Tropical tree rings reveal preferential survival of fast-growing juveniles and increased juvenile growth rates over time

Long-term juvenile growth patterns of tropical trees were studied to test two hypotheses: fast-growing juvenile trees have a higher chance of reaching the canopy ('juvenile selection effect'); and tree growth has increased over time ('historical growth increase'). Tree-ring analysis was applied to test these hypotheses for five tree species from three moist forest sites in Bolivia, using samples from 459 individuals. Basal area increment was calculated from ring widths, for trees < 30 cm in diameter. For three out of five species, a juvenile selection effect was found in rings formed by small juveniles. Thus, extant adult trees in these species have had higher juvenile growth rates than extant juvenile trees. By contrast, rings formed by somewhat larger juveniles in four species showed the opposite pattern: a historical growth increase. For most size classes of > 10 cm diameter none of the patterns was found. Fast juvenile growth may be essential to enable tropical trees to reach the forest canopy, especially for small juvenile trees in the dark forest understorey. The historical growth increase requires cautious interpretation, but may be partially attributable to CO2 fertilization.