Photosynthetic responses of four understory tree species exposed to elevated CO2 and N fertilization in the Duke FACE experiment

In 2005, nitrogen fertilization (11.2 g m^-2 yr^-1 ammonium nitrate) was initiated in the Duke Forest Free Air CO₂ Enrichment (FACE) experiment in order to test the progressive nitrogen limitation hypothesis. We were interested in whether soil N additions would interact with CO₂ to alter photosynthesis of understory tree species, and specifically, whether N or CO₂ differentially affected carboxylation or electron transport.  We examined photosynthetic capacity of four species (Acer rubrum, Carya glabra, Ulmus alata, and Liquidambar styraciflua) of understory trees in June and September of 2006.  Averaged across all species, foliar N concentrations on a mass basis (N_mass) and on an area basis (N_area) both increased by 17% in response to N fertilization. Elevated CO₂ stimulated light-saturated photosynthesis by 41% when averaged across all species and N fertilization stimulated photosynthesis by 13%.  The effects of CO₂ and N fertilization on photosynthesis were additive; we found no interactions between CO₂ and N on photosynthesis of these tree species.  Elevated CO₂ had no significant effect on photosynthetic capacity, but nitrogen fertilization stimulated photosynthetic capacity a small amount by increasing carboxylation efficiency by 5% and electron transport efficiency by 3%.  After ten years of CO₂ enrichment, we found no evidence of photosynthetic down regulation in these four understory tree species grown in elevated CO₂, and the additive effect of N fertilization and CO₂ enrichment on carbon uptake by the shade leaves of these four understory trees species indicated no discernable evidence of progressive nitrogen limitation.