Methane exchange in a California oak savanna: insights from tree stem, soil, and ecosystem fluxes

Savannas remain underrepresented in the global methane (CH 4 ) budget, with flux contributions from soils, vegetation and livestock poorly constrained. Upland savanna soils are generally assumed to be net CH 4 sinks, yet ecosystem-scale quantification is rare, and the role of tree stems, capable of transporting, producing, or consuming CH 4 , remains largely unknown. Episodic livestock emissions and spatio-temporal heterogeneity in soil moisture and vegetation add further complexity. Here we present the first integrated year-round assessment of CH 4 cycling in a California blue oak savanna, combining soil and stem chamber fluxes with understory and overstory eddy covariance (EC), supported by terrestrial laser scanning for structural upscaling. Ecosystem-scale EC measurements showed that the oak savanna functioned as a net CH 4 source (0.3 ± 0.08 g C m -2 yr -1 ), despite soils acting predominantly as CH 4 sinks. Tree stems consistently emitted CH 4 across all sampled heights, challenging the assumption that upland trees are mostly net sinks. Overall, CH 4 dynamics were governed primarily by cumulative thermal and moisture conditions rather than short-term atmospheric variability, with rain-driven soil saturation creating transient production hotspots, and spatial heterogeneity in soil carbon and nitrogen modulating sink strength. Episodic livestock presence introduced important but difficult-to-attribute flux variability, representing the largest source of uncertainty in the ecosystem budget. These findings demonstrate that savanna CH 4 exchange reflects tightly coupled soil, plant, and hydrological processes that require integrated approach to be accurately resolved.