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the Creative Commons Attribution 4.0 License.
| 26 Feb 2024
Tidal influence on carbon dioxide and methane fluxes from tree stems and soils in mangrove forests
Abstract. Mangroves are critical blue carbon ecosystems. Measurements of methane (CH4) emissions from mangrove tree stems have the potential to reduce the uncertainty in the capacity of carbon sequestration. This study is the first to simultaneously measure the CH4 fluxes from both stems and soils throughout tidal cycles. We quantified carbon dioxide (CO2) and CH4 fluxes from mangrove tree stems of Avicennia marina and Kandelia obovata during tidal cycles, which have distinct root structures. The mangrove tree stems served as both net CO2 and CH4 sources. Compared to those of the soils, the mangrove tree stems exhibited markedly lower CH4 fluxes, but no difference in CO2 fluxes. A. marina (with pneumatophores) exhibited significantly higher CO2 and CH4 fluxes than K. obovata. The stems of A. marina exhibited an increasing trend in the CO2 flux from low to high tides, while the CH4 flux showed high temporal variability, with this species functioning as a sink before tidal inundation and becoming a source during low tides after ebbing. In contrast, the stems of K. obovata showed no consistent pattern of the CO2 or CH4 flux. Based on our findings, sampling only during low tides might overestimate the stem CO2 and CH4 fluxes on a diurnal scale. The stem CO2 and CH4 fluxes of A. marina could be 55 % and 194 % less when considering tidal influence, as opposed to ignoring tidal influence. This study highlights species distinctness in the greenhouse gas (GHG) fluxes and the necessity of considering tidal influence when quantifying GHG fluxes from mangrove tree stems.
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RC1: 'Comment on egusphere-2024-533', Anonymous Referee #1, 04 Apr 2024
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General Comments: The manuscript by Yong et al investigated the drivers of mangrove tree stem methane can carbon dioxide fluxes over diurnal and tidal cycles. They also assessed soil fluxes and species differences from four mangrove sites, all located in Taiwan. This represents important work on the ‘new research frontier’ surrounding tree stem methane emissions, and in particular the role of mangrove stems as unaccounted-for CH4 sources within blue carbon ecosystems, which undergo daily soil redox changes due to tidal and diel cycles, that complicate the extrapolation and drivers of tree stem emissions. Although the study captures high-resolution temporal data, a major weakness in the study design is only measuring one tree at one stem height, for each of the four sites. Because tree stem fluxes can have large variability between trees of the same stand/forest and even within axial tree stem heights of the same tree, the upscaled extrapolation and comparison between the sites is unfortunately weak due to this approach, so should not be a focus of the manuscript and either re-worked or removed. The strengths of the manuscript lie within making comparisons to drivers of tree stem fluxes (i.e. tidal influence and diurnal cycles) at each site, the sifts between uptake and emission of methane (on the same tree) and the high-resolution data coupled to WT, so should be the major focus.
Specific Comments:
Line 18 – for the abstract it may be useful to maybe state why you hypothesize sampling at low tide would overestimate stem fluxes.
Line 51 – Seasonal water table height has also been shown to be one of the major factors driving wetland tree stem methane emissions with several studies over the past few years showing clear evidence for this, as this regulates the belowground soil oxygen, redox and methanogenesis conditions. This may be an important consideration for your study as focused on tidal WT fluctuations. See Terazawa 2021, Gauci 2021, Jeffrey 2023 for examples.
Line 62 – I would argue that mangrove soils are not a ‘substantial’ source of soil CH4 emissions and a reason Mangroves are considered ideal Blue Carbon sinks, compared to their freshwater counterparts - due to the abundant supply of sulphate from seawater (mentioned at line 76), favouring microbial sulphate reduction over methanotroph communities in mangrove sediments. Whilst true mangrove sediments can emit methane, it is often several orders of magnitude lower than freshwater wetland soils.
Material and methods - seem to be missing information surrounding the forest stands at each site. Eg density of trees, tree height, average DBH and some background information about the four sites eg pristine vs anthropogenically altered or nutrient-enriched catchment systems. As this is a study of trees it is important to provide some of the basic forestry parameters used in upscaling, and any background site information which may biogeochemistry/ hydrology or nutrient inputs between sites. At line 228 ‘upscaling’ is mentioned but there is no information as to how this was done and what caveats or assumptions this may contain so also needs to be mentioned. Details about any rejection threshold for poor linear fluxes (eg r2 values) should also be included to the methods. Some measurements had large methane uptake rates (very interesting) but it may also be useful to re- assess the starting concentrations of these incubation fluxes to ensure they are near atmospheric methane (1.8ppm) and did not include any accidental ebullition gas influencing the starting concentrations and the subsequent methane flux trend/gradient. CO2 flux is not always a useful proxy in those instances.
