Electrical Resistance Tomography as a Non-Destructive Technique for Heartwood Detection and Quantification in Standing Red Sanders (Pterocarpus santalinus L.f.) Trees

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript uses Electrical Resistance Tomography to measure the characteristics of heartwood, which is significant for enriching wood measurement methods. However, the core issues addressed in the manuscript are not sufficiently prominent, with specific shortcomings as follows:

 

1) The ERT experimental setup lacks any schematic or actual images, making it unclear how the experiment was specifically conducted, particularly regarding the sensor structure.

 

2) It is not specified whether the heartwood used in the experiment was dried or undried. Since ERT is a contact-based measurement method, the manuscript fails to adequately demonstrate how effective contact between the ERT electrodes and the test wood was ensured.

 

3) In Figure 2, the color in the wood cross-section image represents conductivity distribution, but the specific units are not indicated. How was the ERT system calibrated prior to testing?

 

4) Figure 2 also shows resistance values. However, resistance is typically measured only between two electrodes. How were the resistance maps shown in the figure obtained? The same question applies to Figures 3 and 4.

 

5) The wood samples in Figure 1 lost moisture rapidly over a short period, causing significant changes in measured resistance. How did the authors ensure consistency in their measurements?

Author Response

1) The ERT experimental setup lacks any schematic or actual images, making it unclear how the experiment was specifically conducted, particularly regarding the sensor structure.

We agree that a clear illustration of the experimental setup improves the reproducibility and understanding of the methodology. Accordingly, we have revised the manuscript by including Figure 1 a schematic diagram of the Electrical Resistance Tomography (ERT) experimental setup and photographs of the actual experimental arrangement. The newly added figures clearly illustrate the sensor configuration, electrode placement, electrical connections, and the overall measurement procedure.

The revised description specifies that the ERT measurements were performed using a PiCUS TreeTronic system equipped with evenly distributed stainless-steel nail electrodes inserted around the circumference of the wood specimen/tree stem. The electrodes functioned as both current-injecting and voltage-measuring sensors under an adjacent measurement protocol. Sequential current injection and voltage measurements between electrode pairs were automatically controlled by the instrument to reconstruct the internal resistivity distribution. The added schematic and photographs provide a clear representation of the sensor structure, electrode arrangement, and data acquisition process, thereby addressing the reviewer's concern and improving the clarity and reproducibility of the experimental methodology. The importance of clearly documenting the electrode configuration and data acquisition system is well recognized in ERT studies.

2) It is not specified whether the heartwood used in the experiment was dried or undried. Since ERT is a contact-based measurement method, the manuscript fails to adequately demonstrate how effective contact between the ERT electrodes and the test wood was ensured.

 We thank the reviewer for this important observation. The Electrical Resistance Tomography (ERT) measurements were conducted on standing, living trees under natural field conditions. Therefore, all measurements were performed on undried (green) wood, and no dried heartwood samples were used in the experiment. Immediately after ERT scanning, increment cores were extracted from the same trees at the same measurement height (1.37 m) for validation. Consequently, both the ERT measurements and the validation samples represented the natural in situ moisture condition of the wood.

Regarding electrode contact, the PiCUS TreeTronic ERT system uses stainless-steel conductive nails as electrodes. Depending on stem circumference, 8–24 stainless-steel nails were uniformly inserted through the bark until they established firm contact with the conductive sapwood. Before each measurement, the instrument automatically verified the electrical connectivity of all electrodes, and measurements were initiated only after satisfactory contact had been confirmed. During data acquisition, special care was taken to ensure that the electrodes remained firmly seated and that neither the cables nor the tree were disturbed, thereby minimizing contact resistance and ensuring reliable measurements.

To improve clarity, the Materials and Methods section has been revised to explicitly state that the measurements were performed on standing green trees and to describe the electrode installation procedure, contact verification, and quality-control measures adopted during data acquisition.

3) In Figure 2, the colour in the wood cross-section image represents conductivity distribution, but the specific units are not indicated. How was the ERT system calibrated prior to testing?

