Tree and grass competition in savannas: Interactive effects of soil moisture, grass defoliation and grass species identity on the growth and survival of tree seedlings

We experimentally examined the influence of grass competition, grass species identity (taxa) and water availability on the seedling growth and survival of two dominant tree species (Vachellia (formerly Acacia) robusta and V. tortilis) of the Serengeti National Park in Tanzania. Despite being widely distributed, the species have an opposing overstorey dominance across a rainfall and productivity gradient, with V. robusta dominating the more productive mesic sites and V. tortilis in the lower productivity, drier sites. We investigated the role of different grass species, which vary in distribution and abundance across the rainfall gradient, in influencing the growth and survival of Vachellia seedlings. We found a significant effect of grass competition but no effect of grass species identity on the growth or survival of seedlings. Seedling survival was highest in the absence of grass competition, intermediate when grasses were defoliated to simulate grazing and lowest when grown with ungrazed grasses. Grass competition had a more negative effect on the stem diameter of V. tortilis than V. robusta. All V. tortilis seedlings grown under a combination of drought conditions and unclipped grasses died by the end of the experiment. However, reduced grass competition by simulated grazing improved V. tortilis seedling survival to comparable levels achieved by V. robusta species. Our study advances our understanding of tree and grass competition across environmental gradients and suggests that the presence of grass and soil moisture have species-specific effects on tree seedling growth and survival in African savannas.