Precipitation-fire-functional interactions control biomass stocks and carbon exchanges across the world’s largest savanna

Abstract. Southern African woodlands (SAW) are the world’s largest savanna, covering ~3 M km², but their carbon balance, and its interactions with climate and disturbance are poorly understood. Here we address three issues that hinder regional efforts to address international climate agreements: producing a state-of-the-art C budget of SAW region; diagnosing C cycle functional variation and interactions with climate and fire across SAW; and evaluating SAW C cycle representation in land surface models (LSMs). Using 1506 independent 0.5° pixel model calibrations, each constrained with local earth observation time series of woody carbon stocks (C_wood) and leaf area, we produce a regional SAW C analysis (2006–2017). The regional net biome production is neutral, 0.0 Mg C ha^-1 yr^-1 (95 % Confidence Interval –1.7 – 1.6), with fire emissions contributing ~1.0 Mg C ha^-1 yr^-1 (95 % CI 0.4–2.5). Fire-related mortality driving fluxes from total coarse wood carbon (C_wood) to dead organic matter likely exceeds both fire-related emissions from C_wood to atmosphere and non-fire C_wood mortality. The emergent spatial variation in biogenic fluxes and C pools is strongly correlated with mean annual precipitation and burned area. But there are multiple, potentially confounding, causal pathways through which variation in environmental drivers impacts spatial distribution of C stocks and fluxes, mediated by spatial variations in functional parameters like allocation, wood lifespan and fire resilience. Greater C_wood in wetter areas is caused by positive precipitation effects on net primary production and on parameters for wood lifespan, but is damped by a negative effect with rising precipitation increasing fire-related mortality. Compared to this analysis, LSMs showed marked differences in spatial distributions and magnitudes of C stocks and fire emissions. The current generation of LSMs represent savanna as a single plant functional type, missing important spatial functional variations identified here. Patterns of biomass and C cycling across the region are the outcome of climate controls on production, and vegetation-fire interactions which determine residence times, linked to spatial variations in key ecosystem functional characteristics.