The Effect of Competition on the Structure and Dynamics of Acacia Savannas in Southern Africa

Abstract

(1) The effect of competition on the structure and dynamics of Acacia nilotica and A. tortilis communities was examined, using nearest-neighbour analysis. There was a significant positive correlation between nearest-neighbour distance and combined canopy cover for both withinand between-species comparisons, illustrating the importance of competition in the spacing of trees. (2) To verify the findings of the nearest-neighbour analysis experimentally, the response to removal of neighbouring trees was examined for A. nilotica. Those trees whose neighbours within a 5 m radius were removed showed a significant increase in both stem diameter increment and shoot extension when compared with control trees. (3) Seedling establishment relative to canopy cover was examined in both A. nilotica and A. tortilis communities. Two 'types' of species were apparent: (i) seedling establishment associated with under-canopy environments, and (ii) seedling establishment restricted to open or between-canopy environments. (4) To examine the relationship between the spatial pattern of trees, soil moisture and nutrient availability, the slope of the regression between nearest-neighbour distance and combined canopy cover of the nearest-neighbour pair was compared with both available water capacity and total exchangeable bases for nine A. tortilis sites. There was a significant positive relationship between slope of the nearest-neighbour regression and both available water capacity and total exchangeable bases.