Effects Of CO2 Enrichment On The Photosynthetic Light Response Of Sun And Shade Leaves Of Canopy Sweetgum Trees (Liquidambar Styraciflua) In A Forest Ecosystem

To investigate whether sun and shade leaves respond differently to CO₂ enrichment, we examined photosynthetic light response of sun and shade leaves in canopy sweetgum (Liquidambar styraciflua L.) trees growing at ambient and elevated (ambient + 200 μl l⁻¹) atmospheric CO₂ in the Brookhaven National Laboratory/Duke University Free Air CO₂ Enrichment (FACE) experiment. The sweetgum trees were naturally established in a 15-year-old forest dominated by loblolly pine (Pinus taeda L.). Measurements were made in early June 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 unit area, 63% more leaf N per unit leaf area, 27% more chlorophyll per unit leaf area and 77% greater light-saturated photosynthetic rates than shade leaves. Elevated CO₂ strongly stimulated light-saturated photosynthesis of sun and shade leaves in June and August; however, the relative photosynthetic enhancement by elevated CO₂ for sun leaves was more than double the relative enhancement of shade leaves. Elevated CO₂ stimulated apparent quantum yield by 30%, but there was no interaction between CO₂ and leaf position. Daytime 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 period. Thus, the greater CO₂ enhancement of light-saturated photosynthesis in sun 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 effects of increasing atmospheric CO₂ concentrations on forest ecosystems must take account of the complex nature of the light environment through the canopy and how light interacts with CO₂ to affect photosynthesis.