we have chosen to write for a broad audience that might be unfamiliar with certain terminology. In describing recent advances in knowledge it is important to put this into context, but even the “well-established core knowledge” evolves over time, so we have found it easier to try to piece together a coherent description of what we believe are some of the most important aspects about water in wood.
Author Response File: Author Response.docx
Reviewer 4 Report
I reviewed the manuscript Water in wood: a review of current understanding and knowledge gaps with interest and I consider that its contents are within the scope of what we are expecting for this special issue.
Its structure, review of wood-water relations and outlook for the future are in line with the current situation.
The authors are aware of the lack of a theoretical model to satisfy wood-water relations, but until there is one, in addition to continuing research to obtain a model, existing models and their physical interpretation should not be discarded, despite works such as those by Zelinka et al. 2018. In view of this, I would like the authors to consider the possibility of including a paragraph along these lines in section 3.2., such as:
“Until a new theoretical model can definitively satisfy the physics of wood-water relations, we are obliged to continue interpreting the hygroscopic response and thermodynamics of wood using existing models, even though they must be considered approximative interpretation models”.
Author Response
COMMENT: I reviewed the manuscript Water in wood: a review of current understanding and knowledge gaps with interest and I consider that its contents are within the scope of what we are expecting for this special issue. Its structure, review of wood-water relations and outlook for the future are in line with the current situation.
REPLY: Thank you for the positive review of our manuscript.
COMMENT: The authors are aware of the lack of a theoretical model to satisfy wood-water relations, but until there is one, in addition to continuing research to obtain a model, existing models and their physical interpretation should not be discarded, despite works such as those by Zelinka et al. 2018. In view of this, I would like the authors to consider the possibility of including a paragraph along these lines in section 3.2., such as: “Until a new theoretical model can definitively satisfy the physics of wood-water relations, we are obliged to continue interpreting the hygroscopic response and thermodynamics of wood using existing models, even though they must be considered approximative interpretation models”.
REPLY: We disagree with the reviewer that the lack of adequate models justifies the use of existing models to provide physical interpretation of water in wood. However, it can be relevant to use existing models for comparison with older literature. The reviewer refers to Section 3.2 (Thermodynamics of sorption) which we are confused about. We would like to stress that we do not criticize the use of the Clausius-Clapeyron equation, which often relies on a model for empirical interpolation of sorption data. With regards to how our wood science community should move forward, we would argue that focus should be on experimental documentation of wood-water interactions using the broad suite of techniques that has become available since most of the existing theoretical models were developed 50+ years ago. We believe that this opinion is covered by the existing text, e.g. Section 9.
Author Response File: Author Response.docx
Round 2
Reviewer 2 Report
I insist on the range of years of reference. This manuscript is mainly focused on the most recent work improving the understanding of water in wood, but recent is a very vague word, does it mean the last 5 years or the last 10 years? The author can declare in the manuscript the range of years of the main work on which it is focused.
Author Response
COMMENT: I insist on the range of years of reference. This manuscript is mainly focused on the most recent work improving the understanding of water in wood, but recent is a very vague word, does it mean the last 5 years or the last 10 years? The author can declare in the manuscript the range of years of the main work on which it is focused.
REPLY: Thank you for the comment. Our previous reply might have been slightly misleading, in that case we are sorry. The entire premise for our manuscript is that as science progresses, new data are generated that contradicts, reaffirms, or bring new nuances to our existing ideas about water in wood. Therefore, it is necessary from time to time to rethink and reformulate our understanding of water in wood - reflecting on the sum of data generated over time - which is exactly what this review intends to (as described in lines 45-55). No matter the insistence of the reviewer on the range of years, the comment is irrelevant for this review as the intension is to write a coherent review of important knowledge about water in wood which includes findings from across the years 1855 to 2022 (only from 1896 for the first reference specific for wood). We have therefore not added the range of years, and we do not intend to do so no matter the insistence of the reviewer. Such a statement would be misleading for the reader in terms of the intention of the manuscript which is otherwise very clearly formulated towards the end of Section 1.
