Using Radon ( ²²² Rn) to Trace Variability in Greenhouse Gas Emission From Tree Stems Across Species, Seasons and Stem Heights

ABSTRACT Emissions from trees are an important component of the global methane (CH 4 ) cycle, but their spatial origins (soil vs. in‐stem), transport pathways and environmental influences are not well constrained. To address these issues, this field study characterized spatial and temporal variability in stem emissions of biologically inert radon ( 222 Rn), which is naturally enriched in soil and groundwater compared to air and surface water, and emissions of CH 4 and carbon dioxide (CO 2 ), which can be produced and consumed biologically in soil and trees. We assessed the influences of tree species ( Acer rubrum , Taxodium distichum , Fagus grandifolia and Liriodendron tulipifera ), season (summer and winter), flux measurement height (40 and 140 cm) and surficial soil moisture on stem fluxes of 222 Rn, CH 4 and CO 2 . Fluxes of all three gases showed broadly similar patterns of variability: They were greater from the wetland species T. distichum than the three upland tree species, greater at 40‐cm compared to 140‐cm stem height and greater in the summer than in the winter. However, CH 4 emissions showed a greater magnitude of variability across tree species and measurement heights than either 222 Rn or CO 2 . More detailed exploration of the similarities and differences in stem emissions of 222 Rn, CO 2 and CH 4 may help resolve uncertainties in forest greenhouse gas sources and spatiotemporal flux dynamics, enabling more accurate modelling of gas transport through the soil–plant–atmosphere continuum.