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.