In the revised manuscript, the colour scale in Figure 3 has been modified to explicitly indicate the measurement unit of electrical resistivity (Ω), thereby improving the interpretation of the tomographic images. The figure caption has also been revised to clarify that the colour gradient represents the spatial distribution of electrical resistivity, with blue indicating low resistivity (higher moisture content) and dark brown indicating high resistivity (lower moisture content).

Regarding calibration, the PiCUS TreeTronic Electrical Resistance Tomograph does not require conventional laboratory calibration before each measurement. Instead, the instrument performs an internal system check before every scan by verifying electrode connectivity, cable continuity, and electrical contact quality. Measurements were initiated only after all electrodes passed the built-in quality assessment. Prior to each measurement, the stem circumference, number of electrodes, and tree identification details were entered into the software, and the instrument automatically optimized the measurement sequence based on the selected electrode configuration. Electrodes showing poor electrical contact were repositioned until satisfactory connectivity was achieved. These quality-control procedures ensured reliable and reproducible resistivity measurements throughout the study.

The Materials and Methods section has been revised accordingly to describe these procedures in greater detail.

4) Figure 2 also shows resistance values. However, resistance is typically measured only between two electrodes. How were the resistance maps shown in the figure obtained? The same question applies to Figures 3 and 4.

We agree that Electrical Resistance Tomography (ERT) does not directly measure the resistance at every location within the wood cross-section. Rather, the PiCUS TreeTronic system injects low-frequency electrical currents sequentially between multiple pairs of electrodes installed around the stem while recording the corresponding voltage responses at the remaining electrodes. These measurements constitute a set of transfer resistance data between electrode pairs.

The colour maps presented in Figures 3–5 are not direct measurements of point resistance. Instead, they are tomographic reconstructions generated by the PiCUS TreeTronic software using an inverse finite-element reconstruction algorithm. The software combines all measured transfer resistance data to estimate the spatial distribution of electrical resistivity throughout the stem cross-section. The radial resistance profiles shown in Figures 3–5 were subsequently derived by extracting the reconstructed resistivity values along a line passing through the centre of the tomogram (from one side of the stem to the opposite side) and averaging these values across the sampled trees. Therefore, the figures represent reconstructed internal electrical resistivity distributions rather than direct electrode-to-electrode resistance measurements.

To avoid misunderstanding, the manuscript has been revised to replace the term "resistance values" with "reconstructed electrical resistivity values" wherever appropriate and to clarify the tomographic reconstruction procedure in the Materials and Methods section.

5) The wood samples in Figure 1 lost moisture rapidly over a short period, causing significant changes in measured resistance. How did the authors ensure consistency in their measurements?

We would like to clarify that the increment core samples shown in Figure 2 were not used for Electrical Resistance Tomography (ERT) measurements. All ERT measurements were performed first on standing, living trees under their natural moisture conditions. The increment cores were extracted only after completion of the ERT scan and were used solely for validating the ERT-derived heartwood diameter.

To minimize the effect of moisture loss on the validation measurements, each increment core was measured immediately after extraction in the field. Heartwood and sapwood boundaries were identified visually based on their distinct natural colour differences, and bark thickness, sapwood width, and heartwood width were measured without allowing the samples to dry. The interval between core extraction and measurement was only a few minutes, thereby minimizing moisture loss and ensuring consistency among samples. Consequently, changes in moisture content of the extracted cores did not influence either the ERT measurements or the validation results.

To clarify this point, the Materials and Methods section has been revised accordingly.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

Dear Authors, hope this finds you well. please kindly find my detailed comments.

ERT is not a completely non-destructive technique

Throughout the manuscript the authors repeatedly describe Electrical Resistance Tomography (ERT) as a non-destructive technique. I do not completely agree with this terminology. The ERT system used in this study requires inserting 8–24 stainless steel electrodes (conductive nails) into the tree stem. Therefore, the method cannot be considered completely non-destructive. It is more appropriate to describe it as a minimally invasive or semi-destructive technique. I strongly recommend revising this terminology throughout the manuscript, including the title, abstract, introduction, discussion, and conclusions.

