Stem-methane emissions from the Amazon floodplains: controls and variability

Abstract

Methane emission from wetland trees is an overlooked source of methane, with poor resolution of their global significance and mechanisms. Our ongoing work in the Amazon basin has revealed that wetland trees are the largest source of methane, emitting the equivalent of all the methane emitted from the Arctic. Factors controlling and mechanisms driving these emissions remain unclear. Tree stem surfaces are no longer considered passive conduits for soil-produced methane; instead, they are active surfaces driving both methane production and oxidation.Over the past five years, using methane flux measurements, wood incubation experiments, stable carbon isotopic composition of methane measurements and wood structure and traits analysis, we attempt to unravel the following questions:  Where is methane produced? How is methane transported and emitted from the tree stems? What controls the flux strength of methane eventually released at the stem surface?So far, results suggest that soil is the predominant source of tree stem-released methane; however, certain tree species display strong internal methane production, increasing from the wet to dry season. The methane transport pathway is also tree species-specific, with some trees showing strong evidence of diel variability and others displaying minimal to zero diel variability. Internal wood methane concentration and stable isotopic measurements corroborate this. A strong presence of tree-methane oxidation was observed, which again was tree species-specific, despite the net fluxes measured at the stem surface always being positive. Methane oxidation within the tree stems was dominant, with methane oxidation in the bark only playing a minor role. Wood structure and traits analysis revealed that wood density could be used as a proxy to predict stem methane fluxes at an ecosystem level. However, species-level variability was controlled by other species-specific wood traits, making it harder to fully explain the variability we observe in methane emitted at the stem surface.