What was done
In August 1996, circular FACE plots (30-m diameter) receiving 360 and 560 ppm CO2 were established in a 15-year-old loblolly pine plantation in North Carolina, USA.  Although the plantation is dominated by loblolly pine, several hardwood species are present in the understory beneath the primary coniferous canopy.  This paper describes the photosynthetic responses of four such hardwood tree species to elevated CO2 after one-year of differential CO2 treatment.

What was learned
The modest 200-ppm increase in the atmospheric CO2 concentration resulted in significant sustained increases in light-saturated rates of net photosynthesis in all hardwood species studied.  In fact, the CO2-induced photosynthetic stimulation was approximately 50, 75, 100, and 160% for saplings of Acer rubrum, Carya glabra, Liquidambar styraciflua, and Cercis canadensis, respectively.

What it means
As the CO2 content of the air rises, it is likely that forest understory species will exhibit significant increases in net photosynthesis, in spite of the reduced light levels that reach their leaves.  Indeed, the authors stated that "shaded saplings growing under elevated CO2 may be more efficient at using sunflecks than those growing under current ambient conditions."  Thus, with greater rates of photosynthesis and carbohydrate production, it is likely that understory trees will increase their growth rates and accumulate more biomass as the air's CO2 concentration continues to rise.