The vegetation of Manyara : scale-dependent states and transitions in the African Rift Valley

This study focuses on scale dependency, both temporal and spatial, of vegetation changes. At different spatial levels, starting from the level of the individual plant, via patch or stand, plant community, and, finally, at the level of ecosystems, the changes in the vegetation of an East African savanna in the Rift Valley are related to the time span considered. The dominant tree Acacia tortilis in Lake Manyara National Park plays an important role in plant-herbivore relationships, and was therefore taken as a typical case in this respect.

Acacia tortilis seed germination and seedling establishment is most successful on bare patches, in the absence of other vegetation. For the reconstruction of tree establishment in the past, tree growth characteristics were studied. Changes in floristic composition between the 1970s and the 1990s were predominantly caused by changes in abiotic factors. The influence of herbivores on the vegetation appears to be minimal on the long-term. The structure of the herbivore assemblage in Lake Manyara National Park, however, is more likely to be determined by changes in the vegetation. Although faunistic and floristic changes occur, these changes must be considered as fluctuations within the ecosystem, and do not lead to a change of the ecosystem itself. It is proposed that scale, both temporal and spatial, must be incorporated in state and transition models.