Diurnal and seasonal comparisons of assimilation, phyllode conductance and water potential, of three Acacia and one Eucalyptus species in the wet-dry tropics of Australia

Four species of tropical tree (Acacia auriculiformis Cunn. ex Benth., A. mangium Willd., A. crassicarpa Cunn. ex Benth. and Eucalyptus pellita F.Muell.) were studied at a site on Melville Island, off the north coast of the Northern Territory of Australia, in the wet-dry tropics. Rates of light-saturated assimilation were measured every 2 months, in the morning and afternoon, concurrently with g(s) (stomatal conductance) and microclimate (air temperature, relative humidity and photosynthetic photon flux density). Phyllodes were also sampled far subsequent nitrogen determination. Pre-dawn and diurnal phyllode water potentials were measured at the end of the wet and dry seasons. Tree height and canopy area were recorded at the end of 50 months of growth. Assimilation was found to decline substantially in the afternoon compared with the morning in the dry season but not the wet season. This was not due to diurnal declines in phyllode water potential but was attributed partially to decreased g(s) resulting from increased leaf-to-air vapour pressure difference. However, an interpretation of the C-i/C-a (ratio of internal to ambient CO2 concentrations) data suggested that additional factors, other than g(s), may be involved in causing the afternoon decline in assimilation rate. There was a linear relationship between pre-dawn water potential and A(max) (maximum assimilation) and an inverse relationship between A(max) and tree height, a result attributed to differences between species in allocation of carbon within the tree.