GETTING TO THE CANOPY: TREE HEIGHT GROWTH IN A NEOTROPICAL RAIN FOREST

Abstract

There is still limited understanding of the processes underlying forest dy- namics in the world's tropical rain forests, ecosystems of disproportionate importance in terms of global biogeochemistry and biodiversity. Particularly poorly documented are the nature and time scale of upward height growth during regeneration by the tree species in these communities. In this study, we assessed long-term height growth through ontogeny for a diverse group of canopy and emergent tree species in a lowland neotropical rain forest (the La Selva Biological Station, northeastern Costa Rica). Species were evaluated based on annual height measurements of large samples of individuals in all postseedling size classes, over a 16-yr period ( .11 000 increments). The study species were seven nonpi- oneers (Minquartia guianensis, Lecythis ampla, Hymenolobium mesoamericanum, Sima- rouba amara, Dipteryx panamensis, Balizia elegans,and Hyeronima alchorneoides ) and two pioneers (Cecropia obtusifoliaand Cecropia insignis). For each species, inherent height growth capacity was estimated as the mean of the five largest annual height increments (from different individuals) in each juvenile size class (from 50 cm tall to 20 cm in diameter). All species showed marked ontogenetic increases in this measure of height growth potential. At all sizes, there were highly significant differences among species in height growth potential. The two Cecropia species consistently showed the highest observed maximum height increments as smaller juveniles (#4 cm in diameter). Among the nonpioneers, Sim- arouba had the highest growth potential across all juvenile sizes. For all species, mean annual height increments in all juvenile size classes were very much lower than the species' potential growth rates and reflected the impacts of frequent periods of zero growth and major height losses from physical damage. Because of the same factors, maximum net height increments declined over increasing measurement intervals (1-15-yr periods). With only one exception (Simarouba amara saplings #1 cm in diameter that survived 10 yr), the annual height growth increments of these species showed no significant temporal au- tocorrelation. For the seven nonpioneer species, we estimated the minimum time required to grow from 50 cm tall saplings to 10 cm diameter trees, based on their greatest net height increments over increasing intervals. Estimated passage times increased from 7-33 yr, when based on maximum 5-yr height increments, to 37-83 yr, when based on maximum net height growth over 15 yr for the six longest studied (nonpioneer) species. Given the erratic height growth trajectories found for these juvenile trees in this 16-yr study, it is clear that much longer term observation is required. Still unknown are the actual growth trajectories that characterize successful regeneration in each of our focal species, how much time successful trees spend as suppressed juveniles, and the number of times they experience and recover from major physical damage.