Light capture in the forest is determined by the quantity and spatial distribution of direct and diffuse radiation, and the architectural arrangement of leaves within the plant’s crown. These factors are modified by plant size. Different forest environments result in major changes in incident light and directional traits (diffuse vs. direct) of sunlight.. Our question was how direct and diffuse radiation contribute to whole-seedling foliage photochemistry and how their relative contribution varies as seedlings grow in their contrasting recruiting light environments. We postulate that as seedlings grow, reaching more illuminated areas, an increase in diffuse radiation inside the canopy is produced, determining an increment in the whole-seedling photochemical efficiency. Forty plants were digitalized in a range of height (5-60 cm) growing under the canopy (<5% and 30-35% canopy openness, respectively). We used the model Y-plant integrated with chlorophyll fluorescence and CO₂ assimilation evaluations to examine the effect of plant size and light availability in the energy flux partitioning of direct and diffuse radiation in Nothofagus nitida (Phil.) Krasser plants in the Chilean temperate rainforest.

Results/Conclusions

In both environments, the interception of direct radiation increased with plant size in greater proportion than diffuse radiation interception. In gaps, the photochemical flux by direct and diffuse radiation was similar and increased with plant size. However, direct radiation generates a greater proportion of non-photochemical flux than diffuse radiation, increasing with plant size. In understorey, the photochemical flux by diffuse radiation was higher than by direct radiation. This difference increased with plant size. There were no changes in projected and displayed efficiencies, self-shading, and diffuse, direct and total light capture efficiency with seedling height in both light environments. Regardless of the shoot architectural features, there was a 21% increase in seedling photochemical efficiency with the ontogeny in gap environments, which is mainly caused  by an ontogenetic increment in photochemical efficiency of the leaf fraction that receive diffuse radiation. The importance of shaded foliage in tree photosynthesis may be higher than previously thought. This functional response can be advantageous for N. nitida during its ontogeny, when established in more competitive habitats, as open sites. (FONDECYT-1070551 and CONICYT AT-24071033).