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The decline and restoration of riparian and hilltop forests in the Serengeti National Park, Tanzania Sharam, Gregory J.

Abstract

The riparian and hilltop forests of Serengeti National Park, Tanzania, have been in rapid decline since the early 1970s. Fifty percent of riparian forests and 85% of hilltop forests have been converted to grassland in that time. This thesis investigates the causes of this decline and the conditions under which forests will stabilize and recolonize grassland areas. Fire is the main cause of decline, particularly affecting the upwind side of rivers and removing seedlings and canopy trees at the forest-grassland boundary. The effects of fire depend on the type of forest and forest edge. Closed-canopy forests with dense dripline edges are more resistant to fire than open-canopy, advancing edges. Mortality of seedlings and canopy trees in closed-canopy forests increased only when stands were burned in four successive years, while seedlings and canopy tree mortality occurred after only one year of burning in forests with open-canopy advancing edges. Germination of forest trees within forests is limited by grass abundance and conditions in the grassland, but is increased by recent fires, floods and grass removal. Subsequent survival and growth of seedlings is limited by fires at the forest periphery and by antelope browsing. Survival of large trees is reduced by fire and by elephants; however, elephant damage was insufficient to limit replacement of the forest canopy although elephants did damage canopy trees. Seedling establishment is poor in the grassland adjacent to forests, despite the removal of fire, grass and antelope browsers. However, isolated stands of savanna trees can act as nurse trees and facilitate the establishment of riparian forests by excluding grass, fire and browsers, and increasing dry season soil moisture. Moreover, current forest stands probably developed from previous nurse stands. The history of Serengeti forests is reviewed and its implications for forest conservation and management are discussed.

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