Mechanisms of coexistence among neotropical pioneer tree species
The coexistence of plant species in highly diverse communities is one of the enduring questions in ecology. The Hutchinsonian niche is defined as an n-dimensional hypervolume incorporating every aspect of the life history of a plant but in practice very few of those aspects have been examined in detail for a suite of coexisting tropical forest plants. In this thesis I examine the germination and seedling growth and mortality responses of up to 20 species of neotropical pioneers in relation to abiotic arid biotic factors in their environment. Variation in the germination response to the environmental indicators of the presence of a gap was strongly related to seed mass and the environmental constraints seed mass places on regeneration. Smaller seeded species required irradiance to stimulate germination and emergence occurs from the superficial depths of soil that receive irradiance. Both within the small-seeded group of species and in comparison with the larger seeded species, seed mass was positively related to the degree of discrimination in requirements to stimulate germination. Seed mass was related to the amount of resources and the amount of physical defences, both of which enhance seed longevity. Larger seeded species (seed mass > 1.5 mg) were able to persist in the seed bank and emerge from lower in the soil profile. Some large-seeded species possessed a positive germination response to increasing magnitude of diel temperature fluctuation, and measurements suggested that temperature fluctuations penetrated lower in the soil profile than irradiance. The seedling growth and mortality of three species in relation to gap size predicted the known differences in adult distribution for these species at my study site. Differences between species in response to gap size were enhanced but not created by herbivory. Trade-offs between rate of growth at high irradiance and growth and survival rates in low irradiance, and between growth at high irradiance and growth when water and light were simultaneously limiting contributed to the contrasts in response to gap size among six species. I found no evidence of maintenance of the rank of species' growth rates across irradiance treatments or for a trade-off between maximum rate of growth and susceptibility to herbivory. These results illustrate contrasting responses to environmental heterogeneity within the pioneer functional group at my study site. These differences represent a potential mechanism for coexistence of ecologically similar species at one site. I conclude that niche differentiation contributes to the maintenance of tropical forest diversity.