What was done
Over a period of four years (1998-2001), the authors studied the effects of atmospheric CO2 enrichment (to 200 ppm above ambient) on the stomatal characteristics of sun and shade leaves of overstory sweetgum (Liquidambar styraciflua L.) trees growing in the Duke Forest FACE facility near Chapel Hill, North Carolina, USA.

What was learned
With respect to long-term developmental change, Herrick et al. determined that the stomatal densities of both sun and shade leaves were unaffected by elevated CO2.  However, they found that CO2 enrichment decreased stomatal conductances by an average of 31% in sun leaves and 25% in shade leaves, which when scaled to a CO2 enrichment of 300 ppm correspond to approximate reductions of 46% and 38%, respectively.  They also found that these responses were not affected by leaf position or time of year, and that elevated CO2 did not alter the sensitivity of stomatal conductance to either soil moisture or vapor pressure deficit.  Last of all, their work revealed that the sensitivity of stomatal conductance to CO2 did not subside over the course of the four-year study.

What it means
Herrick et al. conclude that "decreases in leaf-level stomatal conductance caused by CO2 enrichment are relatively consistent across a variety of environmental conditions, and do not abate over time."  Indeed, the responses are well-defined and stable.