Effects of CO2 enrichment on the photosynthetic light response of sun and shade leaves of canopy sweetgum trees (Liquidambar styraciflua) in a forestecosystem

To investigate whether sun and shade leaves respond differently to CO2 enri chment, we examined photosynthetic light response of sun and shade leaves i n canopy sweetgum (Liquidambar styraciflua L.) trees growing at ambient and elevated (ambient + 200 mu l l(-1)) atmospheric CO2 in the Brookhaven Nati onal Laboratory/Duke University Free Air CO2 Enrichment (FACE) experiment. The sweetgum trees were naturally established in a 15-year-old forest domin ated by loblolly pine (Pinus taeda L.). Measurements were made in early Jun e and late August 1997 during the first full year of CO2 fumigation in the Duke Forest FACE experiment. Sun leaves had a 68% greater leaf mass per uni t area, 63% more leaf N per unit leaf area, 27% more chlorophyll per unit l eaf area and 77% greater light-saturated photosynthetic rates than shade le aves. Elevated CO2 strongly stimulated light-saturated photosynthesis of su n and shade leaves in June and August; however, the relative photosynthetic enhancement by elevated CO2 for sun leaves was mon than double the relativ e enhancement of shade leaves. Elevated CO2 stimulated apparent quantum yie ld by 30%. but there was no interaction between CO2 and leaf position. Dayt ime leaf-level carbon gain extrapolated from photosynthetic light response curves indicated that sun leaves were enhanced 98% by elevated CO2, whereas shade leaves were enhanced 41%. Elevated CO2 did not significantly affect leaf N per unit area in sun or shade leaves during either measurement perio d. Thus, the greater CO2 enhancement of light-saturated photosynthesis in s un leaves than in shade leaves was probably a result of a greater amount of nitrogen per unit leaf area in sun leaves. A full understanding of the eff ects of increasing atmospheric CO2 concentrations on forest ecosystems must take account of the complex nature of the light environment through the ca nopy and how light interacts with CO2 to affect photosynthesis.