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

We compared the CO- and light-dependence of photosynthesis of four tree species (Acer rubrum, Carya glabra, Cercis canadensis, Liquidambar styraciflua) growing in the understory of a loblolly pine plantation under ambient or ambient plus 200 µl l CO. Naturally-established saplings were fumigated with a free-air CO enrichment system. Light-saturated photosynthetic rates were 159-190% greater for Ce. canadensis saplings grown and measured under elevated CO. This species had the greatest CO stimulation of photosynthesis. Photosynthetic rates were only 59% greater for A. rubrum saplings under CO enrichment and Ca. glabra and L. styraciflua had intermediate responses. Elevated CO stimulated light-saturated photosynthesis more than the apparent quantum yield. The maximum rate of carboxylation of ribulose-1,5-bisphosphate carboxylase, estimated from gas-exchange measurements, was not consistently affected by growth in elevated CO. However, the maximum electron transport rate estimated from gas- exchange measurements and from chlorophyll fluorescence, when averaged across species and dates, was approximately 10% higher for saplings in elevated CO. The proportionately greater stimulation of light-saturated photosynthesis than the apparent quantum yield and elevated rates of maximum electron transport suggests that saplings growing under elevated CO make more efficient use of sunflecks. The stimulation of light-saturated photosynthesis by CO did not appear to correlate with shade-tolerance ranking of the individual species. However, the species with the greatest enhancement of photosynthesis, Ce. canadensis and L. styraciflua, also invested the greatest proportion of soluble protein in Rubisco. Environmental and endogenous factors affecting N partitioning may partially explain interspecific variation in the photosynthetic response to elevated CO.