Upland tree woody surfaces are the major conduits of atmospheric methane
Methane measurement from mature trees in the dry season of 2021 in the Gigante Litter Manipulation Project, South of Barro Colorado Island, Central America
Methane flux measurements from mature trees stems (n = 50 per plot) at five stem heights (20, 60, 100, 140 and 180 cm above the soil surface) were performed during a period of transition towards the dry season when the water tables in both the plots were more than 10 m below the soil surface. They were done between 15 and 25 March. Methane fluxes were measured using static chambers as described in refs. 3,22, with air samples from the static flux chambers drawn using 30 ml syringes and immediately transferred to a 12 ml exetainer (Exetainer) for later analysis of CH4 using the modified LGR CH4 laser-based analyser3 (LGR UGGA).
Measurements in semi-evergreen tropical forest were carried out between 18 and 27 November 2015 in the five control plots of the Gigante Litter Manipulation Project, approximately 5 km south of Barro Colorado Island, Panama, Central America. The refs give a full description of the experiment. 36,37. There is a strong dry season from December to April, and the annual rainfall at the Barro Colorado Island weather station is more than two thousand millimetres.
Thirty liters of gas samples were collected from the Negro and Solimes floodplain forests in the dry season of 2021. Samples were taken from air and from flux chambers on the soils surface and tree stem surface at 5 m above the forest floor using gas-tight syringes and then transferred to pre-evacuated 12 ml borosilicate vials fitted with double wadded caps (Exetainer). Vials were over-pressurized to prevent ingress of air from pressure or temperature changes during transport to the laboratory. The 13C values of CH4 were analysed using a piece of equipment that is similar to a ring-down strontium microscope with a custom-built Auto-sampler. The instrument was calibrated for δ13C-CH4 using isotopic reference gases with isotope ratios of −23.9‰, −54.5‰ and −66.5‰ (Isometric Instruments). The analytical precision based on replicate measurement of gases was 0.4.
For global-scale extrapolation, we used a combination of satellites. This approach allowed us to estimate woody surface areas across the world’s ecosystems, integrating our allometric model with global forest cover data.
The Nature Conservancy has an Ecoregions map that we used to find the extents of the world’s biomes. There is a 1 km consensus land cover map as well as the volute Continuous Fields (VCF49) product, which can be used to determine forest cover. A map of the global view of vegetation cover was provided by this dataset.
In order to account for the climate variations in our model, we used the monthly climate dataset. The data provides mean monthly temperatures. The CH4 application was done across the globe since the woody surface area is in each piece of information.
