Canopy architecture, photosynthetic dynamics and the importance of sunflecks for understory sapling performance in ambient and elevated carbon dioxide

The carbon balance of forest understory plants ultimately determines whether a plant will survive in the light-limited environment. Carbon gain in the understory is affected by the interplay of (a) the leaf-level light environment impacted by the sun's position, weather, overstory canopy, and architectural characteristics of the plant itself, (b) the steady state and dynamic photosynthetic characteristics of leaves, and (c) fractional plant carbon allocation to leaves. In this study, 1 employed models to estimate daily photosynthesis as a function of differences in daily PFD (photosynthetic photon flux density) courses, species differences in both steady state and dynamic photosynthetic characteristics, and growth atmospheric CO2 concentration. The co-occurring species (Acer rubrum, Comus florida, Liquidambar styraciflua, Liriodendron tulipifera) indeed differed in their dynamic responses to changes in PFD and these differences had ramifications for daily photosynthesis. Elevated CO2 in free air (FACE) impacted daily photosynthesis via a direct enhancement of photosynthesis and slightly changed dynamic responses. The effects of dynamic responses and CO2 were strongest in their impact on performance in low light microsites. In these microsites daily photosynthesis can be reduced around 30% due to stomatal and biochemical limitations to sunfleck photosynthesis. Further, elevated atmospheric CO2 (+200 ppm) can enhance daily photosynthesis by up to a factor of 2.5 in these sites compared to around 1.5 in moderate light microsites. In addition, differences in sunfleck frequency and intensity as well as diffuse light PFD within microsites of similar daily PFD exert an additional strong influence on daily photosynthesis. Thus, to adequately estimate daily carbon gain in forest understory plants, fine-scale information on light regimes as well as photosynthetic dynamics is needed. Scaling this daily carbon gain to entire sapling canopies over 24 hours further revealed that differences in carbon allocation to leaf area between the species and CO2 treatment largely determined sapling canopy carbon gain. In conclusion, while leaf-level species comparisons revealed significant impacts of differences in photosynthetic characteristics, at the plant level these differences were overshadowed by light microsite effects and allocational differences among species.