Spatiotemporal variability and origin of CO₂ and CH₄ tree stem fluxes in an upland forest

The exchange of multiple greenhouse gases (i.e., CO₂ and CH₄ ) between tree stems and the atmosphere represents a knowledge gap in the global carbon cycle. Stem CO₂ and CH₄ fluxes vary across time and space and are unclear, which are their individual or shared drivers. Here we measured CO₂ and CH₄ fluxes at different stem heights combining manual (biweekly; n = 678) and automated (hourly; n > 38,000) measurements in a temperate upland forest. All trees showed CO₂ and CH₄ emissions despite 20% of measurements showing net CH₄ uptake. Stem CO₂ fluxes presented clear seasonal trends from manual and automated measurements. Only automated measurements captured the high temporal variability of stem CH₄ fluxes revealing clear seasonal trends. Despite that temporal integration, the limited number of automated chambers made stand-level mean CH₄ fluxes sensitive to "hot spots," resulting in mean fluxes with high uncertainty. Manual measurements provided better integration of spatial variability, but their lack of temporal variability integration hindered the detection of temporal trends and stand-level mean fluxes. These results highlight the potential bias of previous studies of stem CH₄ fluxes solely based on manual or automated measurements. Stem height, temperature, and soil moisture only explained 7% and 11% of the stem CH₄ flux variability compared to 42% and 81% for CO₂ (manual and automated measurements, respectively). This large unexplained variability, in combination with high CH₄ concentrations in the trees' heartwood, suggests that stem CH₄ fluxes might be more influenced by gas transport and diffusivity through the wood than by drivers of respiratory CO₂ flux, which has crucial implications for developing process-based ecosystem models. We postulate that CH₄ is likely originated within tree stems because of lack of a consistent vertical pattern in CH₄ fluxes, evidence of CH₄ production in wood incubations, and low CH₄ concentration in the soil profile but high concentrations within the trees' heartwood.