CO ₂ and CH ₄ 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.