The reduction in canopy cover of the Seronera woodlands since the mid- 1960s can be largely attributed to the destruction of mature Acacia tortilis trees by elephants. The development of the tree regeneration that has colonized the gaps in the mature canopy is being suppressed by giraffe browsing and periodic burning. A simple model is presented which simulates these impacts upon the dynamics of the A. tortilis population. Height-specific impact rates of these three agents are quantified. Between 1968 and 1977, mature trees were lost at a mean annual rate of some 6%, although this figure showed considerable annual variation (13.5-2%). Combinations of impact rates, simulating possible natural developments within the Serengeti woodlands, are programmed into the model to assess their effects upon the tree population structure. The results suggest that measures to promote regeneration development (fire protection and/or giraffe culling) are more effective in the long-term to encourage mature canopy recovery than are measures to reduce mature tree mortality (elephant culling). The management implications of these results are considered, and their extrapolation to other wildlife conservation areas experiencing similar declines in mature canopy woodland is discussed. It is suggested that the effects of the combination of low elephant densities and high giraffe densities prevalent in the Serengeti produce a dynamic system in which the woodland structure oscillates between mature canopy and open regeneration-grassland phases. The woodland structure desired by Park management to ensure continuity of mature tree canopy represents an unstable transitional stage between these two phases.
