Large emissions from floodplain trees close the Amazon methane budget

Abstract

Wetlands are the largest global source of atmospheric methane (CH), a potent greenhouse gas. However, methane emission inventories from the Amazon floodplain, the largest natural geographic source of CH in the tropics, consistently underestimate the atmospheric burden of CH determined via remote sensing and inversion modelling, pointing to a major gap in our understanding of the contribution of these ecosystems to CH emissions. Here we report CH 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 CH emissions. Methane fluxes from Amazon tree stems were up to 200 times larger than emissions reported for temperate wet forests and tropical peat swamp forests, representing the largest non-ebullitive wetland fluxes observed. Emissions from trees had an average stable carbon isotope value (δC) 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 CH 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 CH emissions of 42.7 ± 5.6 teragrams of CH a year for the Amazon basin, based on regular vertical lower-troposphere CH profiles covering the period 2010-2013. We find close agreement between our 'top-down' and combined 'bottom-up' estimates, indicating that large CH emissions from trees adapted to permanent or seasonal inundation can account for the emission source that is required to close the Amazon CH budget. Our findings demonstrate the importance of tree stem surfaces in mediating approximately half of all wetland CH emissions in the Amazon floodplain, a region that represents up to one-third of the global wetland CH source when trees are combined with other emission sources.

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

“…). They were, however, much lower than tropical Amazon floodplain tree CH 4 fluxes that were one to two orders of magnitude higher than our data (Pangala et al ., ). The only other published standing dead tree stem CH 4 emissions (600 μmol m −2 d −1 ), were from a restored coastal freshwater wetland of North Carolina (Carmichael et al ., ), which was similar to our dead stem fluxes (Fig.…”

“…Furthermore, there is no consensus for the biophysical mechanisms transporting CH 4 along the soil–tree–atmosphere continuum (Barba et al ., ), with debate continuing as to whether trees: act as passive pipes (diffusion) connecting the rhizosphere to the atmosphere; participate as active pipes (via xylem flow); or produce CH 4 internally in the heartwood (Covey et al ., ; Barba et al ., ). Tree species, tree size, ecological adaptations, seasonality and hydrogeophysical context all likely play a role in the production and pathways of tree stem CH 4 flux (Keppler et al ., ; McLeod et al ., ; Pangala et al ., ; Pitz et al ., ). As an estimated 3.04 trillion trees exist globally (Crowther et al ., ), with c .…”

Impact of forest plantation on methane emissions from tropical peatland

“…Our results show substantial and apparent diurnal variation in the NEE‐CH 4 over the natural forest where GWL are high, highlighting the importance of high‐frequency ecosystem‐scale flux measurements. There is increasing evidence that woody vegetation can emit a significant CH 4 amount to the atmosphere (Barba et al, ; Covey & Megonigal, ; Pangala et al, , ; Pitz & Megonigal, ; Rice et al, ; Wang et al, ; Welch, Gauci, & Sayer, ). In tropical peatlands, the majority of root biomass occurs within the upper 50 cm of the peat column (Brady, ; Sulistiyanto, ), and dissolved CH 4 in the root zone can be significant (100–1,500 µmol/L; Hoyt, ; Pangala et al, ).…”

Longitudinal Gradients in Tree Stem Greenhouse Gas Concentrations Across Six Pacific Northwest Coastal Forests

“…The omission of GHG fluxes from trees may overestimate the sink potential (Pitz & Megonigal, ) and underestimate the release (Covey et al, ; Wang et al, ) of GHGs from upland and wetland forests, respectively. For instance, tree‐mediated CH 4 emissions have been shown to contribute 9–27% (Pangala et al, ) and 44–65% (Pangala et al, ) of the total ecosystem CH 4 flux in forested temperate and tropical wetlands, respectively. In the broader context of global carbon budgets, forest ecosystems were estimated to emit 6.7 ± 1.1 Pg C/year as CO 2 from tree stems (Yang et al, ).…”