Isotopic evidence for axial tree stem methane oxidation within subtropical lowland forests.

Knowledge regarding mechanisms moderating methane (CH ) sink/source behaviour along the soil-tree stem-atmosphere continuum remains incomplete. Here, we applied stable isotope analysis (δ C-CH ) to gain insights into axial CH transport and oxidation in two globally distributed subtropical lowland species (Melaleuca quinquenervia and Casuarina glauca). We found consistent trends in CH flux (decreasing with height) and δ C-CH enrichment (increasing with height) in relation to stem height from ground. The average lower tree stem δ C-CH (0-40 cm) of Melaleuca and Casuarina (-53.96‰ and -65.89‰) were similar to adjacent flooded soil CH ebullition (-52.87‰ and -62.98‰), suggesting that stem CH is derived mainly by soil sources. Upper stems (81-200 cm) displayed distinct δ C-CH enrichment (Melaleuca -44.6‰ and Casuarina -46.5‰, respectively). Coupled 3D-photogrammetry with novel 3D-stem measurements revealed distinct hotspots of CH flux and isotopic fractionation on Melaleuca, which were likely due to bark anomalies in which preferential pathways of gas efflux were enhanced. Diel experiments revealed greater δ C-CH enrichment and higher oxidation rates in the afternoon, compared with the morning. Overall, we estimated that c. 33% of the methane was oxidised between lower and upper stems during axial transport, therefore potentially representing a globally significant, yet previously unaccounted for, methane sink.