Beyond the regeneration phase: differentiation of height–light trajectories among tropical tree species

Abstract

Summary A height–light trajectory (HLT, a fitted curve relating canopy exposure to tree height) was determined for populations of individuals of each of 47 tree species in a Liberian lowland rainforest. The HLTs were compared and related to tree allometry and adult stature. Crown exposure was measured for 7460 trees and related to tree height using a multinomial regression analysis. Individual trees were followed for 2.8–9.8 years. The trajectories of the 47 species were compared with the average vertical light profile in the forest canopy. Evidence was found for the existence of all nine trajectories hypothesized on the basis of three possible light environments (high, intermediate and low) for juveniles and adults. The classical paradigm of pioneer vs. shade tolerant, based on seed and seedling responses, does not therefore apply to post‐seedling stages. The majority of the species followed the vertical light profile in the forest canopy, starting in low light environments in the juvenile stage and ending up in high light environments in the adult stage. Only two species complied with the classic notion of whole‐life shade tolerants and whole‐life shade intolerants (one each). The predictable vertical light gradient in the forest canopy has led to a close association between adult height, light trajectories and allometric traits. Large‐stature species tend to have relatively slender stems and narrow crowns, and therefore realize a faster temporal and height‐related increase in crown exposure. Tree species have different height–light trajectories when they grow from seedling to adulthood. This may have profound repercussions for our current views on plasticity and adaptation, light partitioning and species coexistence, and on silviculture and management.