Photosynthetic capacity of overstory Liquidambar styraciflua trees exposed to elevated CO2 and N fertilization in the Duke FACE experiment

In 2005, the Duke Forest Free Air CO₂ Enrichment (FACE) experiment initiated nitrogen treatments (11.2 g m^-2 yr^-1 ammonium nitrate) to test the progressive nitrogen limitation hypothesis. During 2005 and 2006, we measured leaf photosynthetic capacity and foliar N of Liquidambar styraciflua (sweetgum) sun leaves in order to detect potential nitrogen limitations and its consequences on photosynthetic regulation.  Fertilization significantly increased foliar N concentration on a mass basis (N_mass) by 35% in the control CO₂ plots and by 7% in the elevated CO₂ plots (ambient +200 µl l^-1).  This effect of CO₂ treatment was primarily caused by CO₂-induced increases in leaf mass per area.  Fertilization stimulated N concentration on an area basis (N_area) by about 9% regardless of CO₂ treatment. This change in foliar N had no effect on light saturate photosynthetic rates.  Elevated CO₂ stimulated photosynthesis (A_sat) by as much as 144% in the unfertilized plots and by as much as 71% in fertilized treatments. There were no effects of CO₂ or N fertilization on carboxylation rate (Vc_max).  Electron transport (J_max) was significantly higher in elevated CO₂ plots, but only in 2006.  Thus, we found no evidence of down regulation of photosynthetic capacity of sweetgum sun leaves due to the long term CO₂ treatment.  We had expected a greater enhancement of photosynthesis by CO₂ in the N fertilized treatment.  Instead, we observed no interaction of CO₂ and N on photosynthesis, indicating no discernable progressive nitrogen limitation on carbon uptake by the sun leaves of these overstory sweetgum trees.