Photosynthetic responses to CO2 enrichment of four hardwood species in a forest understory

We compared the CO2- and light-dependence of photosynthesis of four tree sp ecies (Acer rubrum, Carya glabra, Cercis canadensis, Liquidambar styraciflu a) growing in the understory of a loblolly pine plantation under ambient or ambient plus 200 mu l l(-1) CO2. Naturally-established saplings were fumig ated with a free-air CO2 enrichment system. Light-saturated photosynthetic rates were 159-190% greater for Ce. canadensis saplings grown and measured under elevated CO2. This species had the greatest CO2 stimulation of photos ynthesis. Photosynthetic rates were only 59% greater for A. rubrum saplings under CO2 enrichment and Ca. glabra and L. styraciflua had intermediate re sponses. Elevated CO2 stimulated light-saturated photosynthesis more than t he apparent quantum yield. The maximum rate of carboxylation of ribulose-1, 5-bisphosphate carboxylase, estimated from gas-exchange measurements, was n ot consistently affected by growth in elevated CO2. However, the maximum el ectron transport rate estimated from gas-exchange measurements and from chl orophyll fluorescence, when averaged across species and dates, was approxim ately 10% higher for saplings in elevated CO2. The proportionately greater stimulation of light-saturated photosynthesis than the apparent quantum yie ld and elevated rates of maximum electron transport suggests that saplings growing under elevated CO2 make more efficient use of sunflecks. The stimul ation of light-saturated photosynthesis by CO2 did not appear to correlate with shade-tolerance ranking of the individual species. However, the specie s with the greatest enhancement of photosynthesis, Ce. canadensis and L. st yraciflua, also invested the greatest proportion of soluble protein in Rubi sco. Environmental and endogenous factors affecting N partitioning may part ially explain interspecific variation in the photosynthetic response to ele vated CO2.