the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Aug 2025
Ecological and environmental controls on plant wax production and stable isotope fractionation in modern terrestrial Arctic vegetation
Abstract. Terrestrially-derived plant waxes and their compound-specific stable carbon (δ13C) and hydrogen (δ2H) isotope ratios are valuable tools for inferring past changes in vegetation and hydrology in sedimentary archives. Such inferences require knowing the ecological (i.e. plant growth form) and environmental (i.e. temperature, precipitation, relative humidity, elevation) mechanisms that govern the production of different plant wax carbon chain-lengths and the fractionation of stable isotopes. These mechanisms, however, are uncertain in the Arctic, limiting our ability to infer past vegetation and hydrology changes. To address this, we produced terrestrial plant n-alkanoic acid and n-alkane abundance and δ13C and δ2H data from a latitudinal environmental gradient along the Eastern Canadian Arctic (105 individuals), which we compiled with published data across the Arctic (additional 281 individuals). We compared this dataset with environmental parameters to assess the mechanisms that govern plant-wax production and isotope fractionation. We found that total plant wax concentrations and Average ChainLength (ACL) were statistically different between vascular (trees, shrubs, forbs, ferns, graminoids) and non-vascular plants (mosses, liverworts) and lichens, whereas δ13C values and δ2H apparent fractionation relative to growing season precipitation δ2H often did not differ significantly between plant growth forms. Correlations between plant wax indices and mean of the months above freezing (MAF) environmental parameters were generally weak (r ≤ 0.4), and/or not significant (p > 0.05). These results suggest that a fundamental assumption to paleoclimate research holds in the Arctic: for individual plant taxa and plant communities, the abundance, ACL, and δ13C/δ2H isotopic fractionation of both n-alkanoic acids and n-alkanes is independent of temperature, precipitation, humidity, and elevation. Instead, changes in sedimentary plant wax distributions reflect changes in plant taxa present through time, and changes in plant wax δ2H reflect changes in source water δ2H. Therefore, plant waxes can be used to infer past changes in climate and ecology.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-3849', Anonymous Referee #1, 05 Sep 2025
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RC2: 'Comment on egusphere-2025-3849', Anonymous Referee #2, 18 Sep 2025
Review of egusphere-2025-3849.
Ecological and environmental controls on plant wax production and stable isotope fractionation in modern terrestrial Arctic vegetation. By Kurt R. Lindberg, Elizabeth K. Thomas, Martha K. Raynolds, Helga Bültmann, and Jonathan H. Raberg
Dear associate editor and authors, I have read this manuscript with great pleasure, it is an interesting topic. I do have some questions though, some more scientific and some more technical. I will start with a scientific question, what about meltwater? If I understand the authors correctly they looked at relatively early growth, so when the snow and ice is still melting? The hydrogen isotope composition of snow is, as far as I know, different from rain. Plus, snow and ice maybe derived from other seasons than the precipitation during the growing season. Of course, different plant types might have different access to meltwater, some have roots and can access ground water that I guess in part is meltwater, some might live on rocks or trees and have less access to meltwater and some might live in bogs (or even “lakes” and have access to meltwater much longer time span. Could this affect the hydrogen isotopic composition of the different compounds measured and for instance have kept the 2H values much more stable than expected based on precipitation values?
That brings me to my second comment or question. I really like the Bowen model and tool to reconstruct or estimate precipitation 2H values. However, the quality depends largely on the proximity of measuring stations. The interpolations between measuring stations can be very good if the landscape is fairly boring, as soon as you get elevation differences, for instance, they might be off. This made me think that the strong dependence on epsilon values might over simplify things. Has this been considered?
The authors give an overview of things hat might affect the 2H values, the one I was missing was the amount effect. I don’t know if that plays a role in the settings discussed here, but I think there are elevation differences, perhaps there are also areas with way more precipitation than others? By the way, also a reason why sometimes the 2H model might have the 2H of precip. wrong, the amount effect.
My last main comment is on the M and M section, I think the technical section the 2H measurements is a bit light. What reference materials were measured on the same machine to ensure data quality? Did the authors consider that peaks have to have a decent size for a reliable measurement? Measuring samples or compounds multiple times at roughly the same low peak height (or area) will give you reproducible values, not necessarily the “correct” values. I agree that this doesn’t need to be in the manuscript, but the lack of technical details made me wonder. Of course, the authors have put a lot of effort in the statistical analysis of the results, but if the quality of the results can not be judged by the reviewer and other readers the statistics are also not so useful.
In an Australian study, I think leaf water enrichment and relative humidity were determined to be the most important, I think. They measured along a transect with very little difference in source water 2H and came to that conclusion. I think a paper by Ansgar Kahmen and or his group. Just a suggestion, I know very different environment.
