Ecology of a Miombo Site, Lupa North Forest Reserve, Tanzania: III. Effects on the Vegetation of Local Cultivation Practices

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Abstract
The normal practice of shifting cultivation in the Tanzania miombo consists of cutting the woody vegetation at or above ground level, burning the debris and cultivating with the hand hoe for some 3-5 years until the extra nutrients from the burned vegetation are used up and crop yields begin to fall. Cultivation of this type destroys some but not all of the very large number of suffrutices present in mature miombo (numbers before cultivation in the 2 areas studied were of the order of 10,000/ha comprising some 90% of the total number of plants of tree species). When cultivation is abondoned the remaining suffrutices rapidly grow up, eventually forming local thickets, but in the 1st few years grass growth is stronger than before cultivation and fires are consequently fiercer. The changed conditions of the 1st year of re-growth enable some pioneer species to become more numerous and to grow more rapidly than before cultivation. After some 10-20 years competition between the woody plants becomes fiercer and the canopy and underwood species typical of mature miombo begin to re-assert their dominance. The behavior of the pioneers varies at this stage. Some such as Terminalia sericea disappear almost entirely, others become less numerous than before but remain in special sites often in 1 of the lower layers of the vegetation (e.g. the shrub Hymenocardia acida), while others, e.g. Pterocarpus angolensis, continue to com- pete with the canopy dominants. As regrowth proceeds the species restricted to the shrub and underwood vegetation layers of mature miombo cease height growth before the canopy species. Canopy species continue to increase in size for up to about 100 years which seems to be the maximum life span of most miombo trees, so that a miombo stand can be described as mature when 100 years have elapsed since cultivation was abondoned. Mature miombo is characterized by a basal area of the woody plants at 130 cm from the ground of the order of 14 m2/ha. Values from 6 sample sites throughout Tanzania had a mean of 13.8[plus or minus]0.4 m2/ha. The annual dry matter production of mature miombo is probably of the order of 1000 kg/ha above ground.

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