Coexistence and bush encroachment in African savannas: the role of the regeneration niche.
Globally there is increasing concern over the biodiversity and economic losses associated with bush encroachment. The dominant species responsible for bush encroachment in Africa belong to the mimosoid genera Senegalia and Vachellia, with previous research identifying Vachellia as the more aggressive invader. We asked whether these genera have evolved different traits to enable germination and establishment among competitively superior grasses. We hypothesised that these traits enable coexistence at local scales and underpin differences in invasiveness. We conducted a large greenhouse pot experiment to quantify functional trait differences between the two genera during seedling establishment both in the presence and absence of grasses. We also examined differences in seed morphology and tested whether co-occurrence of the two genera at the plot scale is greater than expected by chance. Our results showed that Vachellia grew faster and taller than Senegalia in the absence of competition from grasses. In the presence of grasses, however, Senegalia increased root tissue density while Vachellia did not. The seed coats of Vachellia species were thicker than those of Senegalia, and their seeds were spherical while those of Senegalia were discoid. The two genera coexist at a local (plot) scale. We speculated that the thicker seed coat and spherical seed shape of Vachellia reflect its primarily endozoochorous dispersal mode, while the thinner seed coat and more discoid seeds of Senegalia reflect its use of wind dispersal. Since animal dispersal is directed, promoting the movement of seed into sites frequented by animals, and in which competition from grasses and fire has been removed, animal dispersal may contribute to the greater invasiveness of Vachellia. Once established in such sites, coexistence at the local scale is facilitated by differences in root tissue density with low-density root tissue allowing for rapid resource acquisition and growth in Vachellia, and high-density root tissue enabling slower but more persistent growth in Senegalia. Where previous research has focussed on environmental filtering, competition, predation or facilitation as explanations of plot-scale phylogenetic overdispersion, our work suggests a potential role for regeneration niche in structuring local co-occurrence.