RADIATION BALANCE, TRANSPIRATION AND PHOTOSYNTHESIS OF AN ISOLATED TREE

The radiation balance of an isolated walnut tree was measured using an experimental Whirligig device. The total amount of all-wave radiation absorbed by the tree canopy was used to estimate transpiration rates using a Penman-Monteith model. The results compared favourably with tree water use, measured by the heat-pulse technique. The total amount of photosynthetically active radiation (PAR) absorbed by the tree canopy was combined with a photosynthetic light response curve to estimate net photosynthesis rates. The results compare favourably with published data from other tree canopies. Daily energy balance calculations showed that on average, about two-thirds of the total radiant energy absorbed by the tree canopy was dissipated as latent heat in the form of transpiration. The dominant environmental variable influencing transpiration was the vapour pressure deficit of the air. Almost two-thirds of the net latent heat flux was attributable to the vapour pressure deficit component, with the remainder owing to the radiation component. Daily transpiration-assimilation ratios varied from day to day in response to changing environmental conditions, but generally decreased with increasing net photosynthesis and with increasing transpiration. This appears to be the first time that such a direct measurement of the energy balance and photosynthesis of a single tree has been made.