Abstract

The abstract states:

"The findings demonstrate that ERT is a reliable, rapid, and minimally invasive technique..."

I have two comments regarding this statement.

First, the authors claim that the method is rapid, but no quantitative comparison has been provided. How much faster is ERT compared with increment coring or other conventional methods? Did the authors calculate the total acquisition time? If not, I recommend avoiding the word "rapid" or supporting it with quantitative evidence.

Second, I recommend relying more on the actual findings of this study rather than making general conclusions. The abstract should emphasize the measured results (R², similarity percentage, resistivity ranges, etc.) instead of broad statements about the capabilities of ERT.

Introduction

The introduction repeatedly discusses heartwood quantity estimation.

However, according to the methodology and results, the study estimates heartwood diameter, not heartwood quantity (volume or percentage). Please clarify this terminology throughout the manuscript.

References

Please carefully review the entire manuscript and provide references for all statements that require citations.

Examples include (but are not limited to):

Page 2, lines 59–60

Page 2, lines 69–70

Page 8, lines 317–320

Page 9, lines 321–323

Several review-type statements are currently presented without supporting references.

Introduction (Lines 60–64)

The manuscript states that destructive methods are impractical mainly because they damage the tree. I think this discussion is incomplete. Increment coring and destructive methods have additional disadvantages besides being destructive. For example:

they only provide information at the drilling location rather than the whole cross-section,

they require more field time,

they increase operational costs,

they depend on operator experience,

they may not represent the overall internal condition of the tree.

I recommend expanding this discussion rather than focusing only on the destructive nature of these methods.

Abbreviations

After introducing Electrical Resistance Tomography (ERT) once, please use only the abbreviation "ERT" throughout the remainder of the manuscript. Please check the entire manuscript for consistency.

Materials and Methods

Please specify during which season the field measurements were performed. Electrical resistivity strongly depends on moisture content, and seasonal variations may significantly influence ERT measurements.

Electrode spacing

Page 4, line 151: The manuscript states: "...the stem at equal angular intervals." Please specify the actual angular interval between adjacent electrodes. In addition, please briefly explain why this particular electrode configuration (8–24 electrodes) was selected.

Image reconstruction parameters

Page 4, lines 166–170: The manuscript states that image reconstruction was performed using mesh fitness = 8, smoothness parameter = 20.  Please explain these parameters in more detail. Why were these values selected? What is their influence on the inversion results? Readers unfamiliar with the PiCUS software cannot understand the significance of these parameters.

Figure 2

The quality of Figure 2 should be improved. The colour bars are difficult to read and the image resolution is relatively low. I also recommend using the same colour scale for all ERT results to make comparison between different figures easier. If available, it would be very useful to present one representative increment core corresponding to one ERT image. Showing the bark, sapwood, and heartwood thickness together with the ERT image would considerably strengthen the validation.

Figures 3 and 4

I recommend presenting Figures 3 and 4 together for easier comparison.

More importantly, I strongly recommend analysing the relationship between

tree diameter (DBH) and maximum ERT resistivity.

A dedicated analysis investigating this relationship could significantly improve the scientific contribution of the manuscript and may even represent an additional novelty.

Relationship between ERT and wood properties

Page 8, lines 313–315: The manuscript states: "Thus, ERT measurements indirectly reflect variations in wood density and mechanical properties." I do not completely agree with this conclusion. ERT directly measures electrical resistivity, which is influenced by several interacting factors, including moisture content, electrolyte concentration, extractive content, temperature, and anatomical characteristics. The manuscript does not directly measure wood density or mechanical properties. Therefore, this conclusion is stronger than the presented evidence and should be moderated.

Discussion structure

I recommend moving Section 3.3 (Implication of ERT application and wood properties) into the Introduction or reducing it considerably. Similarly, the subsections: Specific Gravity, Wood Density, Vessel Morphology, Ray Morphology, Wood Fiber Morphology, mainly summarize previously published knowledge rather than presenting results obtained in this study. These sections interrupt the flow of the Results and Discussion and would fit better in the Introduction.

