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 (CH₄) in forested wetlands. This raises the possibility that CH₄ 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 CH₄ production and emission. Here, we examine CH₄ 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 CH₄ yr^-1 to the atmosphere in addition to inundated tree emissions of approximately 12.7-21.1 Tg CH₄ yr^-1. Applying our approach to all tropical wetland broad-leaf trees yields an estimated non-flooded floodplain tree flux of 6.4 Tg CH₄ yr^-1 which, at 17% of the flooded tropical tree flux of approximately 37.1 Tg CH₄ yr^-1, demonstrates the importance of these ecosystems in extending the effective CH₄ emitting area beyond flooded lands. This article is part of a discussion meeting issue 'Rising methane: is warming feeding warming? (part 2)'.

Keywords: Amazon; floodplain; methane; riparian; soils; trees.

Figure 1.

Map showing the locations of the three sampling sites within the floodplains of Solimões (S; white water) Negro (N; black water) and Tapajós (T; white water) rivers.

Figure 2.

Box and whisker plots of seasonal changes in tree CH₄ flux measured at 20–50 cm above the forest floor (either soil or water depending on the state of flood) for the three study plots in each catchment (a–c). Figure (d–f) demonstrates the corresponding water table for each location at each seasonal time point. Campaigns 1 through 4 were carried out, respectively, during rising (campaign 1; April 2017), peak (campaign 2; July 2017), receding (campaign 3; October 2017) and low water table conditions (campaign 4; January 2018). Error bars indicate the 10th and 90th percentiles.

Figure 3.

The dependence of total tree flux on water table. Measurements are shown as black dots. z_WT〈0, and〉0 refer to water table below and above the soil surface, respectively. Linear regressions of ln(CH_4tree) = M.z_WT + C (equation (3.1)) are applied to all site data when the water table is at or above the soil surface (z_WT ≥ 0), and applied to all site data but limiting z_WT in equation (3.1) to be no higher than the soil surface (i.e. z_WT ≤ 0) – see text for details. Mean and 95% confidence intervals are shown with solid and dashed lines, respectively.