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

<p>Knowledge regarding processes, pathways and mechanisms that may moderate methane (CH₄) sink/source behaviour along the sediment - tree stem - atmosphere continuum remains incomplete. Here, we applied stable isotope analysis (&#948;¹³C-CH₄) to gain insights into axial CH₄ transport and oxidation in two common and globally distributed subtropical lowland forest species (<em>Melaleuca quinquenervia</em> and <em>Casuarina glauca</em>). We found consistent trends in CH₄ flux (decreasing with height) and &#948;¹³C-CH₄enrichment (increasing with height) in relation to stem height from the ground. The average lower tree stem (0-40 cm) &#948;¹³C-CH₄of <em>M. quinquenervia</em> and <em>C. glauca</em> flooded forests (-53.96 &#8240; and -65.89 &#8240;) were similar to adjacent flooded sediment CH₄ebullition (-52.87 &#8240; and -62.98 &#8240;), suggesting that CH₄ is produced mainly via sedimentary sources. Upper stems (81-200 cm) displayed distinct &#948;¹³C-CH₄enrichment (<em>M. quinquenervia</em> -44.6 &#8240; and C. glauca -46.5 &#8240; respectively) compared to lower stems. Coupled 3D photogrammetry and novel 3D measurements on <em>M. quinquenervia</em> revealed that distinct hotspots of CH₄ flux and isotopic fractionation were likely due to bark anomalies where preferential pathways of gas efflux were likely enhanced. By applying a&#160; fractionation factor (derived from previous lab based tree stem bark experiments), diel experiments revealed greater &#948;¹³C-CH₄enrichment and higher oxidation rates in the afternoon relative to the morning. Overall, we estimate CH₄ oxidation rates between the lower to upper stems across both species ranged from 1 to 69 % (average 33.1 &#177; 3.4 %), representing a substantial tree-associated CH₄ sink occurring during axial transport.</p>