Validation strategy

The validation is based on a single increment core extracted from each tree. However, ERT produces a two-dimensional tomographic image, whereas one core represents only one radial direction. Please discuss this limitation and explain why a single core is sufficient for validating the entire tomographic reconstruction.

Boundary determination

Please explain how the heartwood boundary was determined from the ERT images. Was it identified manually? Was a resistivity threshold applied? Was the process automatic? This is an important methodological detail that is currently missing

Discussion of limitations

The manuscript mainly focuses on the advantages of ERT. A discussion of the limitations would improve the scientific quality of the paper. For example: seasonal moisture variations, electrode contact quality, stem irregularities, species dependency, inversion uncertainty.

Conclusions

Several conclusions are stronger than the presented evidence. Statements suggesting that ERT accurately predicts wood density, mechanical properties, or should become a standard method are not fully supported by the current study. I recommend moderating these conclusions and limiting them to what has actually been demonstrated by the presented results.

Regards,

Author Response

ERT is not a completely non-destructive technique

Throughout the manuscript the authors repeatedly describe Electrical Resistance Tomography (ERT) as a non-destructive technique. I do not completely agree with this terminology. The ERT system used in this study requires inserting 8–24 stainless steel electrodes (conductive nails) into the tree stem. Therefore, the method cannot be considered completely non-destructive. It is more appropriate to describe it as a minimally invasive or semi-destructive technique. I strongly recommend revising this terminology throughout the manuscript, including the title, abstract, introduction, discussion, and conclusions.

We respectfully submit that, in the present study, Electrical Resistance Tomography (ERT) may appropriately be described as a non-destructive technique. The PiCUS TreeTronic system employs 2 mm diameter conductive nails (using cordless nailer), which produce only very small punctures in the outer stem. These punctures are superficial, do not remove wood tissue, do not alter the internal structure of the stem, and have no measurable effect on tree stability, heartwood formation, or timber quality. Consequently, the tree remains intact and fully functional after the measurements.

Unlike increment coring or drilling, which remove wood tissue and create relatively large wounds, the ERT procedure involves only temporary insertion of fine electrodes to establish electrical contact. The resulting punctures naturally seal through the tree's compartmentalization processes and are widely regarded as causing negligible injury. For this reason, ERT has been extensively described in the literature as a non-destructive technique for assessing the internal condition of standing trees.

To avoid ambiguity, we have clarified in the revised manuscript that the ERT measurements were performed using 2 mm nails as electrodes, which produce only negligible puncture wounds while preserving the structural integrity and commercial value of the tree.

Abstract

The abstract states:

"The findings demonstrate that ERT is a reliable, rapid, and minimally invasive technique..."

I have two comments regarding this statement.

First, the authors claim that the method is rapid, but no quantitative comparison has been provided. How much faster is ERT compared with increment coring or other conventional methods? Did the authors calculate the total acquisition time? If not, I recommend avoiding the word "rapid" or supporting it with quantitative evidence.

Comment 1 – Use of the term "rapid". We agree that our study did not include a formal comparison of the acquisition time between Electrical Resistance Tomography (ERT) and conventional methods such as increment coring. Therefore, we have removed the word "rapid" from the abstract and conclusions to avoid making an unsupported comparative claim. Instead, we now state that each ERT measurement required approximately 10–15 minutes per tree, as recorded during the field investigation, without implying superiority over other techniques.

Second, I recommend relying more on the actual findings of this study rather than making general conclusions. The abstract should emphasize the measured results (R², similarity percentage, resistivity ranges, etc.) instead of broad statements about the capabilities of ERT.

Comment 2 – Emphasis on study findings. We agree that the abstract should focus more on the quantitative outcomes of the present study. Accordingly, we have revised the abstract to emphasize the measured results, including the observed resistivity ranges, the coefficient of determination (R² = 0.98) between ERT-estimated and measured heartwood diameters, and the average similarity of 91.5%. General statements regarding the broader capabilities of ERT have been reduced so that the conclusions are based primarily on the findings of this investigation.

