Variability of tree methane emissions across regions of the Amazon rainforest

Trees are recently understood to emit large quantities of CH₄ through their stems, particularly in tropical wetland environments. There are still large uncertainties of the processes driving tree CH₄ emissions, however, the primary mechanism is thought to be through transfer of CH₄ produced in soil into tree biomass and then to the atmosphere. Another possible mechanism is via anaerobic decomposition of rotting tree biomass in stems. In the Brazilian Amazon, very little is known about sources and variability of tree CH₄ emissions and how they may vary across different flooded regions.

Across regions of the Amazon we aim to understand the variation of CH₄ emissions from trees. These regions are characterised as white water flooded forest (Várzea region) and black water flooded forest (Igapo region). Using two tree species of similar ages across two regions, we measured tree CH₄ emissions and surrounding porewater CH₄ concentrations for two flooded seasons. Across all study locations and tree species we found large but variable net CH₄ emissions ranging from 0.01 to 84 mg m^-2 hr^-1. These variations in emissions are significantly influenced by the tree species. Furthermore, we measured significantly different fluxes when measuring the same tree species across two regions, suggesting there could be vast alterations in flux when attempting to measure emissions across the Amazon region.

Our work also revealed that CH₄ emission was highest at the base of trees (30 cm) compared to measurements made higher up the stem (70 cm). This is consistent with radial diffusion of soil derived CH₄ up the stem and also stongly suggests the source of CH₄ is soil derived. Porewater concentrations of CH₄ throughout the soil column further supports tree CH₄ emission deriving from soil.

Furthermore, we analysed the stable isotopic carbon values of emitted CH₄ and demonstrate that this vertical reduction in emitted CH₄ is also in part a product of biological oxidation of CH₄ by methanotrophic bacteria located in woody material. The isotopic profile varied between two tree species and at the base of the tree compared with higher up the stem. We also noted individual tree species had isotopic variability across the two sites.

These results show significant CH₄ emissions from trees to atmosphere in the Amazon. By using common tree species of similar ages we demonstrate that the strength and variability of these emissions are strongly influenced by site specific variables that require further investigation.