Tree stem methane emissions from subtropical lowland forest (Melaleuca quinquenervia) regulated by local and seasonal hydrology.
Tree stem mediated methane emissions represent a potentially important yet poorly constrained source of atmospheric methane. Here we present the first ever quantification of tree stem methane emissions from Melaleuca quinquenervia, a widespread iconic Australian lowland tree and globally invasive species. Under two distinct hydrological conditions (wet and dry) we captured 431 tree stem flux measurements encompassing six different vertical stem heights along a 50 m topo-gradient transect, separated into three distinct hydrological zones (upper, transitional and lower). The tree stem methane fluxes closely reflected local topography/hydrology and ranged from - 30.0 to 123,227 µmol m-2 day-1, with the maximum values amongst the highest values reported to date. The highest methane emissions were observed during wet conditions, within the inundated lower zone and from near the tree stem bases and water table. The average methane flux per tree (scaled to 1 m of stem) for the transitional and lower zones was 52-fold and 46-fold higher during wet conditions compared to dry, whereas the upper zone emissions changed little between seasons. Adjacent soil fluxes followed similar trends along the hydrology gradient with the upper zone tree stem emissions offsetting the adjacent soil methane sink capacity. A clear trend of sharply decreasing methane emissions with stem-height suggests a soil methane origin. A 45-h time-series of two trees within the lower zone revealed three to fourfold diel variability, with elevated morning-time fluxes. Overall, the study revealed that seasonal hydrological conditions and topo-gradient substantially regulated the methane emissions from M. quinquenervia and that this previously overlooked pathway should be accounted for within wetland methane budgets, especially during inundated conditions.