Methane flux from living tree stems in a northern conifer forest

Methane (CH₄) is the second-largest contributor to human-induced climate change, with significant uncertainties in its terrestrial sources and sinks. Tree stems play crucial roles in forest ecosystem CH₄ 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 measured fluxes every two weeks and at three heights from April to November 2024 to capture a range of environmental conditions. Tree species influenced CH4 flux more than any of the environmental variables considered. Among environmental variables, soil moisture was the most important driver of CH₄ flux, and our models suggested a significant interaction between soil moisture and soil temperature, such that the effect of higher soil moisture was greater at warmer soil temperatures. 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 the sampling period, with rare, isolate measurements of minimal uptake. The magnitude of flux varied by species: red maple stems were the largest emitters (1.946 ± 5.917 nmol m⁻² s⁻¹, mean ± SD), followed by red spruce (0.031 ± 0.065) and eastern hemlock (0.016 ± 0.027). 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.