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.