Are methane emissions from mangrove stems a cryptic carbon loss pathway? Insights from a catastrophic forest mortality

Summary Growing evidence indicates that tree‐stem methane (CH 4 ) emissions may be an important and unaccounted‐for component of local, regional and global carbon (C) budgets. Studies to date have focused on upland and freshwater swamp‐forests; however, no data on tree‐stem fluxes from estuarine species currently exist. Here we provide the first‐ever mangrove tree‐stem CH 4 flux measurements from  >50 trees ( n  = 230 measurements), in both standing dead and living forest, from a region suffering a recent large‐scale climate‐driven dieback event (Gulf of Carpentaria, Australia). Average CH 4 emissions from standing dead mangrove tree‐stems was 249.2 ± 41.0 μmol m −2  d −1 and was eight‐fold higher than from living mangrove tree‐stems (37.5 ± 5.8 μmol m −2  d −1 ). The average CH 4 flux from tree‐stem bases ( c . 10 cm aboveground) was 1071.1 ± 210.4 and 96.8 ± 27.7 μmol m −2  d −1 from dead and living stands respectively. Sediment CH 4 fluxes and redox potentials did not differ significantly between living and dead stands. Our results suggest both dead and living tree‐stems act as CH 4 conduits to the atmosphere, bypassing potential sedimentary oxidation processes. Although large uncertainties exist when upscaling data from small‐scale temporal measurements, we estimated that dead mangrove tree‐stem emissions may account for c . 26% of the net ecosystem CH 4 flux.