What was done
The authors grew seedlings of two shade-tolerant northern hardwood tree species - sugar maple (Acer saccharum Marsh.) and American beech (Fagus grandifolia J.F. Ehrh.) - as well as seedlings of two shade-intolerant northern hardwood tree species - black cherry (Prunus serotina J.F. Ehrh.) and paper birch (Betula papyrifera Marsh.) - for two full growing seasons inside open-top chambers maintained at either ambient (383 ppm) or elevated (658 ppm) atmospheric CO2 concentrations within an overarching 90-year-old nitrogen-limited northern hardwood forest located in Northern Lower Michigan, USA, to determine their responses to atmospheric CO2 enrichment in two contrasting degrees of shade: moderate shade (14.2 µmol photons m^-2 s^-1 = 5.6% full sun) and deep shade (6.5 µmol photons m^-2 s^-1 = 2.2% full sun).

What was learned
It was determined, according to Sefcik et al., that "the magnitude of enhancement from exposure to elevated CO2 was similar for both shade-tolerance groups," with the elevated CO2 treatment increasing the mean light-saturated net photosynthetic rate by 63% in the shade-tolerant species and by 67% in the shade-intolerant species. More importantly, however, they found that "seedlings grown in deep shade, regardless of shade-tolerance group, showed a greater long-term photosynthetic enhancement to elevated CO2 than those grown in moderate shade," with the mean long-term enhancement being 47% in moderate shade and a whopping 97% in deep shade.

What it means
Noting that the same type of photosynthetic response "has also been found in a number of other studies, suggesting that the impact of a CO2-enriched atmosphere increases as light becomes more limiting (Hattenschwiler, 2001; Granados and Korner, 2002; Leakey et al., 2002)," Sefcik et al. conclude that "if long-term enhancement of photosynthesis in elevated CO2 and deep shade translates into greater survival [and it seems only logical that it would], especially for shade-intolerant species [ditto], this could have profound successional implications for nitrogen-limited northern hardwood forest composition in a future higher CO2 atmosphere," with the likely end result, we would suggest, of enhancing local species richness.

References
Granados, J. and Korner, C. 2002. In deep shade, elevated CO2 increases the vigor of tropical climbing plants. Global Change Biology 8: 1109-1117.

Hattenschwiler, S. 2001. Tree seedling growth in natural deep shade: functional traits related to interspecific variation in response to elevated CO2. Oecologia 129: 31-42.

Leakey, A.D.B., Press, M.C., Scholes, J.D. and Watling, J.R. 2002. Relative enhancement of photosynthesis and growth at elevated CO2 is greater under sunflecks than uniform irradiance in a tropical rain forest tree seedling. Plant, Cell and Environment 25: 1701-1714.