Tropical tree seedling dynamics: recruitment patterns and their population consequences for three canopy species in Panama

Abstract

A study of seedling demography of three shade-tolerant canopy tree species (Quararibea asterolepis, Trichilia tuberculata, and Tetragastris panamensis) was initiated to integrate with long-term studies of tree fruit production and of tree population dynamics on Barro Colorado Island, Panama. Over a five-year period, all seedlings (height <50 cm) and small saplings (height ≥50 cm to dbh 1 cm) were measured and monitored in permanent tree-centred transects (N = 10–11 trees per species). Survival rates increased with plant size class and were similar among species. Maximum height growth rates increased with increasing plant size, but average growth rates did not; this disparity suggests the importance of release from understorey suppression for long-term recruitment success. Among the three species, Quararibea had the lowest standing seedling densities and almost no sapling recruitment, whereas Tetragastris had the highest densities of both seedlings and saplings; Trichilia seedling and sapling densities were intermediate. In all three species, a few trees produced very high seedling and sapling densities in comparison with the sample average. All three species exhibited a year of exceptionally high new seedling recruitment during the study period; these good years were not coincident among the species but instead reflected the species' phenological differences. Since seedling survival becomes relatively constant and high after the first few years of life (c. 80% y−1), such large new cohorts persist as a year-class effect in the seedling population and thus maintain seedling numbers over time. The interspecific differences in seedling and sapling dynamics were consistent with overall 10-year trends of a declining Quararibea population, a stable Trichilia population, and an increasing Tetragastris population.