Anthropo-Zoogenic Impact on the Structure and Regeneration of a Submontane Forest in Kenya

Abstract

Submontane forests of the East African high mountains are being threatened by settlement and cultivation. Only small areas of such forests are being protected in the National Parks, although, even there, disturbance cannot be completely prevented.Reasons for the degradation of submontane forest (c. 80 km2) in one of the National Parks in which a rhino sanctuary has recently been established, namely in the Salient of the Aberdare National Park, were investigated in the course of a study of the vegetation of this Park.The vegetation changes which have taken place in the Salient have been principally initiated by man and intensified or maintained by large mammals. Shifting cultivation and livestock grazing have led to the widespread destruction of primary forest. Since the gazetting of the National Park (1950), secondary bushland has developed on previously settled land (c. 25 km2); to date, no recolonization by forest has taken place here, although considerable regeneration has been observed in adjacent forests.The structure and composition of the present forest vegetation of the Salient suggests that most forests are secondary, disturbed by both man and wildlife. The forests containing Juniperus procera are regarded as a subclimax which has been subject to selective felling of the emergent trees which, in the absence of fire, will develop into Cassipourea forest. Primary, or, at least, little-disturbed forest was only found in the higher parts of the Upper Salient.The present density of wildlife in the Salient is not enough to cause large-scale destruction of the forest, with the exception of the area around Treetops, where seasonally high concentrations of herbivores build up as a consequence of human activities. Here elephants directly destroy the forest. In addition, herbivores contribute in a catalytic manner to the destruction of forest by extensive trampling which compactss the topsoil so that surface run-off is increased and the oxygen supply to the fine roots is decreased and by debarking of trees (by elephants only). The weakened trees are less resistant to felling by wind and to pushing over by elephants.The present herbivore density however, does seem to impede the regeneration of forest in areas covered by secondary bushland. Here the tree seedlings are often exposed to browsing and trampling pressure after the removal of fallen trees by man. This, in combination with the slow growth of trees above 1920 m, seems to be responsible for the lack of regeneration.The shrub-density of the vegetation of the glades in the Salient seems to be dependent mainly on herbivore densities, which in turn are controlled by drought, disease and predators.Another important factor causing large-scale destruction of forest is fire. However, fire has played only a minor role in the Salient since the cessation of shifting cultivation.KeywordsVegetation ChangePark BoundaryLower SalientCedar ForestBuffalo PopulationThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.