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

Abstract

Savannas remain underrepresented in the global methane (CH4) budget, with flux contributions from soils, vegetation and livestock poorly constrained. Upland savanna soils are generally assumed to be net CH4 sinks, yet ecosystem-scale quantification is rare, and the role of tree stems, capable of transporting, producing, or consuming CH4, 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 CH4 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 CH4 source (0.3 ± 0.08 g C m-2 yr-1), despite soils acting predominantly as CH4 sinks. Tree stems consistently emitted CH4 across all sampled heights, challenging the assumption that upland trees are mostly net sinks. Overall, CH4 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 CH4 exchange reflects tightly coupled soil, plant, and hydrological processes that require integrated approach to be accurately resolved.