Cross-validating sun-shade and 3D models of light absorption by a tree-crop canopy

International audience; Sun-shade models proved to be simple, fast and reliable tools for estimating the fraction of absorbed PAR (fAPAR) and the photosynthesis of low and simple canopies (e.g. wheat). Applications on tall canopies, or non-ideal canopies (e.g. row-planting, azimuthal heterogeneity, clumping, non-spherical leaf angle distribution, large proportion of non-green elements) were limited so far. We attempted to apply the Sun-shade model proposed by de Pury and Farquhar [de Pury, D.G.G., Farquhar, G.D., 1997. Simple scaling of photosynthesis from leaves to canopies without the errors of big-leaf models. Plant Cell Environ. 20, 537–557] at the scale of the non-ideal canopy of adult coconut palms. The gap fractions of the whole cover, fraction of intercepted PAR (fIPAR), clumping index and leaf orientation derived from LAI-2000 (PCA) were closely matching the simulations of a reference 3D architectural model (3DM) used for cross-validation. Moreover, 3DM supplied a relationship for down-scaling the gap-fractions from the whole cover to green elements of the canopy. The derived Sun-shade simulations of fAPAR by green leaves agreed within 5% with 3DM, on a half-hourly time-step and for 1 year, confirming combined PCA and Sun-shade methods as a fast and reliable tool, even for tall or complex canopies. fIPAR and plant area index (PAI) were compared with the coconut literature and an empirical model was proposed for estimating fIPAR from age and planting density. The coefficient of extinction, K, was adjusted to 0.33 for the regular range of planting density.