How important are annual tree stem methane emissions to the total wetland flux? A seasonal case study of subtropical lowland Melaleuca forest.

Tree stem methane emissions are gaining rapid research momentum, with about one-third of all literature in this field published in 2021 alone. Long term and seasonal studies are currently rare, limiting our ability to constrain spatial and seasonal emissions variability, and to resolve the tree stem contribution to the total wetland methane flux i.e. soil, water and trees. Here we present preliminary methane flux data from a lowland Melaleuca quinquenervia forest. We measured emissions in situ along a ~3 m amplitude topo-gradient, encompassing forest in lower, transitional and upper elevation zones. Eight (ongoing) field campaigns at monthly intervals, captured flooded to dry to re-flooded site conditions. We measured the stem fluxes from 30 trees at four stem heights, along with 30 adjacent soil and water CH₄ fluxes. Ancillary parameters such as pore water (CH₄, DO%, pH, temp, redox, EC), water table depth, and soil moisture (VWC %) were also measured. Tree stem fluxes ranged several orders of magnitude between hydrological seasons and topo-gradient zones (ranging from negligible to 17, 426 mmol ha^-1 d^-1). Soil fluxes were similar in amplitude and shifted from maximal CH₄ emissions during the wet conditions, to CH₄ uptake in dry locations. The importance of tree stem flux to the net ecosystem flux (NEF) differed between campaigns and hydrological zones, but were most substantial during flooded conditions and ranged from 36-75% of the NEF in the lower and transitional zones during peak emissions respectively. In the upper zone, the tree stem emissions offset the soil sink capacity by ~50% when the water table was closest to the soil surface. This study shows the importance of quantifying lowland tree stem CH₄ emissions to the total wetland flux. This data provides important baseline readings for southern hemisphere and Australian wetland forests, that generally experience dynamic rainfall and soil redox oscillations between flooding and droughts.