Large emissions from floodplain trees close the Amazon methane budget

Wetlands are the largest global source of atmospheric methane (CH4), a potent greenhouse gas. However, methane emission inventories from the Amazon floodplain, the largest natural geographic source of CH4 in the tropics, consistently underestimate the atmospheric burden of CH4 determined via remote sensing and inversion modelling, pointing to a major gap in our understanding of the contribution of these ecosystems to CH4 emissions. Here we report CH4 fluxes from the stems of 2,357 individual Amazonian floodplain trees from 13 locations across the central Amazon basin. We find that escape of soil gas through wetland trees is the dominant source of regional CH4 emissions. Methane fluxes from Amazon tree stems were up to 200 times larger than emissions reported for temperate wet forests6 and tropical peat swamp forests, representing the largest non-ebullitive wetland fluxes observed. Emissions from trees had an average stable carbon isotope value (δ13C) of −66.2 ± 6.4 per mil, consistent with a soil biogenic origin. We estimate that floodplain trees emit 15.1 ± 1.8 to 21.2 ± 2.5 teragrams of CH4 a year, in addition to the 20.5 ± 5.3 teragrams a year emitted regionally from other sources. Furthermore, we provide a ‘top-down’ regional estimate of CH4 emissions of 42.7 ± 5.6 teragrams of CH4 a year for the Amazon basin, based on regular vertical lower-troposphere CH4 profiles covering the period 2010–2013. We find close agreement between our ‘top-down’ and combined ‘bottom-up’ estimates, indicating that large CH4 emissions from trees adapted to permanent or seasonal inundation can account for the emission source that is required to close the Amazon CH4 budget. Our findings demonstrate the importance of tree stem surfaces in mediating approximately half of all wetland CH4 emissions in the Amazon floodplain, a region that represents up to one-third of the global wetland CH4 source when trees are combined with other emission sources

Opportunities and challenges for an Indonesian forest monitoring network

© 2019, INRA and Springer-Verlag France SAS, part of Springer Nature. Key message: Permanent sampling plots (PSPs) are a powerful and reliable methodology to help our understanding of the diversity and dynamics of tropical forests. Based on the current inventory of PSPs in Indonesia, there is high potential to establish a long-term collaborative forest monitoring network. Whilst there are challenges to initiating such a network, there are also innumerable benefits to help us understand and better conserve these exceptionally diverse ecosystems

Physiological processes affecting methane transport by wetland vegetation - a review

Wetland plants transport oxygen to belowground tissues to survive in anoxic sediments, and simultaneously conduct methane (CH4) from the sediment to the atmosphere. Although plant-mediated transport is the main CH4 emission pathway in vegetated wetlands, the contribution of vegetated areas to total emissions in wetlands remains uncertain. To accurately quantify these emissions, understanding the physiological processes driving plant-mediated CH4 transport is crucial. This review describes the state of the art understanding of CH4 transport through trees, emergent, floating-leaved, and submerged freshwater macrophytes. Gas transport mechanisms in plants include diffusion, pressurized flow, and transpiration-driven flow. Pressurized flow in the gas-filled aerenchyma leads to higher gas transport rates than diffusion, and mostly occurs in plants standing in deeper water. Transpiration-driven flow occurs in the xylem tissue of trees, whereby dissolved CH4 is transported by sap flow. Pressurized flow and transpiration-driven flow both result in diel cycles in CH4 emission, with higher emissions during the day than at night. The total CH4 emission through a wetland plant depends on its growth stage, transport mechanisms and the balance between sediment and in-plant CH4 production and oxidation. Although plants contribute substantially to total CH4 emissions, soil carbon content, soil temperature, nutrient availability, and water depth are often stronger driving factors than plant species. Nevertheless, accurate quantification of emissions from vegetated wetlands requires standardization of measurement protocols which capture diurnal and seasonal variation in emissions. Knowledge on CH4 transport through trees and submersed and free-floating macrophytes is scarce and warrants further research