MEASURING AND MODELING WHOLE-TREE GAS-EXCHANGE

Diurnal patterns of CO2 and water vapour exchange were determined for Macadamia integrifolia and Litchi chinensis trees enclosed in whole-tr ee gas exchange chambers at Alstonville, New South Wales (28.5 degrees S) during October and November 1991. Whole-tree gas exchange response s to photon irradiance (I), ambient partial pressure of CO2 (C-a) and vapour pressure deficit (D) were similar to those normally observed fo r individual leaves. Nevertheless, at a given I (above approximately 5 00 mu mol quanta m(-2) s(-1)) stomatal conductances (g(s)) and CO2 ass imilation rates (A) were higher under overcast, as opposed to clear sk y conditions. This difference was maintained even when A and g(s) were examined as a function of sun angle. Combined with a simple light int erception model, nested quadratic equations combining stomatal respons es and biochemical characteristics of individual leaves were found to provide excellent descriptions of the gas exchange responses of the is olated trees. This indicates a close to optimal partitioning of photos ynthetic machinery throughout the canopy. From the whole-tree gas exch ange model it was found that higher A and g(s) under overcast conditio ns are attributable to both lower T-1 and D as well as to a more unifo rm distribution of irradiance across the canopy surface.