Spatiotemporal variability and origin of CO<sub>2</sub> and CH<sub>4</sub> tree stem fluxes in an upland forest
Josep Barba, Rafael Poyatos, Margaret Capooci, Rodrigo Vargas
Abstract
Abstract The exchange of multiple greenhouse gases (i.e., CO 2 and CH 4 ) between tree stems and the atmosphere represents a knowledge gap in the global carbon cycle. Stem CO 2 and CH 4 fluxes vary across time and space and are unclear, which are their individual or shared drivers. Here we measured CO 2 and CH 4 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 CO 2 and CH 4 emissions despite 20% of measurements showing net CH 4 uptake. Stem CO 2 fluxes presented clear seasonal trends from manual and automated measurements. Only automated measurements captured the high temporal variability of stem CH 4 fluxes revealing clear seasonal trends. Despite that temporal integration, the limited number of automated chambers made stand‐level mean CH 4 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 CH 4 fluxes solely based on manual or automated measurements. Stem height, temperature, and soil moisture only explained 7% and 11% of the stem CH 4 flux variability compared to 42% and 81% for CO 2 (manual and automated measurements, respectively). This large unexplained variability, in combination with high CH 4 concentrations in the trees' heartwood, suggests that stem CH 4 fluxes might be more influenced by gas transport and diffusivity through the wood than by drivers of respiratory CO 2 flux, which has crucial implications for developing process‐based ecosystem models. We postulate that CH 4 is likely originated within tree stems because of lack of a consistent vertical pattern in CH 4 fluxes, evidence of CH 4 production in wood incubations, and low CH 4 concentration in the soil profile but high concentrations within the trees' heartwood.