Nipping the bud hypothesis : a comparison of savannah and forest tree responses to canopy scorch

Tree mortality, following surfaces fires, is much greater in forests than in savannas, despite similarities in the bark thickness of tree trunks. This difference is thought to influence the distributions of forest and savanna biomes. There is no consensus and very little research on how fires actually kill trees. Though tree canopies are too high above the flames to be charred, leaves are often scorched over the full tree height; hence the focus on differences in tree canopy recovery. I explored two possible mechanisms for differences in crown resprouting following canopy scorch: (1) differences in bud banks and their insulation, (2) differences in the vulnerability to disruption of the hydraulic system by heat scorch. I simulated canopy scorch using nine congeneric savanna-forest pairs in three Reserves in KwaZulu Natal, South Africa. Contrary to predictions, there was no difference in the presence of a viable bud bank between congeneric species. The resprouting response varied between congeneric pairs with only the Combretum, Rhus and Acacia pairs showing savanna species with a significantly greater ability to resprout. The vulnerability to hydraulic disruption was measured using wood density as a proxy for conduit reinforcement. The results are inconclusive and possibly affected by the growth rates of certain shade-tolerant forest trees. The differences in crown resprouting following canopy scorch of forest and savanna trees is not the result of differences in bud banks and resprouting ability. The differences in insulation of vulnerable tissue by bark may be related to the protection of the hydraulic system rather than the buds and/or cambium