Spatial and Temporal Distribution of Photosynthesis and Transpiration by Single Leaves in a Rainforest Tree, Argyrodendron peralatum

Abstract

The light environments and rates of photosynthesis by leaves of Argyrodendron peralatum (F. M. Bailey) I.H. Boas were examined at eight positions within a natural rainforest canopy. Daily integrals of irradiance at leaf surfaces expressed as a percentage of that reaching a horizontal sensor in the open decreased from about 50% in the uppermost leaves to about 20% at a point 2 m below the top of the crown and to 10-15% in the lower crown and understory. Leaves in the upper crown had maximum instantaneous rates of apparent photosynthesis of 11.5 µmol CO₂ m^-2 s^-1, whilst leaves in the lower crown of the same tree, and in understory saplings had maximum photosynthesis rates of 8.2 µmol m^-2 s^-1. These rates were not significantly different, and are within the range observed in a number of rainforest species characteristic of later successional stages. Under conditions of adequate water supply, photosynthetic rate was light-saturated by 500 µmol quanta m^-2 s^-1, irrespective of level in the canopy.