Spatial pattern analysis and competition between Acacia karroo trees in humid savannas

Abstract

It is unclear whether inter-tree competition plays a role in determining the woody plant cover of humid savannas. Spatial point-pattern analysis can give insights to the underlying processes affecting the individuals in a population. We assessed the spatial patterns of Acacia karroo trees from savannas in KwaZulu-Natal, using conventional nearest neighbour analysis and second-order spatial statistics such as Ripley’s K- and L-functions, and the univariate and bivariate O-ring statistics. We predicted that juvenile trees would be spatially aggregated, because of facilitation between shrubs when zones of overlap are relatively small, while adult trees would be regularly spaced because of the effects of density-dependent mortality (i.e. consistent with the honeycomb rippling model). We found that juvenile trees were more aggregated than expected by chance, and the overall spatial distribution of all trees was also found to be aggregated, with no evidence of regularity among large individuals. Nearest neighbour analysis, however, revealed significant positive correlations between the sum of the distances to the four nearest neighbours and the sum of the canopy diameters of the target tree and its four nearest neighbours, indicating the presence of competition. In sum, these findings suggest that competitive interactions between A. karroo trees at these sites are relatively weak, and result in decreased performance (smaller canopy diameters) rather than mortality, thus preventing a regular pattern of tree distribution. We advocate the use of both methods of detecting competitive interactions in the field, especially if the effects of competition are too subtle to result in differential mortality.