Introduction

The introduction repeatedly discusses heartwood quantity estimation. However, according to the methodology and results, the study estimates heartwood diameter, not heartwood quantity (volume or percentage). Please clarify this terminology throughout the manuscript.

We agree that the terminology used in the original manuscript was not sufficiently precise. The present study estimates heartwood diameter in standing trees using Electrical Resistance Tomography (ERT) and does not directly quantify heartwood volume, mass, or percentage. Therefore, the term "heartwood quantity" has been replaced with "heartwood diameter" or "heartwood size", as appropriate, throughout the manuscript, particularly in the Introduction, to accurately reflect the scope and objectives of the study. These revisions ensure consistency between the Introduction, Methodology, Results, and Conclusions and avoid any ambiguity regarding the parameter being estimated.

References: Please carefully review the entire manuscript and provide references for all statements that require citations. Examples include (but are not limited to): Page 2, lines 59–60; Page 2, lines 69–70; Page 8, lines 317–320; Page 9, lines 321–323. Several review-type statements are currently presented without supporting references.

Performed a comprehensive scientific reference audit and identify every statement that requires citation.

Introduction (Lines 60–64)

The manuscript states that destructive methods are impractical mainly because they damage the tree. I think this discussion is incomplete. Increment coring and destructive methods have additional disadvantages besides being destructive. For example: they only provide information at the drilling location rather than the whole cross-section, they require more field time, they increase operational costs, they depend on operator experience, they may not represent the overall internal condition of the tree. I recommend expanding this discussion rather than focusing only on the destructive nature of these methods.

We have substantially revised the relevant paragraph in the Introduction to provide a more comprehensive discussion of the limitations of conventional heartwood assessment methods. In addition to their destructive nature, we now discuss the practical and technical limitations of destructive and semi-destructive approaches, including their localized sampling that may not represent the entire stem cross-section, increased field time and operational costs, dependence on operator expertise, potential sampling bias, and the risk of reducing timber quality and tree health through repeated increment coring. This expanded discussion better highlights the need for non-destructive techniques such as Electrical Resistance Tomography (ERT) for comprehensive heartwood assessment in standing trees.

Abbreviations: After introducing Electrical Resistance Tomography (ERT) once, please use only the abbreviation "ERT" throughout the remainder of the manuscript. Please check the entire manuscript for consistency.

We have carefully reviewed the entire manuscript to ensure consistent terminology. The term Electrical Resistance Tomography (ERT) is now defined at its first occurrence, and the abbreviation ERT is used consistently throughout the remainder of the manuscript. All instances of the full term appearing after its initial introduction have been replaced with the abbreviation to improve readability and maintain consistency.

Materials and Methods

Please specify during which season the field measurements were performed. Electrical resistivity strongly depends on moisture content, and seasonal variations may significantly influence ERT measurements.

We have revised the Materials and Methods section to specify the period during which the field measurements were conducted. The manuscript now clearly states that all ERT measurements were performed during the dry season to minimize the influence of seasonal moisture fluctuations on electrical resistivity and to ensure consistent measurement conditions across all sampled trees. We have also included a brief discussion acknowledging that wood moisture content has a significant influence on electrical resistivity and that seasonal variation may affect ERT measurements, which should be considered when interpreting the results.

Electrode spacing

Page 4, line 151: The manuscript states: "...the stem at equal angular intervals." Please specify the actual angular interval between adjacent electrodes. In addition, please briefly explain why this particular electrode configuration (8–24 electrodes) was selected.

We have revised the Materials and Methods section to clarify the electrode configuration used in the study. The manuscript now states that eight stainless-steel electrodes (nails) were installed uniformly around the stem circumference at 45° angular intervals (360°/8). The electrodes were inserted perpendicular to the tree trunk (90° to the stem surface) to ensure consistent electrical contact.

We have also clarified that the use of eight electrodes was standardized for all trees included in the study because preliminary standardization trials showed that increasing the number of electrodes did not result in any appreciable change in the measured electrical resistance or estimated heartwood size. Therefore, the eight-electrode configuration was adopted as it provided reliable tomographic reconstruction while minimizing field time and operational complexity.

