Causes and Consequences of Savanna Tree Cover Change

The structure and productivity of savannas has undergone major transitions over the past century, with widespread increases in the cover of small trees and shrubs recorded for sites across the world. Rapid increases in cover have occurred throughout southern Africa, some starting in the late 1800's, and are referred to locally as "bush encroachment". Recent studies show that "bush encroachment" has occurred rapidly in many areas over the past decades in South Africa. At the same time, the abundance of tall trees has been declining in certain, particularly in protected areas where elephants occur. These changes have importance consequences for biodiversity and ecosystem services. Many theories have been proposed to explain these changes, but robust evidence to support them is lacking. Altered fire regimes, overgrazing, declining abundance of browsing, and increased concentrations of atmospheric CO2 have all been proposed as drivers of "bush encroachment“. Evidence supporting the importance of each is limited and often site-specific, while interactive effects have rarely been considered. Critical evidence that is lacking from almost all studies is that of changes in the population dynamics of dominant woody species. This is also a critical gap of knowledge for understanding the decline of tall trees, and the long-term impacts of altered elephant populations. This project aims to provide the long-term demographic data needed to determine past and current changes in the tree layer of savannas. Such data is also critical for modelling future changes. The project is currently implemented at 10 sites in the central lowveld region of South Africa.