The Effects of Fire on the Characteristics of Woody Vegetation and Encroachment in an African Savanna

Abstract

African savannas have experienced considerable woody encroachment over the last century, presenting an increasing problem from both ecological and socioeconomic viewpoints. Despite decades of work by savanna ecologists, the reasons for woody encroachment remain unclear. A major barrier to understanding the causes is the difficulty of disentangling the effects of broader-scale environmental changes, such as climate change and associated increases in atmospheric CO₂, from localised effects such as fire. In this thesis I examine the effects of sixty years of experimental burning on the characteristics of woody vegetation in two climatically distinct African savannas, a wet and a dry savanna, to examine how long-term burning interacts with other potential drivers of woody encroachment. I examine tree abundance, woody cover, tree structure, diversity and community composition under four different fire regimes: annual, biennial, triennial and fire exclusion. Differences between sites and plots subject to different burning regimes are compared along with changes in these differences through time. Additionally, variation in the densities of Acacia, Combretum, Terminalia and Dichrostachys species were examined to establish how dominant species, particularly those responsible for encroachment, are affected by fire. Overall, I found that the effects of fire depend on savanna type. Fire lowered tree abundance and woody cover much more in the wet savanna than in the dry savanna. However, the maximal height of trees was much more constrained by increased fire frequency in the dry savanna than in the wet savanna. Woody encroachment occurred across both savanna types during the sixty year time period, but was much more rapid at the wet savanna. Additionally, encroaching species of Dichrostachys cinerea and Terminalia sericea in the wet savanna were shown to be more difficult to manage using fire. Overall as fire regimes were kept constant over the last sixty years, yet woody encroachment occurred across all fire treatments, it is most likely that an external driver is responsible. Rainfall change in both areas was minimal over the duration of the study, thus increased atmospheric CO₂ would appear to be the most likely cause of woody encroachment. However, the magnitude and characteristics of woody encroachment are strongly mediated by fire and rainfall. Wet savannas would appear to be much more vulnerable to woody encroachment and existing management strategies are likely to become increasingly ineffective at keeping woody cover below potential maximum levels. Overall this thesis demonstrates that the effects of fire on woody vegetation in savannas vary depending on regional differences in rainfall and that processes of woody encroachment differ depending on savanna type.