Overall, I think this is extremely valuable study on differences in dune grass traits to inform modeling. However, I think the current framing of the study narrows the audience of the paper more than is needed. Some careful reframing of the introduction and discussion would make the paper more interesting to readers who are not intimately familiar with dune models. For example, I think the discussion could elaborate on how the findings alter our understanding of dune resistance and recovery. Additionally, can these findings inform modeling in dunes with different grass species or other coastal ecosystems?

I found the introduction overly lengthy. I have some specific suggestions below, but I generally think the readability of this section could be improved if extra details were removed. This paper is likely written for someone familiar with sand dunes, and certainly someone familiar with coastal vegetation, so think about what background is common knowledge to the reader. Briefly, you want to convey that dune vegetation traits modulate sediment dynamics, and that some traits are not well understood while others vary with season, mechanical stressors, and soil properties. Some of these main points are currently buried because there’s so much info in the intro. Also, there is no mention of different grass components and how these are important, but differences between plant parts (sprouts, leaves, stems) are a key part of the study that could use introducing.

Minor Comments

-Line 32: “Short term changes” to what?

-Paragraph 2: The first half of this paragraph feels very drawn out, especially to a reader knowledgeable about dunes or Nbs. I think distilling to the key points would be valuable, especially since vegetation and models are not mentioned until the 2^nd half of the paragraph

Paragraph beginning line 66: This has a lot of good info summarizing what has been done, but it is challenging to read with all the parentheticals and citations. Could it be summarized or presented in a supplementary table? Most readers will not need all this info I don’t think

Intro generally: Why are dune models important and what are current models missing? Addressing this will explain why this paper is so valuable

Paragraph beginning 182: Are these conditions representative of other sites dominated by marram grass? In other words, do you expect that the properties measured in this study will apply to marram grass elsewhere?

Methods: Great distilling of methods and use of supplementary information.

Line 252: the name of the universal testing machine did not proof correctly

Line 260: define Young’s modulus.

Line 273: elaborate on why these specific metrics are important. What does a different EI mean for dune growth, for instance. It may be helpful to mention this earlier in the bending test section.

Line 278: I support grouping into summer and winter seasons, but in Fig. 2 and 3 the terms spring and autumn are used, which does not match this grouping

Fig 4: mention in caption that no flowering was observed in winter so it doesn’t look like a bar is missing from the graph

Section beginning Line 230: Present these results in the order they appear in Fig 5 or re-order fig. 5 to match the presentation of results here. Going from sprouts to leaves to stems felt odd when the figure was sprouts, stems, leaves. Same for the stiffness figure

Fig 5: Explain why there are no winter stems in the caption

Figures 8-13: I’m wondering if the figures with the same metric can be combined. For instance, can Fig. 8 and 9 be combines with the Dune Ridge site shown in darker colors than the Cusp Dune site? This will facilitate better comparisons between the sites and eliminate the need for Fig. 14

All data figures: You can absolutely disagree with this, but I think it may be beneficial to remove some of the extreme values from the graphs. This would allow you to keep the scales consistent and zoom in on the majority of the data. For example, in Figure 11, it’s hard to see how the data differs because the plots are so small. I think you can say “Seven extreme values from Brown Leaves fell outside the scale of this figure and were excluded to enable better visualization.”

Discussion: Generally somewhat repetitive of the results section. I’d recommend removing some areas that only restate the results and instead focusing on the implications of these findings. What does this finding mean for dune evolution or dune modeling? What critical factors were learned here and how do these findings improve our understanding of dune systems?

Section 4.5: I think these considerations could be summarized in a single paragraph, this section is very detailed for a general reader

Conclusions: Can the conclusions highlight concrete guidance rather than a summary of the results? For example, how should models be adjusted to account for seasonal variation. Should managers collect data on plant parts in order to accurately model these dynamics?

This study addresses a relevant and timely subject: refined biomechanical parameterization of a common coastal sand dune plant. With increasing concern about climate change, sea-level rise, and the role of dunes in coastal hazard mitigation, there are growing calls for robust modelling of dune dynamics. As the authors note, this requires appropriate parameterization of vegetation.

I really like the concept behind this study, and the selection of parameters to measure is robust and well-justified. However, the authors need to make it clear when they are referring to studies on salt marsh vegetation. These studies are heavily drawn upon to justify the research, but it requires familiarity with the literature to recognize that the authors are referring to different coastal environments with distinct physical processes. The transferability of findings from salt marsh systems to dune ecosystems is not always straightforward and should be explicitly discussed.

