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To investigate whether sun and shade leaves respond differently to CO 2  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 CO 2  in the Brookhaven National Laboratory/Duke University Free Air CO 2  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 CO 2  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 CO 2  strongly stimulated light-saturated photosynthesis of sun and shade leaves in June and August; however, the relative photosynthetic enhancement by elevated CO 2  for sun leaves was more than double the relative enhancement of shade leaves. Elevated CO 2  stimulated apparent quantum yield by 30%, but there was no interaction between CO 2  and leaf position. Daytime leaf-level carbon gain extrapolated from photosynthetic light response curves indicated that sun leaves were enhanced 98% by elevated CO 2 , whereas shade leaves were enhanced 41%. Elevated CO 2  did not significantly affect leaf N per unit area in sun or shade leaves during either measurement period. Thus, the greater CO 2  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 CO 2  concentrations on forest ecosystems must take account of the complex nature of the light environment through the canopy and how light interacts with CO 2  to affect photosynthesis.

tree physiology

Effects of CO 2  enrichment on the photosynthetic light response of sun and shade leaves of canopy sweetgum trees ( Liquidambar styraciflua ) in a forest ecosystem