Factors affecting post-dispersal seed and seedling survival of common tree species in the Amazon Forest / Renato Cintra.

Tese (Ph.D.)- Duke University, Durham, 1994

Most plants colonize new or disturbed habitats through succesful seed dispersal and seedling establishment. A throrough grasp of these central processes, especially their spatial components, is crucial to understand the distribution and abundance of a plant species. In a three-year study of two common Peruvian rain forest trees, the palm Astrocaryum murumuru (Mart) and the legume Dipteryx micrantha, I investigated how physical and biological factors act at different spatial scales to affect post-dispersal seed and seedling survival. I simulated seed and seedling dispersal using experimental seedas and seedlings in the field. Seed and seedling perfomance and survival were followed from 1991 until 1993 and analyzed using standar statistical methods of survival analysis. In the first chapter, I investigate whether 'safe sites' exist for A. murumuru or D. micrantha seeds or seedling and what factors determine these sites. At a small spatial scale (1 m² plots), leaf litter quantity and light penetration affected seed survival. The forest floor provided numerous safe sites for Astrocaryum and few for Dipteryx. Diptery survival was lowest in sites with the highest surrounding plant density ( all plants),suggesting sensitivy to light or nutrient competition. One important source of environmental heterogeneity in a tropical forest is leaf fall; its depth and quality may vary widely over short distances. ?Chapter two describes the effects of natural and experimentally modified leaf litter depth on seeds and seedlings. Seed survival of both species was found to be enhanced by the preence and thickness of leaf litter. Chapter three focuses on how seedling survival is affected by distance to the parent tree and the local density of conspecific seedlings. This expriment provides a test of the still controversial Janzen-Connel model (Janzen 1970, Connel 1971) for these two species. In general my results support the model. Astrocaryum seed and seedling survival were respectively density and distance-dependent in relation to adult trees. Dipteryx seed and seedling survival showed distance-dependence only. Finally, chapter four examines the importance of tree-fall gaps to the success of Astrocaryum and Dipteryx seeds and seedlings. I compared the performance of seeds and seedlings placed in gaps and in closed forest to test the colonization hypothesis of Howe and Smallwood (1982). Both species showed increased seed survival in tree-fall gaps. In conclusion, seed dispersal is advantageous for these two tree populations. Patterns of seed and seedling survival were complex and dependent on multiple factors such as offspring age (seed or seedling), spatial scale, qualities of microsites, type of habitat, distance from a conspecific adult, presence of gap, type of predator and year considered.