Overall, I found the situating of this study within the broader aeolian dune literature to be a weakness. One specific issue was the classification of the two sampling sites as "fixed" and "dynamic." While this might be accurate for the broader landscape in which they were sampled, the biomechanical parameters of the plants themselves will reflect local patterns of sediment deposition and wind exposure. Dynamic dunes can contain areas with low sediment accumulation, while parts of fixed dunes (e.g., the crest of foredunes) can experience relatively high deposition. It is unclear from the site descriptions or the provided photos whether the sampling locations differ substantially in sediment deposition/erosion regimes. This is important because sand deposition is a strong determinant of marram vigour. Consequently, the conclusion that biomechanical properties of marram are broadly transferable between fixed and dynamic dunes seems somewhat overstated. Greater consideration of the relationship between sand deposition and above-ground plant properties in dune environments, as well as aeolian processes, is needed.

I also found the seasonal characterization of summer accretion and winter erosion to be overly simplistic. It needs to be made clear that you refer specifically to wave erosion, not aeolian erosion. Not all coastal systems exhibit strong seasonal patterns, and further, the link between above-ground biomechanical properties and wave erosion is not explicit. I can see how a focus on regrowth following scarping may have local effects on the most seaward plants, or how frequent overwash could alter above-ground properties, but it is not clear how this applies to the two study sites. Moreover, if the authors are attempting to link seasonal differences in abiotic disturbance regimes to seasonal biomechanical properties, then they need to explicitly describe the conditions at their study sites over the duration of the study. If winter erosion did not occur during the sampling period, then no conclusions can be drawn about seasonal erosion/accretion cycles and their effects on biomechanical properties. I recommend focusing this section more directly on seasonal growth patterns rather than attempting to link them to broad seasonal erosion trends.

The results are presented as many pairwise comparisons in boxplots. As a minor comment, I found these somewhat difficult to follow. The authors should consider how to consolidate some figures and emphasize the results that show significant differences. The appendices contain several results, and it should be reconsidered whether these should be included in the main text. Additionally, the manuscript lacks justification for the selection of the Mann-Whitney test over other statistical approaches. This is critical for interpreting the significance of the findings. Furthermore, the full results of the Mann-Whitney test need to be provided. The appendices currently include only p-values, which are insufficient for assessing the practical significance of the tests.

Overall, while I have no particular issues with the sampling strategy or characterization of biophysical properties, I find that the significance of the findings and their broader application is overstated. My concerns fall into two main areas:

1)    The study’s perspective on marram behavior, and dune system dynamics in general, is quite Eurocentric. White and grey dunes need to be clearly defined, as these terms are not widely used outside of Europe—no equivalent grey dune classification exists in New Zealand, for example. Based on my experience with marram and its congeners, seasonal growth patterns vary considerably between arid environments (such as Australia), temperate regions (such as New Zealand), and areas with cold winters (such as the Great Lakes). Therefore, the absolute biomechanical values provided at the end of the manuscript as potential modelling inputs may not be representative of environments beyond the study area.

2)    Despite statistically significant differences in several measured parameters, the magnitude of these differences is small. Examination of the boxplots shows substantial overlap in the range of measured values, and the reported differences in means or medians (it is unclear which are used in the text) are relatively minor. Given that the study is framed as addressing a need for improved vegetation proxies in aeolian modelling, it is unclear whether these differences are meaningful in a practical sense. Does an average canopy height difference of 6 cm between summer and winter significantly impact aeolian processes? The authors do not explore this question. The paper would benefit greatly from either testing the importance of these differences within an aeolian model or at least conceptually discussing what constitutes a meaningful variation for modelling purposes. As it stands, the study can only conclude that significant differences exist in some variables, and models should take these into account—but without further analysis, it cannot definitively state the practical implications of these differences.

Presentation-wise, the manuscript is well-organized, with a logical flow from background to methods, results, and discussion. However, a large amount of information is placed in the appendix, and it should be reconsidered whether some of this should be integrated into the main text. The introduction is rather long and could be streamlined to focus more directly on the study’s key contributions while maintaining necessary background context. Additionally, the discussion section should move beyond simply restating results to consider the practical implications for modelling. Specifically, the authors should address whether the observed differences will have a meaningful impact on model outputs. If so, what do the results suggest about the most important vegetation attributes—e.g., height, flexibility, or horizontal coverage?

Finally, the inclusion of soil data seems out of place. Since it is not linked to the main findings, I suggest removing it. Additionally, the wind data for the active site should be presented in terms of windward and leeward exposure, or another metric that conveys wind impact. Simply listing wind directions does not provide sufficient insight into exposure levels, making the significance of the results difficult to interpret.

Overall, this study presents useful data on dune vegetation biomechanics with clear relevance for coastal modelling. Its main contribution lies in providing a framework to decompose the different components of plant structure that influence sedimentation and in highlighting the need to consider population-level variation. It takes an important step toward identifying which biomechanical traits matter most, but the discussion does not provide sufficient generalization beyond the two studied populations. Additionally, the study must better address the practical significance of the identified statistical differences in terms of sediment capture and aeolian processes.