What was done
The authors measured photosynthetic rates in leaves of four hardwood saplings growing beneath the canopy of a Pinus taeda forest, several portions of which were exposed to either ambient or enriched (ambient + 200 ppm) atmospheric CO2 concentrations in a FACE study spanning two complete years.  The measurements were made under conditions of both low and moderate light intensity (less than 3% and 10-20% of the light intensity at the top of the canopy, respectively).  These photosynthetic rates were then used to validate a model for predicting photosynthetic carbon uptake in response to intermittent illumination by sunflecks.  Hence, the authors measured and modeled the effects of elevated CO2 on photosynthetic performance in several hardwood saplings under the low and variable light conditions that prevail beneath the canopies of mature forest species.

What was learned
The elevated atmospheric CO2 concentration stimulated daily photosynthetic carbon uptake more at the lower, rather than the higher, light intensity.  Indeed, the extra 200 ppm of CO2 generally enhanced daily photosynthetic carbon uptake by more than two-fold in three of the four species studied.

What it means
As the atmospheric CO2 content increases, understory trees growing beneath mature forest canopies will likely exhibit enhanced daily photosynthetic carbon gains, even in low-light environments supported by intermittent sunflecks.  The authors thus conclude that "elevated CO2 could have profound impacts on individual species' performance in marginal understory sites" and that "rising CO2 may benefit plants growing in poor light microsites relatively more than at better microsites."  In agreement with the general finding of Idso and Idso (1994), the present data once again demonstrate that elevated CO2 generally stimulates plant growth to a greater extent in low, rather than high, light environments.

Reference
Idso, K.E. and Idso, S.B.  1994.  Plant responses to atmospheric CO2 enrichment in the face of environmental constraints: a review of the past 10 years' research.  Agricultural and Forest Meteorology 69: 153-203.