Grass competition overwhelms effects of herbivores and precipitation on early tree establishment in Serengeti

1. Savanna ecosystems span a diverse range of climates, edaphic conditions and disturbance regimes, the complexity of which has stimulated long-standing interest in the mechanisms that maintain tree-grass coexistence. One hypothesis suggests that tree establishment is strongly limited by one or several demographic bottlenecks at early stages of the tree life cycle. A major impediment to testing this hypothesis is the lack of data on the relative strengths of different bottlenecks across key environmental gradients.
2. To identify demographic bottlenecks that limit early tree establishment (0-18 months), we conducted a series of transplant experiments with two savanna trees species (Acacia robusta and A. tortilis) across a natural rainfall and soil fertility gradient in the Serengeti ecosystem, Tanzania. We tested the interactive effects of precipitation, herbivory, seed scarification, grass competition, water limitation and tree species identity on two key life stages: germination and ...