THE ROLE OF RESOURCES, DISTURBANCES, AND THEIR INTERACTION IN DETERMINING THE CARBON BALANCE OF TROPICAL SAVANNAS

Abstract

Tropical savannas are playing an increasingly important role in the carbon balance of the world. However, the dynamic response of plant communities to resources and disturbances combined with the capacity for communities to influence resources and disturbances creates complex feedbacks. This thesis examines both the independent and interactive effects of fire and herbivory on savanna ecosystem carbon storage, and how these effects may be influenced by nutrients and rainfall. I expand on the potential role of nutrients by considering how fire-nutrient interactions at the individual, community, and ecosystem scale.

Chapter 2 presents an empirical evaluation of how fire affects carbon and nutrients and the degree to which changes in nutrients trigger shifts in the nutrient status of plants in an African savanna. The impact of disturbance on the plant community is broadened to include the interactions between fire and disturbance by herbivores in Chapter 3. Chapter 4 evaluates whether fire-nutrient feedbacks emerge at the ecosystem scale in a wetter South American savanna. Finally, Chapter 5 presents a broader global analysis of how fire may determine the community composition of plants by selecting for plants with different nutrient acquisition strategies.

The final chapter synthesizes the findings of this thesis to argue that carbon storage in savanna is a balance between disturbance induced losses and resources available for plant recovery. An interaction between disturbance and resources emerges, such that fire-driven nutrient losses alter the biogeochemistry of savannas and thus potential resources for vegetation recovery. However, the effect of fire-driven nutrient losses on the plant community is not straightforward and raises the issue as to what determines the resilience of a community to fire-driven nutrient losses.