The functional morphology of light capture and carbon gain in the Redwood forest understorey plant Adenocaulon bicolor Hook

1. A three-dimensional geometric simulation model of crown architecture was utilized to investigate the efficiency of light capture and its relationship to whole-plant CO2 assimilation of Adenocaulon bicolor. 2. Positioning of the leaves by the combined effects of ontogenetic variations in petiole length and angle and leaf size, and the leaf divergence angles were shown to be effective in minimizing self shading. The efficiency of light absorption varied from 0.64 to 0.70 among individual plants that were sampled. 3. Plant to plant variation in simulated daily carbon gain was strongly influenced by variations in the direct and diffuse PFD received by the individual plants. When simulations were run for all plants under a single common light environment, the carbon gain was strongly dependent on the efficiencies of light absorption of the different plants. 4. Simulations in which petiole length was varied showed a non-linear dependence of light absorption efficiency on petiole length. When both petiole length and leaf size were varied in a way that maintained a constant biomass then an optimal petiole length that corresponded to the observed petiole length was apparent. The observed divergence angle between successive leaves also maximized light absorption efficiency as compared to greater or lesser angles, but increases in internode length had no significant effect. 5. The results of this study provide evidence for selection for an 'optimal design' of crown architecture in Adenocaulon bicolor that maximizes light capture.