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

Growing evidence indicates that tree-stem methane (CH4) 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 CH4 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 CH4 emissions from standing dead mangrove tree-stems was 249.2 +/- 41.0 mu mol m(-2) d(-1) and was eight-fold higher than from living mangrove tree-stems (37.5 +/- 5.8 mu mol m(-2) d(-1)). The average CH4 flux from tree-stem bases (c. 10 cm aboveground) was 1071.1 +/- 210.4 and 96.8 +/- 27.7 mu mol m(-2) d(-1) from dead and living stands respectively. Sediment CH4 fluxes and redox potentials did not differ significantly between living and dead stands. Our results suggest both dead and living tree-stems act as CH4 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 CH4 flux.