• Hettwer, Christian
  UMaine School of Forest Resources
  

Methane (CH4) is the second-largest contributor to human-induced climate change, with significant uncertainties in its terrestrial sources and sinks. Tree stems, both living and dead, play crucial roles in forest ecosystem CH4 and carbon dioxide (CO2) flux dynamics, yet much remains unknown regarding the environmental drivers of fluxes. We measured CH4 flux from three tree species (Picea rubens, Tsuga canadensis, Acer rubrum) along an upland-to-wetland gradient at Howland Research Forest, a net annual sink of CH4, in Maine USA. We also measured CH4 and CO2 fluxes from standing dead stems (snags). We measured fluxes every two weeks and at three heights from April to November 2024 to capture a range of environmental conditions.

While species identity was the largest driver for living stems, CH4 flux was most strongly influenced by soil moisture among environmental variables, and our models suggest a significant interaction between soil moisture and soil temperature as a predictor of CH4 flux rates. We determined a “breakpoint” in soil moisture along the upland-to-wetland gradient at ~60% volumetric water content, above which CH4 flux rates increased dramatically. All stems measured were net CH4 sources throughout sampling, with rare, isolated measurements of minimal uptake. The magnitude of flux varied by species: red maple stems were the largest emitters , followed by red spruce and eastern hemlock. This study highlights the contribution of these species to ecosystem CH4 fluxes. Our results establish the sensitivity of stem flux rates to projected increases in regional precipitation and temperature, potentially shifting the site from a net CH4 sink to a source.

We fit nonlinear models to CO2 and CH4 fluxes from snags in order to quantify their responses to important environmental predictors (soil moisture, soil temperature, air temperature). Gas fluxes from snags increased with increasing temperature, yet CO2 flux peaked at moderate soil moisture levels (~ 30%), while CH4 peaked at the highest moisture levels. CH4 fluxes were overwhelmingly net positive, suggesting that snags represent an important pathway for wetland gas emission. We also found that CH₄ flux is relatively insensitive under low soil moisture and temperature, but increases with rising soil temperature when soil moisture is high, suggesting that methanogenesis depends on anaerobic moisture conditions. Results also suggest that CO2 flux may co-vary 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 emission to CH4 emission) occurred at ~ 60% soil moisture. These results, which align with those from previous studies establishing anaerobic moisture thresholds, present direct measurements of gas exchange from snags along a moisture and temperature gradient.