Growth responses of 15 rain-forest tree species to a light gradient: the relative importance of morphological and physiological traits

1. Growth of seedlings of 15 rain-forest tree species was compared under co ntrolled conditions, at six different light levels (3, 6, 12, 25, 50 and 10 0% daylight). 2. Most plant variables showed strong ontogenetic changes; they were highly dependent on the biomass of the plant. 3. Growth rate was highest at intermediate fight levels (25-50%) above whic h it declined. Most plant variables showed a curvilinear response to irradi ance, with the largest changes at the lowest Light levels. 4. There was a consistent ranking in growth between species; species that w ere fast growing in a low-light environment were also fast growing in a hig h-light environment. 5. At low light, interspecific variation in relative growth rate was determ ined mainly by differences in a morphological trait, the leaf area ratio (L AR), whereas at high light it was determined mainly by differences in a phy siological trait, the net assimilation rate (NAR). 6. NAR became a stronger determinant of growth than LAR in more than 10-15% daylight. As light availability in the forest is generally much lower than this threshold level, it follows that interspecific variation in growth in a forest environment is mainly owing to variation in morphology.