Non-flooded riparian Amazon trees are a regionally significant methane source

Abstract

Inundation-adapted trees were recently established as the dominant egress pathway for soil-produced methane (CH 4 ) in forested wetlands. This raises the possibility that CH 4 produced deep within the soil column can vent to the atmosphere via tree roots even when the water table (WT) is below the surface. If correct, this would challenge modelling efforts where inundation often defines the spatial extent of ecosystem CH 4 production and emission. Here, we examine CH 4 exchange on tree, soil and aquatic surfaces in forest experiencing a dynamic WT at three floodplain locations spanning the Amazon basin at four hydrologically distinct times from April 2017 to January 2018. Tree stem emissions were orders of magnitude larger than from soil or aquatic surface emissions and exhibited a strong relationship to WT depth below the surface (less than 0). We estimate that Amazon riparian floodplain margins with a WT < 0 contribute 2.2–3.6 Tg CH 4 yr −1 to the atmosphere in addition to inundated tree emissions of approximately 12.7–21.1 Tg CH 4 yr −1 . Applying our approach to all tropical wetland broad-leaf trees yields an estimated non-flooded floodplain tree flux of 6.4 Tg CH 4 yr −1 which, at 17% of the flooded tropical tree flux of approximately 37.1 Tg CH 4 yr −1 , demonstrates the importance of these ecosystems in extending the effective CH 4 emitting area beyond flooded lands. This article is part of a discussion meeting issue 'Rising methane: is warming feeding warming? (part 2)'.

Large Methane Emissions From Tree Stems Complicate the Wetland Methane Budget

“…This suggests that soil moisture, reducing soil conditions and high porewater CH 4 concentrations (Figure S4 in Supporting Information ) were the major drivers of tree stem CH 4 emissions. This agrees with previous studies showing similar trends along soil moisture gradients (Daniel et al., 2023; Jeffrey et al., 2020a; Moldaschl et al., 2021; Pitz et al., 2018), via artificial flooding events (Pangala et al., 2014; Terazawa et al., 2021) and with seasonal changes in the forest water table heights (Gauci et al., 2022; Sjögersten et al., 2020). In the same M .…”

“…This process was first proposed by Gauci et al. (2022), who found a remarkedly similar correlation in tree stem CH 4 fluxes within an Amazon riparian forest subject to ∼12 m amplitude fluctuations in the water table height. Overall, this shows a significant correlation between the depth of the water table below‐ground and the root‐entrained CH 4 tree stem emissions, whereas tree stem CH 4 fluxes essentially become de‐coupled from the water table dynamics when water levels are above the soil surface (Figure 10).…”

“…Two‐thirds of all tree CH 4 literature was published within the last three years. Tree stem CH 4 emissions have now been reported from various ecosystems including upland forests (Barba et al., 2019b; Machacova et al., 2016, 2023; Pitz & Megonigal, 2017; Wang et al., 2016), mangrove forests (Gao et al., 2021; Jeffrey et al., 2019b; Zhang et al., 2022), riparian forests (Flanagan et al., 2021; Gauci et al., 2022) and standing deadwood, snags or ghost forests (Carmichael & Smith, 2016; Martinez & Ardon, 2021; Warner et al., 2017). The highest tree stem CH 4 emissions, however, are exclusively attributed to forested wetland ecosystems, also known as lowland forests (Gauci et al., 2010; Jeffrey et al., 2021b; Pangala et al., 2013; Terazawa et al., 2007).…”

Evaluation of wetland CH4in the Joint UK Land Environment Simulator (JULES) land surface model using satellite observations

“…The Congo remains one of the most significant global wetland regions but equally remains one of the most challenging to simulate and evaluate, with a significant uncertainty in the CH 4 emissions. Ongoing model development (Gedney et al, 2019) related to the inclusion of methane emissions from trees in flooded areas (Pangala et al, 2017;Gauci et al, 2022) as well as improvements in the soil ancillary data to represent Oxisol and Ultisol soils in this area are expected to improve our ability to more accurately model the CH 4 emissions from the Congo in future work.…”

Evaluation of Wetland CH4 in the JULES Land Surface Model Using Satellite Observations

“…The Congo remains one of the most significant global wetland regions but equally remains one of the most challenging to simulate and evaluate, with a significant uncertainty in the CH 4 emissions. Ongoing model development (Gedney et al, 2019) related to inclusion of methane emissions from trees in flooded areas (Pangala et al, 2017;Gauci et al, 2022) as well as improvements in the soil ancillary data to represent oxisol and ultisol soils in this area are expected to improve our ability to more accurately model the CH 4 emissions from the Congo in future work.…”