Influence of foliar phenology and shoot inclination on annual photosynthetic gain in individual beech saplings: A functional-structural modeling approach

We developed a functional-structural plant model for Fagus crenata saplings and calculated annual photosynthetic gains to determine the influences of foliar phenology and shoot inclination on the carbon economy of saplings. The model regenerated the three-dimensional shoot structure and spatial and temporal display of leaves; we calculated the hourly light interception of each leaf with a detailed light model that allowed us to estimate hourly leaf photosynthetic gain taking leaf age into account. To evaluate the importance of simultaneous foliar phenology and slanting shoots in beech saplings, we calculated the photosynthetic budgets for saplings with contrasting foliar phenologies and shoot inclinations. In our simulations, we distinguished between simultaneous and successive foliar phenologies, upright and slanting shoot inclinations, and environments with and without a vertical gradient in light intensity. Other model parameters (including photosynthesis vs. light curve, leaf size, and leaf shape) were obtained directly from live beech saplings. With no vertical gradient in light intensity, modeled saplings with simultaneous foliar phenology and slanting shoots (as in live beech) had larger annual photosynthetic gains than saplings with other combinations of traits. Hence, simultaneous foliar phenology and slanting shoots are efficient ways to display leaves in the shaded forest understory light regime where beech saplings thrive. In the presence of vertical light gradients, which can occur in canopy gaps, saplings with upright shoots had larger annual photosynthetic gains than counterparts with slanting shoots. Although mean daily photosynthetic gains of saplings with successive foliar phenology were elevated by exposing leaves to strong light when young and productive, the annual photosynthetic budget of these saplings was reduced (compared to saplings with simultaneous foliar phenology) by their relatively short leaf lifespan. Overall, our results suggest that slanting shoots with simultaneous foliar phenology are particularly successful in shaded environments, where beech often dominates, because they appear to maximize the annual carbon budget by avoiding self-shading and extending leaf lifespans.