A 3-DIMENSIONAL CROWN ARCHITECTURE MODEL FOR ASSESSMENT OF LIGHT CAPTURE AND CARBON GAIN BY UNDERSTORY PLANTS

A model simulating the three-dimensional crown architecture of a plant was developed with the objective of assessing the light capture and w hole-plant carbon gain consequences of leaf display in understory plan ts. This model uses geometrical measurements taken in the field to rec onstruct the projected image of a plant so that light absorption from any direction can be assessed. The photon flux density (PFD) from a gi ven direction was estimated from the canopy openness derived from hemi spherical canopy photographs and equations simulating the daily course of direct and diffuse PFD. For diffuse PFD, the directional fluxes an d absorbed PFD were integrated over 160 different directions represent ing 8 azimuth classes and 20 elevation angle classes. Direct PFD absor ption was determined for the time that a solar track on a given day in tersected a canopy gap. Assimilation rate was simulated for the sunlit and shaded parts of leaves separately and then summed to give the who le-plant carbon gain. Comparisons of simulations for a tropical forest edge species, Clidemia octona, and an understory species, Conostegia cinnamomea, illustrate the operation of the model and show that the ed ge species is more efficient at capturing side light while the underst ory species is slightly more efficient at capturing light from directl y above, the predominant light direction in this environment. Self-sha ding within Conostegia crown and steep leaf angles in the Clidemia cro wn reduced light capture efficiencies for light from directly above. W hole-plant daily carbon gain was much higher in the forest edge site, mostly because of the additional PFD available in this site. However, simulations for both species in the understory light environment show that the higher light capture efficiencies of the understory species i n this environment conferred a 27% advantage in carbon gain in this en vironment.