Spatial and temporal distribution of photosynthesis and transpiration by single leaves in a rainforest tree, Argyrodendron peralatum

The light environments and rates of photosynthesis by leaves of Argyrodendron peralatum were examined at 8 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 decresaed from c50% 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 CO ms, whilst leaves in the lower crown of the same tree, and in understory saplings had maximum photosynthesis rates of 8.2 μmol ms. 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 ms, irrespective of level in the canopy. -Author