Effects of long-term fixed fire regimes on African savanna vegetation biomass, vertical structure and tree stem density

Fire plays an integral role in shaping the vegetation structure of savanna ecosystems. However, effects of fire regime characteristics, such as frequency and season of burn, on savanna vegetation structure, biomass and tree abundance across landscape types are largely unknown. We used high-resolution airborne Light Detection and Ranging (LiDAR) to investigate the long-term effects of fire manipulation on savanna vegetation in Kruger National Park, South Africa. We analysed the effects of fire exclusion and experimental burns every 1, 2, 3, 4 and 6 years and during different seasons on aboveground biomass (AGB), tree stem densities and vegetation vertical height profiles across a rainfall gradient and on contrasting geologies. Across savanna types, and especially in drier savannas, fire season was more influential for constraining AGB than was fire frequency. Plots experiencing fires during the late- and mid-dry season had 44.50% and 43.60%, respectively, lower AGB relative to unburnt plots than wet-season fires. However, in mesic savannas, fire frequency interacted with fire season to influence AGB: plots subjected to high frequency, dry season fires had 55.35% lower AGB than unburnt plots, whereas plots burnt in the wet season at lower frequencies had lower AGB (24.40% lower than unburnt plots) than plots subjected to high frequency, wet-season fires (13.74% lower AGB than unburnt plots). Fire regimes had variable effects on tree densities, and effects varied with savanna type. Woody vertical vegetation profiles showed the largest differences in response to dry season fires, with the greatest divergence in vegetation height classes < 5m.  Synthesis and applications. Understanding the influence of fire regimes on vegetation structure has important implications for the management of savanna heterogeneity, and for predicting trajectories of change in savanna vegetation as fire regimes vary with climate change. We show that the magnitude of the effect of fire on woody vegetation structure varies with savanna context. Our results suggest that heterogeneous vegetation structure can be achieved by applying fires in the dry season in mesic savannas, whereas in dry savannas, variation in fire regimes is less consequential for constraining biomass accumulation and altering vegetation structure.