The impacts of a hot fire on the population dynamics of Vachellia erioloba

Abstract

The Savanna Biome is one of the largest in southern Africa, extending across a wide range of rainfall conditions. A major factor in the occurrence of Savannas in Africa is the presence of fire and herbivory disturbances that influence vegetation structure and enable tree-grass coexistence. The Kalahari is an arid Savanna home to many important and endangered species. Vachellia erioloba is one of those species and is essential to the survival of a range of other plants and animals. The role of fire in arid Savannas has been neglected as fire is considered much rarer in arid Savannas compared to mesic systems. Thus, few studies have focused on the impacts of fire on V. erioloba individuals and populations. Additionally, studies focused on the species’ population dynamics have not always considered the long-term dynamics of this long-lived tree. In September of 2021 a large, intense fire burnt through Benfontein Nature Reserve (BNR) causing many large and old V. erioloba trees to topple and even die. This study was conducted across 1100 ha of Kimberley Thornveld in BNR. Individual, burnt adults were surveyed to determine their survival and resprouting abilities after the fire. Sapling traits and densities were measured and compared between 2003 and 2022 following which, the adult population density was analysed from 1940 to 2023 to understand some of the long-term population dynamics and the impacts of the 2021 fire. Finally, the demographic bottleneck index was determined what limits the growth of this population. I determined that V. erioloba trees are able to survive burning but their strategies change with age from fire tolerance to fire resistance and avoidance. Saplings recovered surprisingly well and fast after burning, with a greater density observed in 2022 than in 2003. The adult population showed little change from 1940 to 2017, but the 2021 fire caused a drastic decline. The population’s demographic bottleneck is at the sapling recruitment to adult stage. Fire, herbivory, frost, rainfall, and competition are considered to be drivers of the demographic bottleneck. This research emphasizes that this species and landscape can be slow to change and if the population is going to recover from the fire, then efforts must be made to reduce restrictions to sapling growth and encourage recruitment to the adult life stage.