Tropical wetlands are the largest natural source of atmospheric methane (CH4), with Amazonian floodplain trees alone contributing approximately 15% of global wetland CH4 emissions. Although stem CH4 emissions in the Amazon floodplain exceed those in other tropical regions, they are highly variable across landscapes and individual trees. The mechanisms driving this variability remain poorly understood. To investigate CH4 flux variability at both ecosystem and species levels, fieldwork was conducted across three floodplain systems in the Brazilian Amazon during wet and dry seasons, spanning multiple field campaigns. Stem CH4 fluxes were measured from two tree species with contrasting wood densities (Eschweilera coriacea and Hevea spruceana) and compared with below-ground porewater chemistry and root biomass. Results revealed substantial spatial and seasonal variation in stem CH4 emissions, with fluxes differing across floodplain types but not consistently by species, suggesting that site-level biogeochemistry plays a stronger role than species identity in regulating emissions. Vertical declines in stem CH4 flux were examined alongside dual isotope enrichment (δ13C-CH4 and δ2H-CH4), indicating internal CH4 oxidation occurring within stems at rates comparable to those in subtropical forests, despite differences in overall emission magnitudes. While oxidation did not vary substantially across species or ecosystems, species-specific patterns highlighted variability in oxidation which requires further investigation. Stable isotopes (δ13C), wood decay and radiocarbon (14C) analysis of porewater, wood cores and tree chambers revealed species-level differences that may help explain why some trees emit more CH4 than others. Wood decay was identified as a potential alternative pathway for CH4 production, and sonic tomography is proposed as a promising tool for scaling decay-related emissions across forest systems. However, distinguishing soil-derived from tree-derived CH4 using 14C and δ13C remains challenging due to complex interactions among microbial oxidation, internal transport, and tree-specific traits. By studying two tree species with contrasting wood densities across multiple Amazonian floodplain ecosystems and seasons, this research confirms the significant role of CH4 cycling within trees. CH4 production inside tree stems contributes to emissions year-round and appears to dominate during the dry season, calling for a reassessment of how emissions are measured in tropical floodplain trees and how ecosystem and regional CH4 budgets are estimated. Furthermore, internal oxidation was confirmed in both species and shown to influence net CH4 fluxes by reducing emissions. However, oxidation rates varied across species and sites, suggesting that the underlying controls of this process warrant further investigation due to their implications for ecosystem-level and regional CH4 budgets. These findings have important consequences for process-based modelling and the refinement of regional CH4 budget, emphasising the need to integrate tree-level dynamics into broader assessments of tropical wetlandCH4 fluxes.
Holly Blincow (Thu,) studied this question.