the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Jun 2026
Vegetation structure drives carbon dioxide and methane dynamics in adjacent boreal fen and bog ecosystems
Abstract. Boreal peatlands store substantial soil carbon and differ strongly in greenhouse gas exchange depending on factors such as hydrology and vegetation. Understanding these differences is increasingly important because many boreal peatlands are undergoing fen-to-bog transitions, potentially altering ecosystem carbon cycling and greenhouse gas exchange. We compared active-season (May–October) carbon dynamics between an oligotrophic sedge fen and an ombrotrophic patterned bog located 1.2 km apart within the Siikaneva wetland complex, southern Finland. We analysed fluxes of carbon dioxide (CO2) and methane (CH4) at diurnal, seasonal, and interannual scales, using eddy covariance measurements from 2011–2016. These fluxes were related to abiotic and biotic drivers, including soil temperature, photosynthetically active radiation, water table depth, and vascular plant leaf area index (LAI), with a focus on aerenchymatous LAI (LAIAer).
Across the six-year period both ecosystems acted as CO2 sinks during the active-season and the fen was generally a stronger sink (mean±std NEE -80.8±42.1 g C m-2 season-1) than the bog (-72.2±22.5 g C m-2 season-1). The fen also exhibited higher gross primary production (GPP) and light-use efficiency, consistent with its higher total LAI, indicating vegetation structure and phenology as key drivers of the CO2 sink difference under shared climate. CH4 emissions were substantially higher at the fen (mean±std CH4 10.8±2.3 g C m-2 season-1) than the bog (7.9±1.4 g C m-2 season-1). CH4 fluxes scaled positively with total LAI and LAIAer, supporting the role of substrate supply and plant-mediated transport.
The contrasting carbon dynamics between these adjacent peatland types suggest that ongoing fen-to-bog transitions may alter active-season carbon exchange by reducing CO2 uptake and CH4 emissions as vegetation composition and hydrological conditions shift toward bog-like states. Our results additionally indicate greater temporal variability in carbon fluxes at the fen than the bog. These findings highlight the importance of representing vegetation composition and canopy development in peatland models, rather than relying solely on peatland type.
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RC1: 'Comment on egusphere-2026-3323', Katharina Jentzsch, 21 Jul 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-3323/egusphere-2026-3323-RC1-supplement.pdfCitation: https://doi.org/
10.5194/egusphere-2026-3323-RC1 -
RC2: 'Comment on egusphere-2026-3323', Anonymous Referee #2, 30 Sep 2026
This manuscript focuses on comparing adjacent bogs and fens ecosystems using eddy covariance towers between 2011 and 2016 to isolate the effects of vegetation, hydrology, and peatland type on carbon exchange (CO2 and CH4 fluxes). The authors found that, contrary to their original hypothesis, the fen was a stronger sink in terms of CO2 but also emitted more CH4 than the bog. These differences were mostly attributed to vegetation structure and phenology. The fen had higher LAI, gross photosynthetic uptake and light use efficiency compared to the bog, which led to higher net carbon uptake. At the same time, CH4 emissions were greater in the fen due to plant mediated CH4 transport. I think this study is very timely and interesting, both considering current environmental conditions, as well as future climate projections because it increases our understanding of the role of vegetation on ecosystem fluxes, particularly methane. Overall, I think this manuscript presents a valuable contribution to our understanding of peatland carbon cycling and is suitable for publication after minor revisions.
General comments:
Overall, the manuscript is both data and analysis rich, clear and well written, methods are described in a way that allows for replication, the figures are clear and show relevant information, and the analyses performed are adequate for the objectives presented in the paper. However, some of the results could be synthesized a bit better, and the main results could be highlighted more. Currently, all the figures make it a bit too information dense and hard to follow.
Introduction: Very good introduction. Encompasses the relevant information to understand the paper and highlights research gaps.
Methods:
- Figure 1 is good, but maybe you should consider overlapping the tower footprints. I think that would add important information. If not overlapping in the map, I still think you should add your tower footprints and energy balance closures in the supplementary information.
- Not sure if it’s necessary to add the VPD calculation. It is a fairly standard variable.
Results:
This section is, in general, very informative and well written. However, as I mentioned above, there is quite a lot of figures and repeated information that could be summarised a bit better.
- Figure 3 shows weekly fluxes for the whole study period, and 4 shows monthly diurnal courses, so the text in those sections have repeated information, perhaps you could consider removing the weekly section and just focus on diurnal, seasonal and annual variations across sites. Likewise, Figures 7, 9 and 10 could also be merged as they all show similar information.
- The photosynthetic parameters could be in a table either in the main text or supplementary information, and then briefly compared in the text.
- You mention linear mixed-effect models quite a few times, but don’t provide the model structure or the parameters obtained from those, I think you could add these in the supplementary material as well and just refer to them in the main text.
Discussion:
- You state that ‘the fen acted as a stronger CO2 sink than the bog during the active-season.’ However, the results show that the mean values are relatively similar and the standard deviations overlap considerably. It is therefore not clear whether the difference between ecosystems is statistically significant. I suggest clarifying this point and either presenting a statistical comparison or softening the wording in the Discussion and Conclusions.
- Water table depth was generally higher at the fen compared to the bog, could this also explain the higher CH4 emissions and higher CO2 uptake rather than the vegetation structure alone? Given the extensive literature identifying water table depth as a primary control on peatland CO₂ and CH₄ exchange, I think the relatively limited role of WTD in this study deserves a more thorough discussion. In general, some questions that I think should be discussed are: Was the limited role of WTD due to the wet nature of both sites? Because only one logger was used at each site and EC measurements integrate large spatial heterogeneity? Because vegetation integrates long-term hydrological conditions better than instantaneous WTD?
- The manuscript attributes many of the observed differences to vegetation structure and phenology. While this interpretation is supported by the analyses presented, the two ecosystems also differ in nutrient status, peat chemistry, pH, and microtopography. A brief discussion acknowledging the potential influence of these co-varying factors would strengthen the interpretation.
- Considering you have both CO2 and CH4 for both sites, instead of just analysing them separately, can you say something about the GHG budget in terms of CO2 equivalents? It seems like the fen would have a higher emission in terms of CO2-eq/radiative forcing compared to the bog due to the higher CH4 and I think that should also be discussed. Moreover, I think the discussion on fen-to-bog transitions could be expanded as it was one of the main topics discussed in the introduction, but it is only slightly glossed over in the discussion.
Specific comments:
- Line 403: use ‘than’ instead of ‘then at the fen’
- Line 581: the PCA is figure 5, not 6
- There’s ‘active-season’ and ‘active season’ usage in the manuscript, choose one and standardise.
- Figure 4 caption appears to have a missing letter? ‘Monthly diurnal courses ‘of’…
- Figure 5 caption: "LAI data from the fen was only available", should be "were only available".
Citation: https://doi.org/10.5194/egusphere-2026-3323-RC2
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