Woody dominance in a semi-arid savanna rangeland – Evidence for competitive self-thinning

Self-thinning has been considered an important mechanism facilitating species coexistence in nature. The purpose of this study was to examine the role of self-thinning in regulating the density of woody plants in semi-arid savannas. Previous models (e.g. the ‘honeycomb rippling model’) have postulated that, as trees grow older (inferred by size), the interplant distance, taken to reflect competition-induced mortality, becomes greater and more even. Therefore, the opening of canopies in the long-run promotes species coexistence. To study the role of self-thinning in a semi-arid savanna we selected four savanna patches and sampled the plant density, size, and distance between the tallest woody plant and its nearest conspecific and heterospecific neighbors, along with other environmental variables. Interplant distance was positively influenced by species, tree size, woody species diversity, and nearest neighbor type (conspecific vs. heterospecific). Distances did not increase evenly with increasing size of the tallest tree or shrub, however. We conclude that self-thinning is evident in woody encroached semi-arid savanna patches, as suggested by the honeycomb rippling model. The mechanism may play a significant role for the long-term stability of the savanna by opening dense canopies and facilitating tree–grass coexistence.