Structure and spatial interactions of savanna trees along a local stress gradient in a semi-arid environment

Competition and facilitation vary inversely along environmental stress gradients. Here we ask how environmental stress along a slope gradient influences plant-plant interaction in a semi-arid savanna. We attempted to link spatial patterns and ecological interactions, so we analysed the spatial distribution of three Vachellia (Acacia) tree species and assessed the consequences of their interaction on plant structural variables (height, basal area, biomass and canopy cover) at Matopos Research Station, Zimbabwe. Spatial patterns among and between the tree species were assessed using both univariate and bivariate analysis. To understand the extent of tree species isolation, we applied the species mingling index. Canopy cover varied significantly between species at both ends of the stress gradient, while basal area did not differ among the three species at any slope position. The woodland showed differential recruitment levels for the three species across the catena, which may indicate future changes in composition. The mingling index showed that there was species isolation in the bottom and middle catena. Pair-wise interactions of plants reflected two common spatial patterns, clustering, interpreted as suggesting facilitation, and random, interpreted as neutral. In some cases these patterns changed with spatial scale, indicating that the stress gradient hypothesis (SGH) may be influenced by the spatial scale at which species are interacting. Our univariate analyses corroborate the evidence that plants in semi-arid environments have a clustered distribution and that catenas can produce differential recruitment levels of the different species.