MODELING EFFECTIVE STOMATAL-RESISTANCE FOR CALCULATING TRANSPIRATION FROM AN APPLE TREE

We present an improved method for calculating transpiration from an ap ple tree using the Penman-Monteith equation, based on an improved calc ulation of the effective stomatal resistance for the whole tree. The c alculation has two parts: (1) calculating the distribution of radiatio n over the leaf area of the tree so the distribution of stomatal condu ctance over the leaf area can be estimated; (2) weighting these values according to the estimated distribution of net radiation over the tre e leaf area so the effective stomatal resistance can be calculated. Th e weighting procedure is based on a continuous form of the expression for the effective stomatal resistance of a heterogeneous collection of leaf surfaces, described by McNaughton (1994, Plant Cell Environ., 17 : 1061-1068). Methods are developed to parameterize the photosynthetic photon flux density (PPFD) and net radiation distributions over the t ree's leaf area using the available measurements from PPFD, solar shor twave and net radiation instruments mounted above the tree canopy, and from our 'Whirligig' radiometer which measures directly the total pho tosynthetic photon flux (PPF) and total net radiation absorbed by the tree. The calculations assume that the distribution of leaf orientatio n is spherical. These results were tested in two ways. First, the tota ls of PPF and net all-wave radiation absorbed by the tree, as modelled from our distributions, fitted the measured values from the Whirligig very well over a wide range of sunny and overcast conditions. Second, over the same wide range of conditions, the transpiration rates calcu lated from the Penman-Monteith equation agreed very closely with indep endent measurements of transpiration made by heat-pulse equipment inse rted in the trunk of the tree-this was a substantial improvement over our earlier results.