Photosynthetic capacity in a central Amazonian rain forest

The vertical profile in leaf photosynthetic capacity was investigated in a terra firme rain forest in central Amazonia. Measurements of photosynthesis were made on leaves at five levels in the canopy, and a model was fitted t o describe photosynthetic capacity for each level. In addition, vertical pr ofiles of photosynthetic photon flux density, leaf nitrogen concentration a nd specific leaf area were measured. The derived parameters for maximum rat e of electron transport (J(max)) and maximum rate of carboxylation by Rubis co (V-cmax) increased significantly with canopy height (P < 0.05). The high est J(max) for a single canopy level was measured at the penultimate canopy level (20 m) and was 103.9 mu mol m(-2) s(-1) +/- 24.2 (SE). The highest V -cmax per canopy height was recorded at the top canopy level (24 m) and was 42.8 +/- 5.9 mu mol m(-2) s(-1). Values of J(max) and V-cmax at ground lev el were 35.8 +/- 3.3 and 20.5 +/- 1.3 mu mol m(-2) s(-1), respectively. The increase in photosynthetic capacity with increasing canopy height was stro ngly correlated with leaf nitrogen concentration when examined on a leaf ar ea basis, but was only weakly correlated on a mass basis. The correlation o n an area basis can be largely explained by the concomitant decrease in spe cific leaf area with increasing height. Apparent daytime leaf respiration, on an area basis, also increased significantly with canopy height (P < 0.05 ). We conclude that canopy photosynthetic capacity can be represented as an average vertical profile, perturbations of which may be explained by varia tions in the environmental variables driving photosynthesis.