Image reconstruction parameters

Page 4, lines 166–170: The manuscript states that image reconstruction was performed using mesh fitness = 8, smoothness parameter = 20.  Please explain these parameters in more detail. Why were these values selected? What is their influence on the inversion results? Readers unfamiliar with the PiCUS software cannot understand the significance of these parameters.

We have revised the Materials and Methods section to provide a clearer explanation of the image reconstruction parameters used in the PiCUS TreeTronic software.

The manuscript now explains that mesh fitness controls the density and refinement of the computational mesh used for tomographic image reconstruction. A higher mesh fitness generates a finer mesh with more elements, allowing better representation of internal resistivity variations, but it also increases computational time. A mesh fitness value of 8 was selected because it provides an optimal balance between image resolution, computational efficiency, and reconstruction stability, and is the recommended setting for routine tree tomography.

We have also clarified that the smoothness parameter regulates the degree of spatial smoothing applied during the inversion process. Lower values permit greater local resistivity variation but may amplify measurement noise and produce image artefacts, whereas higher values generate smoother resistivity distributions but may obscure genuine internal boundaries. A smoothness value of 20 was selected to achieve a stable inversion by reducing noise while preserving the major resistivity contrasts associated with heartwood and sapwood. These parameter values were retained consistently for all trees to ensure uniform image reconstruction and facilitate comparison among samples.

Figure 2

The quality of Figure 2 should be improved. The colour bars are difficult to read and the image resolution is relatively low. I also recommend using the same colour scale for all ERT results to make comparison between different figures easier. If available, it would be very useful to present one representative increment core corresponding to one ERT image. Showing the bark, sapwood, and heartwood thickness together with the ERT image would considerably strengthen the validation.

Thank you for your valuable suggestions. We have improved the quality and resolution of Figure 3 to enhance the readability of the colour bars and overall image clarity. The colour scale has also been standardized across all ERT images to facilitate direct comparison between figures.

An attempt was also made to compare a representative ERT tomogram with the corresponding increment core. Although the increment core clearly delineated the bark, sapwood, and heartwood regions, the ERT images could not distinguish the bark from the sapwood due to their similar electrical resistivity properties. Therefore, only the heartwood boundary could be reliably interpreted from the tomograms, and a complete anatomical correspondence between the ERT image and the increment core could not be established. This limitation prevented the implementation of a direct image-to-core comparison.

Figures 3 and 4

I recommend presenting Figures 3 and 4 together for easier comparison. More importantly, I strongly recommend analysing the relationship between tree diameter (DBH) and maximum ERT resistivity. A dedicated analysis investigating this relationship could significantly improve the scientific contribution of the manuscript and may even represent an additional novelty.

Following the reviewer's recommendation, Figures 3 and 4 have been combined into a single composite (figure 4) to facilitate direct comparison between the ERT tomograms and the corresponding heartwood measurements. In addition, we investigated the relationship between tree diameter at breast height (DBH) and the maximum electrical resistivity recorded from each tomogram. A regression analysis was performed, and the results have been incorporated into the Results and Discussion sections as a new figure (Figure 5). This additional analysis demonstrates the association between stem size and maximum resistivity and provides further insight into the applicability of ERT for heartwood assessment, thereby strengthening the scientific contribution of the study.

Relationship between ERT and wood properties

Page 8, lines 313–315: The manuscript states: "Thus, ERT measurements indirectly reflect variations in wood density and mechanical properties." I do not completely agree with this conclusion. ERT directly measures electrical resistivity, which is influenced by several interacting factors, including moisture content, electrolyte concentration, extractive content, temperature, and anatomical characteristics. The manuscript does not directly measure wood density or mechanical properties. Therefore, this conclusion is stronger than the presented evidence and should be moderated.

