Juggling Carbon: Allocation Patterns of a Dominant Tree in a Fire-Prone Savanna

In frequently burnt mesic savannas, trees can get trapped into a cycle of surviving fire-induced stem death (i. e. topkill) by resprouting, only to be topkilled again a year or two later. The ability of savanna saplings to resprout repeatedly after fire is a key component of recent models of tree-grass coexistence in savannas. This study investigated the carbon allocation and biomass partitioning patterns that enable a dominant savanna tree, Acacia karroo, to survive frequent and repeated topkill. Root starch depletion and replenishment, foliage recovery and photosynthesis of burnt and unburnt plants were compared over the first year after a burn. The concentration of starch in the roots of the burnt plants (0.08 ± 0.01 g g⁻¹) was half that of the unburnt plant (0.16 ± 0.01 g g⁻¹) at the end of the first growing season after topkill. However, root starch reserves of the burnt plants were replenished over the dry season and matched that of unburnt plants within 1 year after topkill. The leaf area of resprouting plants recovered to match that of unburnt plants within 4-5 months after topkill. Shoot growth of resprouting plants was restricted to the first few months of the wet season, whereas photosynthetic rates remained high into the dry season, allowing replenishment of root starch reserves. ¹⁴C labeling showed that reserves were initially utilized for shoot growth after topkill. The rapid foliage recovery and the replenishment of reserves within a single year after topkill implies that A. karroo is well adapted to survive recurrent topkill and is poised to take advantage of unusually long fire-free intervals to grow into adults. This paper provides some of the first empirical evidence to explain how savanna trees in frequently burnt savannas are able to withstand frequent burning as juveniles and survive to become adults.