The proposed research project explores relationships between tree species size, measured as the height at onset of reproduction, and leaf-level photosynthetic light response. A positive relationship between adult tree size and maximal light-saturated photosynthetic rate (Pmax) among primary rainforest trees is predicted. This prediction derives from a general correlation across environments between light levels and maximum rate of photosynthesis, and the presence of a strong, predictable light gradient through the rainforest canopy. Recent studies of physiological acclimation to sun and shade conditions suggest that photosynthetic acclimation is relatively limited in primary tropical rainforest trees. If this is so, Pmax values measured for saplings should be similar to Pmax values for adult trees, and the proposed allometric relationship should be detectable from photosynthetic measurements on saplings. Measurement will be made of maximal light-saturated photosynthetic rate (Pmax) for saplings of 35 species for which size and structural allometric data have been gathered. Differences in Pmax between saplings and mature trees of selected species will be assessed. Each genus included in the study spans a wide size range, from understory treelets to canopy-level trees, making possible tests of phylogenetic differences. This study represents the first attempt to examine photosynthetic physiology of tropical rainforest trees in an allometric context.
