Factors affecting post-dispersal seed and seedling survival of common tree species in the amazon forest

Abstract

Most plants colonize new or disturbed habitats through succesful seed dispersai and seedling establishment. A thorough 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 dispersai using experimental seeds and seedlings in the field. Seed and seedling performance and survival were followed from 1991 until 1993 and analyzed using standard statistical methods of survival analysis. In the first chapter, I investigate whether 'safe sites' exist for A. murumuru or D. micrantha seeds or seedlin_gs and what factors determine these sites. At a small spatial scale (1 m2 plots), leaf litter quantity and light penetration affected seed survival. Toe forest floor provided numerous safe sites for Astrocaryum and few for Dipteryx. Dipteryx survival was lôwest insites 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 f all; its depth and quality may vary widely over short distances. Chapter two describes the effects of natural and experimentally mod.ified leaf litter depth on seeds and seedlings. Seed survival of both species was found to be enhanced by the presence and thickness ofleaf litter. Chapter three focuses on how seedling survival is affected by distance to the parent tree and the local density of conspecific seedlings. This experiment provides a test of the still controversial Janzen-Connel m del (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(l 982). Both species showed increased seed survival in tree-fall gaps. In conclusion, seed dispersai 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.