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

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 (Cwood) 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 (Cwood) to dead organic matter likely exceeds both fire-related emissions from Cwood to atmosphere and non-fire Cwood 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 Cwood 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.