The influence of penumbra on canopy photosynthesis II. Canopy of horizontal circular leaves

Abstract

Rate of gross photosynthesis is calculated for canopies of randomly dispersed circular horizontal leaves irradiated by the sun's disk of finite angular radius, for one particular light response of photosynthesis. The theory developed in an earlier paper is used to take account of irradiance distribution in penumbral regions of canopies. Calculations are performed first for zenith sun and then for other solar elevation angles. A parameter, called characteristic leaf area index ( L r), occurs naturally in the theoretical development; L r incorporates leaf size and leaf number density. Probability of penumbra at a given depth, depth distribution of photosynthesis rate and hence canopy photosynthesis rate are found to depend on L r as well as on canopy leaf area index (LAI) and the light scattering coefficient of the leaves. Potential application of the results to problems involving different vertical distributions of foliage is illustrated. Rates thus calculated are compared with rates calculated from parallel solar beam geometry. The discrepancy between the results from the two irradiation geometries, computed from the present theory, is compared with an estimate in the literature based on irradiance measurements under a sunflower canopy of LAI = 1.88. The values compare well for mean photosynthesis rate per unit leaf area at that particular level, but it is shown that the discrepancy between canopy photosynthesis rates calculated for the two irradiation conditions is much smaller than that local rate.