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Zimbabwe: Landuse in Dry Tropical Savannas






Ecosystems

Ecosystems (short for ecological systems)are functional units that result from the interaction of abiotic, biotic and cultural (anthropogenic) components.

Ecozones of Southern Africa

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Ecozones on the other hand are these systems described in a geographical or spatial context. Ecozones are natural units respecting no political boundaries. The Limpopo valley as an ecozone is described as a dry savanna system, a fragile ecosystem characterized by low rainfall, fertile soils and sparse vegetation.

Ecosystem fuctioning

Fire, periodic drought and animals are the driving forces of the dry savanna ecosystems. The impacts of these factors can be clearly seen at Malilangwe Conservation Trust and the adjacent Gona-Re-Zhou National Park. Large mammals, especially elephant have caused widespread destruction of the tree layer, such that some species have all but disappeared as large trees. This has occured in the National Park where elephant numbers have exceeded carrying capacities for a number of years. Large specimens of baobabs (Adansonia digitata) have been particularly hard hit by elephant. The boundary between the National Park and the Conservation Trust can be clearly seen from the air, because of the destruction by elephants in the National Park, but an added feature is fire frequency. In the National Park fires have occurred on an almost annual basis, while in the Trust their has been fire protection (or at least less frequent burning). Most trees are fire-adapted but annual fires can slowly change a woodland to a wooded grassland. Periodic droughts have also played a role in shaping the ecosystem. The droughts of the 1980s saw massive deaths of certain species (e.g. hippo), more so of domestic herbivores where there has been a switch from cattle-dominated systems to the more hardy goat-dominated systems. The droughts have been so severe that even trees have experienced mortality.

Particular combinations of determinants can cause systems to switch from one type to another. For instance in the grazing lands of the communal areas, we saw massive cattle deaths in 1990/91. There were also massive tree deaths in some areas, up to 30% of tree cover in one study site. When trees are thinned grass biomass increases (given good rain). A high grass biomass also results in the absence of the cattle as herbivory is substantially reduced. Thus in a good rainy season there is a massive increase in grass and fuel loads. When the fire went through the system the result was a very intense fire with further tree death.


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