We agree that the original statement was too strong. ERT directly measures electrical resistivity, which is influenced by multiple interacting factors, including moisture content, electrolyte concentration, extractive content, temperature, and wood anatomical characteristics. Since wood density and mechanical properties were not measured in the present study, we have moderated the statement to avoid implying a direct relationship. The revised text now emphasizes that ERT-derived resistivity patterns may be associated with variations in wood properties but should not be interpreted as direct measurements of density or mechanical characteristics.

Discussion structure

I recommend moving Section 3.3 (Implication of ERT application and wood properties) into the Introduction or reducing it considerably. Similarly, the subsections: Specific Gravity, Wood Density, Vessel Morphology, Ray Morphology, Wood Fiber Morphology, mainly summarize previously published knowledge rather than presenting results obtained in this study. These sections interrupt the flow of the Results and Discussion and would fit better in the Introduction.

We agree that the original Section 3.3 contained substantial background information on the physical and anatomical properties of Pterocarpus santalinus that has been established in previous studies rather than generated in the present investigation. Accordingly, we have substantially revised the manuscript. Most of the descriptive background on specific gravity, wood density, vessel morphology, ray morphology, and wood fibre morphology has been moved to the Introduction, where it provides the necessary context for understanding the electrical resistivity patterns observed in this study. The corresponding section in the Results and Discussion has been considerably shortened and now focuses only on interpreting how these established wood properties explain the ERT results obtained in the present study. This revision improves the overall flow of the manuscript and clearly distinguishes background information from the discussion of our findings.

Validation strategy

The validation is based on a single increment core extracted from each tree. However, ERT produces a two-dimensional tomographic image, whereas one core represents only one radial direction. Please discuss this limitation and explain why a single core is sufficient for validating the entire tomographic reconstruction.

We have addressed this limitation in the revised manuscript by explicitly discussing that validation was based on a single increment core extracted from each tree, whereas ERT generates a two-dimensional tomographic image. We acknowledge that a single core represents only one radial direction and therefore cannot fully validate the entire tomographic reconstruction. However, because the primary objective of this study was to validate heartwood diameter, the core was extracted along the radial direction corresponding to the maximum heartwood extent identified in the ERT image. This approach provides a practical and minimally destructive reference for validating the estimated heartwood boundary while avoiding excessive damage to the valuable Pterocarpus santalinus trees. The limitation has been clearly stated in the Discussion section, and we recommend that future studies employ multiple increment cores or stem discs from harvested trees for more comprehensive validation of the complete tomographic reconstruction.

Boundary determination

Please explain how the heartwood boundary was determined from the ERT images. Was it identified manually? Was a resistivity threshold applied? Was the process automatic? This is an important methodological detail that is currently missing

Determination of the heartwood boundary from ERT images: The electrical resistivity tomograms generated using the PiCUS TreeTronic software display the spatial distribution of resistivity using a continuous colour scale, with lower resistivity represented by blue and green colours and higher resistivity by yellow, orange, red, and brown. In Pterocarpus santalinus, the heartwood is characterized by consistently higher electrical resistivity than the surrounding sapwood due to its lower moisture content and higher extractive concentration. Accordingly, the heartwood boundary was identified by visually delineating the transition between the high-resistivity central region and the lower-resistivity peripheral sapwood on each tomogram. No fixed resistivity threshold was applied because absolute resistivity values varied among trees owing to differences in stem size, moisture status, and other biological factors. Likewise, no automatic image segmentation algorithm was used. Instead, the boundary was interpreted manually by experienced operators based on the distinct resistivity contrast, and the estimated heartwood diameter was subsequently validated against measurements obtained from increment cores. This relative interpretation approach follows the standard practice recommended for PiCUS ERT applications, where internal compartment boundaries are identified from resistivity patterns rather than universal resistivity thresholds.

Discussion of limitations

The manuscript mainly focuses on the advantages of ERT. A discussion of the limitations would improve the scientific quality of the paper. For example: seasonal moisture variations, electrode contact quality, stem irregularities, species dependency, inversion uncertainty.

In response to the reviewer's comment, a paragraph has been added before the Conclusion section to address the above-mentioned issue.

