Assessing the fire resilience of the savanna tree component through a functional approach

Abstract

Changes in savanna's fire regimes, either through fire suppression or through an increase in fire frequency, can negatively affect their resilience. We evaluated the extent to which the aboveground biomass, diversity (taxonomic and functional) and resilience (functional redundancy and functional response indices) of savanna tree communities differ between burned and unburned plots. Burned plots experienced two fire events over the ten-years prior to sampling, while unburned plots experienced fire suppression over the same time period. We found that aboveground biomass was 40% smaller in burned plots, indicating that fire regimes must be included as a source of variation in models estimating the potential of savannas to store carbon. Burned plots had a higher functional diversity of vegetative traits but a smaller functional diversity of reproductive traits, indicating that generalizations about the effect of fire on tree functional diversity should be viewed with caution. Periodic fires can benefit savanna tree biodiversity by maintaining the balance between light-demanding and shade-tolerant species but can also increase the dominance of species with less specialized reproductive traits that do not rely on animal interactions. Burned plots had slightly lower functional redundancy but similar functional response diversity compared to unburned plots, suggesting that both communities harbor tree species that might respond positively or negatively to fire and, therefore, will be able to maintain the ecosystem functions considered under a future scenario of fire-suppression or increased fire frequency. Therefore, a longer-term fire suppression (>10 years) or a return fire interval of less than 4 years may be necessary to reduce the resilience of the savanna tree component, considering the ecosystem functions analyzed in this study.