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

Growth of seedlings of 15 rain‐forest tree species was compared under controlled conditions, at six different light levels (3, 6, 12, 25, 50 and 100% 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 light levels (25–50%) above which it declined. Most plant variables showed a curvilinear response to irradiance, with the largest changes at the lowest light levels. 4. There was a consistent ranking in growth between species; species that were fast growing in a low‐light environment were also fast growing in a high‐light environment. 5. At low light, interspecific variation in relative growth rate was determined mainly by differences in a morphological trait, the leaf area ratio (LAR), whereas at high light it was determined mainly by differences in a physiological 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.