VEGETATION CHANGES ON FORMERLY OVERGRAZED HILL SLOPES IN SEMIARID CENTRAL TANZANIA

Composition of hill slope vegetation was studied in a semi-arid part o f upland Tanzania where all grazing had been banned for 12 yr. The hil ls had been severely overgrazed previously and suffered from heavy gul ly and sheet erosion. Eight vegetation types are described. Floristic gradients revealed by numerical ordination techniques were found to be related mainly to degree of erosion, soil type and succession. The mo re or less bare soil that prevailed after grazing had ceased is now co vered by grassland, woodland and immature secondary forest. The grassl ands are still characterized by early successional species and they wi ll probably remain open grassland as long as frequent burning continue s. Brachystegia woodlands may have developed during earlier periods wh en the field layer was sparse due to grazing. The grazing had reduced the frequency of fire which in turn promoted the establishment of Brac hystegia spp. Secondary forests are believed to have developed mainly where fires were not frequent, particularly at higher altitudes.