Conclusions

Several conclusions are stronger than the presented evidence. Statements suggesting that ERT accurately predicts wood density, mechanical properties, or should become a standard method are not fully supported by the current study. I recommend moderating these conclusions and limiting them to what has actually been demonstrated by the presented results.

The conclusions have been revised to better reflect the evidence presented in this study. Statements implying that Electrical Resistance Tomography (ERT) can accurately predict wood density, mechanical properties, or should be adopted as a standard assessment method have been moderated. The revised conclusions emphasize that ERT is an effective non-destructive technique for delineating heartwood boundaries and estimating heartwood dimensions in Pterocarpus santalinus, while acknowledging that electrical resistivity is influenced by multiple interacting factors, including moisture content, extractive composition, electrolyte concentration, and wood anatomy. Accordingly, any relationships between resistivity and wood density or mechanical properties are discussed only as indirect associations supported by previous studies rather than as direct findings of the present work.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Dear Authors,

Thank you for your careful responses to my previous questions and comments. I appreciate the significant effort you have made to revise the manuscript. At this stage, I only have a few minor comments:

Page 17, Line 640: I believe there is a typographical error. The heading Conclusions appears twice. Please check and correct this.

Recommendations section: The recommendations are currently presented as numbered bullet points. I suggest rewriting this section as one or two concise paragraphs to maintain consistency with the style of the manuscript.

Figure 1: The explanation of the colour scale should be interpreted more cautiously. You currently state:

Brown: higher resistance because of low moisture content

Red: high resistance (comparatively lower than brown)

Yellow: medium resistance because of slight moisture content

Light blue: sapwood with low resistance because of moderate water content

Blue: sapwood with low resistance because of high water content

These interpretations are valid for the healthy red sanders trees investigated in this study. However, such colour distributions cannot always be interpreted in this way. For example, high- or low-resistivity anomalies may also result from internal defects such as cavities, decay, or other structural abnormalities, depending on the condition of the tree. I therefore recommend clarifying that these descriptions apply specifically to the healthy tree trunks examined in this study rather than presenting them as universally applicable.

Author Response

 

Reviewer #2 (2^nd round corrections)

Page 17, Line 640: I believe there is a typographical error. The heading Conclusions appears twice. Please check and correct this.

The manuscript was checked thoroughly, and the duplicate “Conclusions” heading at Page 17, Line 640 has been corrected. The manuscript was also carefully checked for similar typographical errors.

Recommendations section: The recommendations are currently presented as numbered bullet points. I suggest rewriting this section as one or two concise paragraphs to maintain consistency with the style of the manuscript.

Thank you for this valuable suggestion. The Recommendations section has been revised accordingly. The numbered bullet points have been consolidated into two concise paragraphs to improve the flow and maintain consistency with the overall style of the manuscript.

Figure 1: The explanation of the colour scale should be interpreted more cautiously. You currently state:

Brown: higher resistance because of low moisture content

Red: high resistance (comparatively lower than brown)

Yellow: medium resistance because of slight moisture content

Light blue: sapwood with low resistance because of moderate water content

Blue: sapwood with low resistance because of high water content

These interpretations are valid for the healthy red sanders trees investigated in this study. However, such colour distributions cannot always be interpreted in this way. For example, high- or low-resistivity anomalies may also result from internal defects such as cavities, decay, or other structural abnormalities, depending on the condition of the tree. I therefore recommend clarifying that these descriptions apply specifically to the healthy tree trunks examined in this study rather than presenting them as universally applicable.

We agree that the interpretation of ERT colour patterns should not be generalized, as electrical resistivity is influenced by several factors, including moisture content, ionic concentration, extractives, anatomical characteristics, and internal defects. Accordingly, the description of the colour scale in Figure 1 has been revised to clarify that the observed colour patterns and their associated interpretations apply specifically to the healthy red sanders trees investigated in the present study. We have also clarified that high- or low-resistivity anomalies in other trees may arise from defects such as cavities, decay, or other structural abnormalities and therefore should be interpreted in conjunction with tree condition and other available evidence.

Author Response File: Author Response.docx