Modeling dynamic understory photosynthesis of contrasting species in ambient and elevated carbon dioxide

Dynamic responses of understory plants to sunflecks have been extensively studied, but how much differences in dynamic light responses affect daily photosynthesis (A_day) is still the subject of active research. Recent models of dynamic photosynthesis have provided a quantitative tool that allows the critical assessment of the importance of these sunfleck responses on A_day. Here we used a dynamic photosynthesis model to assess differences in four species that were growing in ambient and elevated CO₂. We hypothesized that Liriodendron tulipifera, a species with rapid photosynthetic induction gain and slow induction loss, would have the least limitations to sunfleck photosynthesis relative to the other three species (Acer rubrum, Cornus florida, Liquidambar styraciflua). As a consequence, L. tulipifera should have the highest A_day in an understory environment, despite being the least shade tolerant of the species tested. We further hypothesized that daily photosynthetic enhancement by elevated CO₂ would differ from enhancement levels observed during light-saturated, steady-state measurements. Both hypotheses were supported by the model results under conditions of low daily photosynthetic photon flux density (PFD; <3% of the above-canopy PFD). However, under moderate PFD (10-20% of the above-canopy PFD), differences in dynamic sunfleck responses had no direct impact on A_day for any of the species, since stomatal and photosynthetic induction limitations to sunfleck photosynthesis were small. Thus, the relative species ranking in A_day under moderate PFD closely matched their rankings in steady-state measurements of light-saturated photosynthesis. Similarly, under elevated CO₂, enhancement of modeled A_day over A_day at ambient CO₂ matched the enhancement measured under light saturation. Thus, the effects of species-specific differences in dynamic sunfleck responses, and differences in elevated CO₂ responses of daily photosynthesis, are most important in marginal light environments.