Light gradient partitioning among tropical tree species through differential seedling mortality and growth

To characterize species differences in light-dependent mortality and growth, I transplanted seedlings of four related tree species into a range of light environments (<1% to 85% full sun) on alluvial soils at La Selva Biological Station, Costa Rica. With maximum likelihood techniques, I fit species-specific, nonlinear regression models of seedling probability of mortality and radial growth as functions of light availability for Trophis racemosa, Castilla elastica, Pourouma aspera, and Cecropia obtusifolia (Family Moraceae, latter two also classified as Cecropiaceae). Models characterizing mortality in the first year of seedling life showed significant differences among the species, both in functional forms and in model parameter estimates. All species decreased in mortality with increases in light to 20% full sun. Above 20% full sun, the mortality of T. racemosa and Castilla elastica continued to decrease, while that of Cecropia obtusifolia remained constant and that of P. aspera increased. At <10% full sun, there was a fivefold difference in mortality probability; T. racemosa had the highest survivorship, followed by C. elastica, P. aspera, and C. obtusifolia. Mortality models for the initial 4 mo of the experiment revealed significant size effects for all species except P. aspera and increased mortality under high light for P. aspera and T. racemosa. In contrast, radial growth of all species increased with light availability. The radial growth models, based upon only variation in light availability and tree size, accounted for 60-86% of the variation in growth. Cecropia obtusifolia occupied the lowest rank in radial growth at <50% full sun and the highest radial growth at light levels >75% full sun. When considered together, the mortality and growth models revealed that differences in species performance could lead to light gradient partitioning. At distinct light levels, each species showed the highest growth relative to other surviving species. Neither growth nor mortality considered alone revealed this specialization. Light gradient partitioning provides an explanation for successional dynamics and suggests that tropical tree species coexistence and diversity is at least partly maintained through niche differentiation.