Overall, I think the manuscript needs some work, perhaps different water sources, (including precipitation, ice, snow, meltwater, lake water etc.) have been measured and can be compared to the model results for possibly more accurate epsilon values? I definitely would like to be able to better judge the quality of the 2H measurements, so a bit more detail I the M en M section would be appreciated. If these questions have been addressed I think the manuscript is very publishable.
Some more detailed remarks:
Line 4: that govern stable hydrogen isotope fractionation?
Line 16 and 17: independent of precipitation but reflecting source water 2H, so are precipitation and source water different things? Precipitation amount?
Line 35: Baas et al. A comparative study of lipids in Sphagnum species (2000) Organic Geochemistry 31 535- 541. Ficken et al. An n-alkane proxy for the sedimentary input of submerged/ floating freshwater aquatic macrophytes (2000) Organic Geochemistry 31 745-749.
Line 45: higher and lower d13C values rather than more or less depleted or enriched?
Line 57: the precipitation amount, the amount effect?
Figure legend of figure 2, precipitation is in 2e not 2d.
Line 164: I assume the H3+ factor slowly changed from the one to the other value over a significant amount of time and this was not the day to day variation? I do like that you mention the H3+ factor, lots of people have stopped doing that.
Line 247: an average of 30.5 plus or minus 18.6 permil. What drives this range?
Line 294: this is from the Bowen model, right, not measured? I know measurements also have their issues.
Line 378-379: So, what about meltwater?
Line 441: enriched and depleted relative to what? Each other? I love isotope lingo, but there are a lot of potential readers out there that get confused with all these relative terms. It might be better to use delta values and lower and higher.
Line 464-466: Not only the mean annual is important, also the amount that falls in one session (again the amount effect) is important. That is a measure that is not always captured by yearly means and averages, every day a little bit or everything in just 2 days, it makes a difference for the plants, sure, but also for the 2H of the precipitation. Apparent fractionation that varies with 600 mm?
Line 469: Could that be, to some degree, an effect of using the model and it not capturing al the variability there actually is I precipitation 2H? In figure 2d I noticed quite some elevation differences, for instance.
Citation: https://doi.org/10.5194/egusphere-2025-3849-RC2
Data sets
Lindberg_Arctic_terrestrial_plantwax.xslx Kurt Lindberg https://doi.org/10.5281/zenodo.16754381
Interactive computing environment
Arctic terrestrial plant wax code and data v1.0.1 Kurt Lindberg https://doi.org/10.5281/zenodo.16754381
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Lindberg et al. investigate patterns in leaf wax (n-alkane and n-alkanoic acid) distribution in vegetation from the Arctic. The work provides a foundation for interpreting paleoclimate records based on leaf wax molecular proxies in sediment cores. Their assessment of the environmental parameters (meteorological/environmental effects, and vital effects of different species) is thorough, in that it combines their large dataset form the Eastern Canadian Arctic with a pan-Arctic synthesis. Ultimately, they provide a practical tool for paleoclimatologists working in the Arctic.
The manuscript is very clearly written, includes a solid statistically-based discussion, includes pertinent citations, and is essentially ready for publication, although I have two considerations for the authors, as well as a few very minor comments.
The first consideration is that latitude is not tested as an environmental parameter. We don't typically think of latitude as a driving factor, but in the case of Arctic leaf waxes, there has been discussion on if day-length impacts leaf wax hydrogen isotopes. Perhaps their study sites don’t span a substantial gradient in day-length, or the length of the 24-hour daylight season (latitude is a proxy for this), but I think it would be worth at least acknowledging this point. That is, a previous study from Baffin Island by Shanahan et al. (2013) had anomalously small fractionation values between precipitation and leaf wax D/H – how does that previous. In fact, this is one paper that seems like it should be cited, or explained why it is not included in the synthesis.
The second consideration is that among all the environmental parameters tested, they did not include Vapor Pressure Deficit. I wonder, if they combine the air temperature, relative humidity, and added in the modeled leaf temperature, would they find strong gradients in VPD across their sites, and how would this relate to the epsilon value. It has been untested, to my knowledge, but could be potentially revealing as an important environmental control.
Minor comments:
Paragraph at line 244, which refers to Figure S2: Specify again in this paragraph that this includes data points from all plant types. Also, you mention that the pan-Arctic dataset has an n=386. But it does not look like Figure S2 has 386 data points. Can you clarify what is included in this figure?
Figure 7 caption: specify if this is the pan-Arctic dataset or the ECA dataset. (It’s stated in the text, but would help clarify the figure caption.)