A three-dimensional crown architecture model for assessment of light capture and carbon gain by understory plants.

A model simulating the three-demensional crown architecture of a plant was developed with the objective of assessing the light capture and whole-plant carbon gain consequences of leaf display in understory plants. This model uses geometrical measurements taken in the field to reconstruct the projected image of a plant so that light absorption from any direction can be assessed. The photon flux density (PFD) from a given direction was estimated from the canopy openness derived from hemispherical canopy photographs and equations simulating the daily course of direct and diffuse PFD. For diffuse PFD, the directional fluxes and absorbed PFD were integrated over 160 different directions representing 8 azimuth classes and 20 elevation angle classes. Direct PFD absorption was determined for the time that a solar track on a given day intersected a canopy gap. Assimilation rate was simulated for the sunlit and shaded parts of leaves separately and then summed to give the whole-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 edge species is more efficient at capturing side light while the understory species is slightly more efficient at capturing light from directly above, the predominant light direction in this environment. Self-shading within Conostegia crown and steep leaf angles in the Clidemia crown reduced light capture efficiencies for light from directly above. Whole-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 in this environment conferred a 27% advantage in carbon gain in this environment.