Successional Dynamics in an Acacia Nilotica-Euclea Divinorum Savannah in Southern Africa

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Abstract
(1) Three sites were chosen to examine the relationships in time and space between Acacia nilotica, a microphyllous, deciduous tree and Euclea divinorum, an evergreen shrub. The succession in dominance from Acacia nilotica to Euclea divinorum was a function of differences in the timing and distribution patterns of seedling establishment. (2) Establishment of Acacia nilotica seedlings was restricted to open areas with no woody canopy covers, whereas both the distribution of seed and seedling establishment of Euclea divinorum was limited to areas beneath the canopies of established Acacia individuals. (3) The spatial pattern of established Acacia nilotica was a function of within-species competition. Individuals were regularly dispersed, and there was a significant positive correlation between the nearest-neighbour distance and the combined size of the nearest-neighbour pair. (4) Euclea divinorum individuals were aggregated, with patches corresponding to the area over which Acacia nilotica projected canopy cover. However, there was a positive correlation between the nearest-neighbour distance and the combined size of the nearest-neighbour pair for within-species comparisons, suggesting the importance of competition (within-species) on the spacing pattern of Euclea within patches. (5) Euclea divinorum shrubs were found to be regularly dispersed within patches when the dispersion index (R) was recalculated with density based only on the area of Acacia nilotica canopy cover. (6) There was no significant correlation between nearest-neighbour distance and combined size for mixed-species nearest-neighbour pairs of Acacia nilotica and Euclea divinorum. (7) The role of disturbance in species replacement in time is discussed.

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