Automated measurements of greenhouse gases fluxes from tree stems and soils: magnitudes, patterns and drivers

Tree stems exchange CO₂, CH₄ and N₂O with the atmosphere but the magnitudes, patterns and drivers of these greenhouse gas (GHG) fluxes remain poorly understood. Our understanding mainly comes from static-manual measurements, which provide limited information on the temporal variability and magnitude of these fluxes. We measured hourly CO₂, CH₄ and N₂O fluxes at two stem heights and adjacent soils within an upland temperate forest. We analyzed diurnal and seasonal variability of fluxes and biophysical drivers (i.e., temperature, soil moisture, sap flux). Tree stems were a net source of CO₂ (3.80 ± 0.18 µmol m⁻² s⁻¹; mean ± 95% CI) and CH₄ (0.37 ± 0.18 nmol m⁻² s⁻¹), but a sink for N₂O (-0.016 ± 0.008 nmol m⁻² s⁻¹). Time series analysis showed diurnal temporal correlations between these gases with temperature or sap flux for certain days. CO₂ and CH₄ showed a clear seasonal pattern explained by temperature, soil water content and sap flux. Relationships between stem, soil fluxes and their drivers suggest that CH₄ for stem emissions could be partially produced belowground. High-frequency measurements demonstrate that: a) tree stems exchange GHGs with the atmosphere at multiple time scales; and b) are needed to better estimate fluxes magnitudes and understand underlying mechanisms of GHG stem emissions.