The role of topkill in the differential response of savanna woody species to fire

Abstract

Understanding the impact of fire on the demography of savanna trees and shrubs is necessary for understanding human impacts in tropical savannas. In a replicated experiment, we studied the impact of fire and vegetation cover on survival and growth of two subshrubs ( Periandra mediterranea and Protium ovatum), two shrubs ( Miconia albicans and Rourea induta) and three trees ( Myrsine guianensis, Piptocarpha rotundifolia and Roupala montana) of the Brazilian cerrado savannas. Burning increased complete mortality (i.e. death of the individual) of five of the seven species, but primarily among individuals with stem diameters <4 mm. Stem mortality (i.e. topkill) was much more prevalent, primarily affecting individuals with stem diameter <32 mm, though all species experienced some topkill in even the largest size classes. Fires of higher intensity (flame length >2 m) caused greater mortality and topkill than fires of lower intensity (flame length <2 m). Pre-burn vegetation density had little effect on survival or resprout size, but did affect subsequent growth rates. Four species had greater growth rates in open sites, whereas only one species had greater growth rates in dense sites. For the three tree species and one shrub, resprouting individuals did not reach the minimum reproductive size within 1 year of burning, while the other shrub and the two subshrubs were able to reach reproductive size during this time, indicating that growth form largely determines the population response to frequent burning.