The role of Amazonian wetland trees in atmospheric methane emissions

Abstract:

Methane (CH4) emissions from wetland trees are an overlooked source of methane, with poor resolution of their global significance and mechanisms. With 12 field campaigns in the Brazilian Amazon since 2013, we examined the regional significance of tree CH4 emissions, factors controlling their variability and the mechanism driving these emissions. We used a combination of intensive field measurements and controlled lab and mesocosm experiments to understand the production, transport, and emissions of CH4 from wetland trees. First, using CH4 flux measurements from over 2000 trees across the central Amazon basin, we demonstrate that wetland trees emit half of all the CH4 emitted from the Amazon basin, thereby contributing significantly to the regional CH4 budget. Second, during the wet season, tree species traits exert a greater control on the variability of CH4 fluxes from tree stems than the flooding depth. However, repeat measurements of tree stem fluxes from the same trees at 4 hydrological distinct time points (rising, peak, receding and low) reveal that trees continue to emit CH4 even when the water table is ≥ 5 m below the soil surface. Third, measurements of tree stem emissions across a hydrological gradient (flooded to upland) reveal that Amazonian trees can emit both CH4 and N2O from tree stems. High CH4 emissions were observed from the flooded trees compared to low but positive emissions from the upland trees where the water table was ≥ 10 m below the soil surface, conversely, N2O emissions were highest from upland trees. Further measurement of N2O emissions from 800 trees across the central Amazon basin reveal that tree stems are an important source of N2O emissions and represent a major gap in the regional N2O budget. Fourth, flooded trees in the Pantanal region emit significant quantities of CH4, however, rooted macrophytes are the largest contributor of CH4 emissions from this region. Finally, our most recent field campaign, reveal that in-tree CH4 production contributions to CH4 fluxes measured at the tree stem surfaces are low compared to soil-produced CH4. Using wood incubations and stable carbon isotopic composition of CH4, we highlight that CH4 oxidation in wetland tree stems play a major role in regulating the net CH4 fluxes measured. Tree stem surfaces therefore represent a major additional forest CH4 sink term.