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 lig ht environments (<1% to 85% full sun) on alluvial soils at La Selva Biologi cal 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 Morace ae, latter two also classified as Cecropiaceae). Models characterizing mortality in the first year of seedling life showed s ignificant differences among the species, both in functional forms and in m odel 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 obtusi folia remained constant and that of P. aspera increased. At <10% full sun, there was a fivefold difference in mortality probability; T. racemosa had t he highest survivorship, followed by C. elastica, P. aspera, and C. obtusif olia. Mortality models for the initial 4 mo of the experiment revealed sign ificant size effects for all species except P. aspera and increased mortali ty 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 o btusifolia occupied the lowest rank in radial growth at <50% full sun and t he highest radial growth at light levels >75% full sun. When considered together, the mortality and growth models revealed that dif ferences in species performance could lead to light gradient partitioning. At distinct light levels, each species showed the highest growth relative t o other surviving species. Neither growth nor mortality considered alone re vealed this specialization. Light gradient partitioning provides an explana tion for successional dynamics and suggests that tropical tree species coex istence and diversity is at least partly maintained through niche different iation.