Zimbabwe: Landuse in Dry Tropical Savannas

Savanna vegetation

Vast areas of the tropics are classified as savannas. They can be defined as plant formations of tropical regions comprising a virtually continuous stratum of herbaceous plants, especially grasses and sedges, and a stratum of woody plants in varying densities.

Half of Africa is covered by savanna and it is by far the biggest ecozone in southern Africa taking in large parts of all countries except Lesotho. Savannas occur in areas with a dry season of at least five to seven months, resulting in a long period of water shortage. Fire and grazing are critical factor affecting savannas; without them, either grass or trees would dominate. Theories about why grasses and trees occur together in savannas are still developing. The usual explanation is that there is a surface layer of soil where grasses out-complete trees for most of the rain water and a lower layer where deep-rooted trees have exclusive access to the water. The amount of water getting to the sub-soil is enough to support the trees, but not enough to allow them to grow big and dense enough to shade out the grass.

Savannas are changeable and may not automatically return to a previous condition if left alone after a major impact, such as a overgrazing, fire or drought. Open grassland can change into woodland under conditions of decreased fire, or increased feeding or water availability. Woodland can change to grassland if trees are killed and fire, combined with browsing, slows the development of trees to the point where grasses dominate. This process may be part of long-term swing back and forth between the two conditions.

Trees can facilitate nutritions grass growth by providing nutrients and higher soil moisture. Their roots pull in nutrients and water from deep in the soil, much further out than the reach of their branches, and concentrate the nutrients in leaves which fall and fertilise the ground. When they die they also leave nutrient-rich patches. Recent studies show that the best production of nutritious grass occurs with a specific number of trees per hectare, varying from about 10-25 percent cover of trees.

Dry and moist savannas

African savannas have been divided into two types: dry and moist savanna. Dry savannas receive relatively low rainfall, over less than six months, but have higher levels of soil nutrients, while moist savannas are the reverse, though other combinations of nutrients levels and rainfall certainly occur.

Dry savannas are characterised by erratic rainfall, 200-600 mm/yr, generally fertile soils, due to less leaching, and sparse vegetation, mixed grass and trees

In the study area the predominant vegetation is dominated by colophospermum mopane (Photo needed).This species forms almost pure stands over vast areas of southern Africa centred on the great river valleys, such as the Limpopo and Zambezi. C. mopane loses its leaves during the dry season and is favoured browse species by livestock and wildlife. Most arid savanna trees are relatively fire resistant, however, C. mopane vegetation is highly flammable because of the abundance of volatile oils.