Flux measurement – The authors need to explain why they chose 110 cm stem height – as this may have actually represented a location of low methane emissions when compared to lower 10-50 cm stem closer to soils, as similar studies in mangroves and wetland forests have previously demonstrated. Was the tidal cycle expected to reach 1 m up the stem? This looks to be the case based on the SI photos! Also note how disturbance to soils was minimised during the intensive campaign measurements. Details about the volume and surface area of each chamber is also missing. The volume and SA of your chambers can affect the minimum detection limits of your equipment so suggest determining this value using the equations of Wassmann 2018 to be certain: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0191352
Line 163 - How do the authors explain CO2 uptake on tree stems?
Line 227 – The authors mention ‘diurnal’ differences – were night time measurements collected? I may have missed this but could the data also be split into day vs night fluxes? Eg are there differences between night-time low tide vs daytime low tide – a period where tree transpiration, photosynthesis and root oxidation would differ? This data would also be novel and interesting to show instead of comparing the sites within the discussion. The discussion stating ‘distinct’ species variation in methane emissions would, unfortunately, require more than 1 tree replicate - as the variation may be due to the individual tree (age, size, physiological structure etc), the sampling date, the sampling site and soil differences as well.
Line 254-268 – the comparison to others studies are useful but further should discuss why this may be. Temperature, soil or at what stem heights were other comparative studies measured from? Would that explain larger emissions and lack of uptake measurements compared to your results? The data comparison with few other mangrove studies that exist could also be useful when presented in a comparison table. If so, see other mangrove papers also:
https://onlinelibrary.wiley.com/doi/pdf/10.1055/s-2003-42712
https://www.sciencedirect.com/science/article/abs/pii/S0048969724001967?via%3Dihub
Conclusion
As mentioned above, I don’t think you can conclude that one tree species ‘distinctly’ emits more methane than another, as only one tree was measured at each site. Stick to conclusions surrounding the drivers of stem fluxes, temporal variation range, and shifting between uptake vs emissions – which is quite interesting.
Technical Corrections:
Line 30 – change ‘bubbles’ to ‘ebullition’
Line 32 – The Pangala 2017 would be a highly relevant citation here that considered top-down vs bottom-up scaling to compare tree stem contribution to methane budgets
Line 50 – physiological bark-mediated gas transport on wetland trees was also recently shown as a pathway for stem methane: https://nph.onlinelibrary.wiley.com/doi/full/10.1111/nph.19404
Line 55 – add: The contribution of ‘mangrove’ tree stems to the total…
Line 69 – not sure N2O is relevant at all here as this is the only mention of this GHG in the entire paper. Stick to methane and CO2.
Line 117 – add: The ‘second’ cylindrical chamber was….
Line 163 – what is the +/-? SD or SE? please clarify throughout.
Table 1 . A row showing the number of fluxes n=? measured would be useful for readers as well as tree size. Some units are m2/d and others m2/h and switch between umol and mmol. I suggest keep consistency throughout.
Figure 4 – a couple of large outliers for soil fluxes exist in the data. As per the comment about re-assessing your chamber start concentrations on tree stems, suggest double checking the start concentration on those soil chamber outliers in case ebullition was accidentally released into the chambers during measurements. However, these ‘outliers’ have also been shown to occur as natural fluxes (hot spots undergoing hot moments) when soil oxygen and methanotroph communities are limited by the fluctuating water tables etc.
Figure 5 – I would not show trend lines and equations for any non-significant data such as K. obovata. I also suggest the authors try other non-linear regression fits/ curves to the A. marina data as some of the falling trends appear to be non-linear.
Line 244- did you mean pneumatophores ‘were’ intentionally avoided?
Line 277 - change ‘absorption’ to ‘oxidation’ for methane. CO2 would be ‘fixation’.
Line 281 – The net diffusion rate also relates to net oxidation rates during transport.
End of review.
Citation: https://doi.org/10.5194/egusphere-2024-533-RC1
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