Tree Recruitment in the Serengeti Ecosystem: The Role of Resource and Disturbances

Abstract

Tree recruitment involves successful seed germination and establishment as seedlings that grow to become adult individuals. For decades, savanna ecologists have worked diligently to identify mechanisms that explain tree recruitment bottlenecks. Recruitment limitations are driven mainly by bottom-up (rainfall and nutrients) and top-down disturbances (herbivory and fire). However, the focus has been on how bottom-up and top-down drivers shape the tree – grass ratio, tree seedling transition to the adult tree stage, and the savanna woody cover. There is limited knowledge on the role of resources and disturbances on other parts of tree life history, such as seed production and the traits that contribute to the growth and survival of tree seedlings. For example, once seeds are mature on the parent tree, they can reach potential germination sites via various dispersal pathways. However, it is unclear whether the seed germination potential varies with seed pathways such as primary dispersal, seedbank, and endozoochory. The transition from a germinating seed to an established seedling is a challenging life-history stage because of high seedling mortality caused by competition, resource availability, and disturbances. However, it is unclear what traits contribute to their growth and survival under various levels of resources, disturbances, and competition.I conducted observational and experimental research utilizing the Serengeti National Park in Tanzania as a study ecosystem and to understand how resource variability influences pre and post dispersal processes that play a significant role in dominant tree species recruitment, such as fecundity (chapter 1), germination potential (chapter 2), seedling growth (chapter 3) and survival (chapter 4). The results suggest that the role of resources and disturbance on seed production (fecundity), growth, and survival is not uniform among tree species. Seed germination depended on scarification status and varied by dispersal pathway.