the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 Aug 2025
CO2 and CH4 fluxes from standing dead trees in a northern conifer forest
Abstract. Representing 15 – 20 % of aboveground biomass in forests, deadwood is an important, yet understudied, component of ecosystem greenhouse gas (GHG) fluxes. In particular, standing dead trees (snags) can serve as conduits for the atmospheric flux of carbon dioxide (CO2) and methane (CH4), with fluxes varying according to environmental conditions. We measured CO2 and CH4 fluxes from six snags along an upland-to-wetland gradient at Howland Research Forest (Maine, USA) with measurements made every two weeks from April to November 2024. Using nonlinear models, we quantified flux responses to environmental predictors including soil moisture, soil temperature, and air temperature. Gas fluxes increased with increasing temperature, yet CO2 flux peaked at moderate soil moisture (~ 30 %), while CH4 peaked at the highest moisture levels. CH4 fluxes were overwhelmingly net positive, suggesting that snags are important pathways for wetland gas emission. CH4 flux was relatively insensitive under low soil moisture and temperature, but increased with rising soil temperature when soil moisture was high, suggesting that methanogenesis depends on anaerobic moisture conditions. Results also suggest that CO2 flux co-varied with CH4 flux from snags, with decreases in CO2 flux associated with increases in CH4 flux. As soil moisture increased, a pronounced shift in gas fluxes (from CO2 to CH4 emission) occurred at ~ 60 % soil moisture. These results, which align with those from previous studies establishing anaerobic moisture thresholds and provide new insights into CO2 and CH4 fluxes from snags, present direct measurements of gas exchange from snags along a moisture and temperature gradient.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-3480', Anonymous Referee #1, 09 Sep 2025
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RC2: 'Comment on egusphere-2025-3480', Anonymous Referee #2, 22 Sep 2025
General comments:
This study conducted a six-month time series of in-situ measurements of CO₂ and CH₄ fluxes in a Maine forest, along with measurements of key environmental predictors. The results indicated that soil moisture is the primary driver of both gases’ fluxes, while temperature also plays a role.
The study highlights the importance of CO₂ and CH₄ fluxes from snags to the atmosphere. However, even though information on the biomass percentage of the snags was provided at the beginning, no quantitative comparison on the actual CO₂ or CH₄ fluxes were made after they got the fluxes data. I would encourage the authors to include reference CO2 and CH4 fluxes, especially of different forest ecosystems, to allow for a more direct and robust comparison.
Specific Comments:
Line 199-201, I suggest adding some explanation for why CO₂ flux is influenced by air temperature while CH₄ flux is influenced by soil temperature. Currently, the manuscript only states that this comes from the random forest analysis, which lacks the science behind it.
Lines 113–135: For readers who have not conducted this type of research, a schematic illustrating how the flux measurements were conducted would be very helpful.
Figure 2: A 2D heatmap may be more straightforward than a 3D contour plot to show how volumetric soil moisture and temperature influence CO₂ and CH₄ fluxes.
Lines 222–226: The description of maximum CO₂ flux is a bit confusing. Line 222 states, “Assuming maximum observed air temperature, CO₂ flux reached its maximum at 30% volumetric soil moisture,” whereas line 224 says, “CO₂ flux peaked at approximately 25% soil moisture, then declined exponentially until becoming negligible at approximately 60% soil moisture.” The manuscript should clarify which soil moisture corresponds to the maximum flux.
Lines 237–238: “The two-gas model revealed a significant negative interaction between CO₂ flux and soil moisture in predicting CH₄ flux (p < 0.001)” is somewhat misleading, as it sounds like soil moisture negatively affects CH₄ flux, while the results actually show a positive effect.
Lines 305–307: similar to the above comment, the phrase “antagonistic interactions between CO₂ flux and soil moisture in predicting CH₄ flux” is confusing and seems contrary to the results. It may be clearer to rephrase this to reflect the actual relationship observed between the two gases along the upland-to-wetland gradient.
Citation: https://doi.org/10.5194/egusphere-2025-3480-RC2 -
RC3: 'Reply on RC2', Anonymous Referee #2, 22 Sep 2025
typo: it was a 8-month time series, not a 6-month.
Citation: https://doi.org/10.5194/egusphere-2025-3480-RC3
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RC3: 'Reply on RC2', Anonymous Referee #2, 22 Sep 2025
Data sets
CO2 and CH4 fluxes from living and standing dead trees in Howland Research Forest, Maine USA, 2024 C. Hettwer et al. https://doi.org/10.6073/pasta/03586624214245c96a0399509ab4e3cb
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General Comments:
This study investigates spatiotemporal patterns of CO2 and CH4 fluxes from standing dead wood in a North American forest. It provides insights into the relationships of these fluxes with local temperature and soil moisture conditions.
I do not have any technical issues with this paper, but it is not a very compelling narrative. While looking at standing snags instead of fallen deadwood is interesting, the analyses are quite basic and reveal well-established relationships between temperature and moisture (which is just soil moisture in this study) and CO2 and CH4 fluxes from decaying biomass. They do not place their findings in a broader ecological context (e.g., how significant are snag fluxes to overall ecosystem fluxes?), and there is not a lot of analysis of the snag wood itself, which could reveal more interesting and novel mechanisms of GHG production. Similarly, there is likely substantial heterogeneity of fluxes within any given snag, but this is not investigated. It’s as if there are logical next steps in this research that were not taken, and the impact of the work is diminished as a result.
With all that said, I think the writing style and clarity of the manuscript are good. The figures are well constructed. The methods are reproducible.
Specific Comments:
Site Description: Including a plot of the study site and zones of different soil drainage classes in the main text would provide helpful context for the readers.