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

Dynamic responses of understory plants to sunflecks have been extensively s tudied, but how much differences in dynamic light responses affect daily ph otosynthesis (A(day)) is still the subject of active research. Recent model s of dynamic photosynthesis have provided a quantitative tool that allows t he critical assessment of the importance of these sunfleck responses on A(d ay). Here we used a dynamic photosynthesis model to assess differences in f our 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 fl orida, Liquidambar styraciflua). As a consequence, L. tulipifera should hav e the highest A(day) in an understory environment, despite being the least shade tolerant of the species tested. We further hypothesized that daily ph otosynthetic enhancement by elevated CO2 would differ from enhancement leve ls observed during light-saturated, steady-state measurements. Both hypothe ses were supported by the model results under conditions of low daily photo synthetic 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 photosy nthesis were small. Thus, the relative species ranking in A(day) under mode rate PFD closely matched their rankings in steady-state measurements of lig ht-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 i n dynamic sunfleck responses, and differences in elevated CO2 responses of daily photosynthesis, are most important in marginal light environments.