Methane production in the stems of living trees in subtropical forests

Methane was rarely produced in the stems of living trees in the upland forests. However, it was largely produced in the swollen lower stems in the wetland forests. The methane (CH₄) fluxes on aboveground living trees have been widely observed over the last decades. However, aboveground living tree-based CH₄ production has been rarely studied in terrestrial ecosystems. Thus, it is essential to study aboveground CH₄ production in multiple tree species. In this study, we aimed to understand the CH₄ production in the stems of living trees in the subtropical forests from 445 individual living tree stems (1335 upper, middle, and lower tree stem segments). We found that substantial CH₄ concentrations of ≥ 1000 μL L^-1 and detectable CH₄ production of ≥ 0.2 ng g^-1 dry weight (DW) h^-1 occurred only in ~ 5% of all sampled living tree stems in the upland forests. Substantial CH₄ concentrations and detectable CH₄ production occurred frequently in the swollen lower stems (a kind of eco-physiologic adaptation) but rarely in the straight stems of living trees in the wetland forests. Particularly, CH₄ production was significantly higher in the swollen lower stems compared to in the straight stems, with a mean rate of 22.2 ng g^-1 DW h^-1, and the produced CH₄ was demonstrated to be microbial according to its highly depleted ¹³C abundance and its exponential kinetics in response to time. In global wetland forests, the existing eco-physiologic adaptations of living trees are potentially a distinctive CH₄ source, and thereby more research is required for understanding their CH₄ processes and fluxes.