Methane Flux Dynamics in a California Oak Savanna

Methane (CH4) is a potent greenhouse gas, yet the role of trees in the global CH4 budget remains uncertain. While some studies report CH4 emissions from wetland and certain upland trees via soil-derived transport or in-tree production, others suggest that upland forests may function as net atmospheric CH4 sinks. In this study, we investigated CH4 exchange in an oak savanna in California (AmeriFlux site US-Ton) using a multi-scale measurement approach. From August 2024 till October 2025, we have conducted biweekly measurements of stem CH4 and CO2 fluxes on six mature oak trees at three heights (0.4, 1.3, and 2.6 m), alongside soil CH4 flux measurements near each tree using LI-COR 7810 analyzers and a Smart Chamber. Ecosystem-scale CO2 and CH4 fluxes were quantified using eddy covariance with open-path LI-COR 7500 and 7700 analyzers. To assess sub-canopy flux variability, an additional eddy covariance system was deployed below the canopy. Tree surface area for flux upscaling was quantified using terrestrial laser scanning. Tree stems generally acted as small CH4 sources throughout the year, whereas soils consistently functioned as minor CH4 sinks, especially in sun-exposed areas. Stem vertical stem flux profiles did not indicate a direct coupling with soil CH4 dynamics. However, during early spring flooding events, the stem bases of some trees emitted episodically large CH4 fluxes, suggesting that transport of soil-derived CH4, in addition to internal production, can contribute to stem emissions. Ecosystem-scale eddy covariance measurements showed no persistent seasonal pattern in CH4 emissions, although modest increases were observed from spring. At the annual scale, sub-canopy eddy covariance CH4 fluxes were comparable to soil chamber-based estimates, indicating that the under-canopy the soil likely functions as a small CH4 sink. In contrast, above-canopy eddy covariance measurements indicated that the ecosystem as a whole is a small net CH4 source. This discrepancy may be explained by non-microbial CH4 production from oak leaves, supported by incubation experiments and the pronounced increase in ecosystem CH4 fluxes following leaf emergence.